Alexandre Roman | Universidad de La Serena-Chile (original) (raw)

Papers by Alexandre Roman

Research paper thumbnail of From the bulge to the outer disc: StarHorse stellar parameters, distances, and extinctions for stars in APOGEE DR16 and other spectroscopic surveys

Astronomy & Astrophysics

We combine high-resolution spectroscopic data from APOGEE-2 survey Data Release 16 (DR16) with br... more We combine high-resolution spectroscopic data from APOGEE-2 survey Data Release 16 (DR16) with broad-band photometric data from several sources as well as parallaxes from Gaia Data Release 2 (DR2). Using the Bayesian isochrone-fitting code StarHorse, we derived the distances, extinctions, and astrophysical parameters for around 388 815 APOGEE stars. We achieve typical distance uncertainties of ∼6% for APOGEE giants, ∼2% for APOGEE dwarfs, and extinction uncertainties of ∼0.07 mag, when all photometric information is available, and ∼0.17 mag if optical photometry is missing. StarHorse uncertainties vary with the input spectroscopic catalogue, available photometry, and parallax uncertainties. To illustrate the impact of our results, we show that thanks to Gaia DR2 and the now larger sky coverage of APOGEE-2 (including APOGEE-South), we obtain an extended map of the Galactic plane. We thereby provide an unprecedented coverage of the disc close to the Galactic mid-plane (|ZGal| < 1 k...

Research paper thumbnail of The 16th Data Release of the Sloan Digital Sky Surveys: First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra

The Astrophysical Journal Supplement Series

This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the ... more This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17).

Research paper thumbnail of NLTE NH3(J,K) = (1,1) observations towards southern hemisphere compact regions

Symposium - International Astronomical Union

We present NH3(J,K) = (1,1) observations toward CS(2,1) sources in the southern hemisphere, obtai... more We present NH3(J,K) = (1,1) observations toward CS(2,1) sources in the southern hemisphere, obtained with the Itapetinga Radio Telescope, that exhibit departure from LTE conditions. The mechanism of selective trapping in the hyperfine transitions of NH3(J, K) =(2,1)-(1,1) is invoked to explain the non-thermal population in the NH3(J, K) = (1,1) hyperfine states. This effect is relevant only when the width of the hyperfine lines lie between 0.3 and 0.6 km s−1. Due to this restriction, the assumption that the molecular cloud is formed by clumps, which produce spectra within this line-width interval seems to be a natural explanation for the non-thermal population. The observed spectra can be the result of the superposition of individual clump spectra with different central velocities. This model was applied to determine the physical conditions of the observed regions, providing satisfactory results for most of them. However, for some sources the model is not adequate to reproduce the o...

Research paper thumbnail of Massive Stars in the SDSS-IV/APOGEE SURVEY. I. OB Stars

The Astrophysical Journal

In this work, we make use of DR14 APOGEE spectroscopic data to study a sample of 92 known OB star... more In this work, we make use of DR14 APOGEE spectroscopic data to study a sample of 92 known OB stars. We developed a near-infrared semi-empirical spectral classification method that was successfully used in case of four new exemplars, previously classified as later B-type stars. Our results agree well with those determined independently from ECHELLE optical spectra, being in line with the spectral types derived from the "canonical" MK blue optical system. This confirms that the APOGEE spectrograph can also be used as a powerful tool in surveys aiming to unveil and study a large number of moderately and highly obscured OB stars still hidden in the Galaxy.

Research paper thumbnail of The APOGEE-2 Survey of the Orion Star-forming Complex. I. Target Selection and Validation with Early Observations

The Astrophysical Journal Supplement Series

The Orion Star Forming Complex (OSFC) is a central target for the APOGEE-2 Young Cluster Survey. ... more The Orion Star Forming Complex (OSFC) is a central target for the APOGEE-2 Young Cluster Survey. Existing membership catalogs span limited portions of the OSFC, reflecting the difficulty of selecting targets homogeneously across this extended, highly structured region. We have used data from wide field photometric surveys to produce a less biased parent sample of young stellar objects (YSOs) with infrared (IR) excesses indicative of warm circumstellar material or photometric variability at optical wavelengths across the full 420 square degrees extent of the OSFC. When restricted to YSO candidates with H < 12.4, to ensure S/N ∼ 100 for a six visit source, this uniformly selected sample includes 1307 IR excess sources selected using criteria vetted by Koenig & Liesawitz and 990 optical variables identified in the Pan-STARRS1 3π survey: 319 sources exhibit both optical variability and evidence of circumstellar disks through IR excess. Objects from this uniformly selected sample received the highest priority for targeting, but required fewer than half of the fibers on each APOGEE-2 plate. We fill the remaining fibers with previously confirmed

Research paper thumbnail of A Chemical and Kinematical Analysis of the Intermediate-age Open Cluster IC 166 from APOGEE and Gaia DR2

The Astronomical Journal

IC 166 is an intermediate-age open cluster (OC) (∼1 Gyr) that lies in the transition zone of the ... more IC 166 is an intermediate-age open cluster (OC) (∼1 Gyr) that lies in the transition zone of the metallicity gradient in the outer disk. Its location, combined with our very limited knowledge of its salient features, make it an interesting object of study. We present the first high-resolution spectroscopic and precise kinematical analysis of IC 166, which lies in the outer disk with R GC ∼12.7 kpc. High-resolution H-band spectra were analyzed using observations from the SDSS-IV Apache Point Observatory Galactic Evolution Experiment survey. We made use of the Brussels Automatic Stellar Parameter code to provide chemical abundances based on a line-by-line approach for up to eight chemical elements (Mg, Si, Ca, Ti, Al, K, Mn, and Fe). The α-element (Mg, Si, Ca, and whenever available Ti) abundances, and their trends with Fe abundances have been analyzed for a total of 13 high-likelihood cluster members. No significant abundance scatter was found in any of the chemical species studied. Combining the positional, heliocentric distance, and kinematic information, we derive, for the first time, the probable orbit of IC 166 within a Galactic model including a rotating boxy bar, and found that it is likely that IC 166 formed in the Galactic disk, supporting its nature as an unremarkable Galactic OC with an orbit bound to the Galactic plane.

Research paper thumbnail of StarHorse: a Bayesian tool for determining stellar masses, ages, distances, and extinctions for field stars

Monthly Notices of the Royal Astronomical Society

Understanding the formation and evolution of our Galaxy requires accurate distances, ages and che... more Understanding the formation and evolution of our Galaxy requires accurate distances, ages and chemistry for large populations of field stars. Here we present several updates to our spectro-photometric distance code, that can now also be used to estimate ages, masses, and extinctions for individual stars. Given a set of measured spectrophotometric parameters, we calculate the posterior probability distribution over a given grid of stellar evolutionary models, using flexible Galactic stellar-population priors. The code (called StarHorse) can acommodate different observational datasets, prior options, partially missing data, and the inclusion of parallax information into the estimated probabilities. We validate the code using a variety of simulated stars as well as real stars with parameters determined from asteroseismology, eclipsing binaries, and isochrone fits to star clusters. Our main goal in this validation process is to test the applicability of the code to field stars with known Gaia-like parallaxes. The typical internal precision (obtained from realistic simulations of an APOGEE+Gaia-like sample) are 8% in distance, 20% in age, 6% in mass, and 0.04 mag in A V. The median external precision (derived from comparisons with earlier work for real stars) varies with the sample used, but lies in the range of [0, 2]% for distances, [12, 31]% for ages, [4, 12]% for masses, and 0.07 mag for A V. We provide StarHorse distances and extinctions for the APOGEE DR14, RAVE DR5, GES DR3 and GALAH DR1 catalogues.

Research paper thumbnail of The 13th Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-IV Survey Mapping Nearby Galaxies at Apache Point Observatory

The Astrophysical Journal Supplement Series

Where a licence is displayed above, please note the terms and conditions of the licence govern yo... more Where a licence is displayed above, please note the terms and conditions of the licence govern your use of this document. When citing, please reference the published version. Take down policy While the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has been uploaded in error or has been deemed to be commercially or otherwise sensitive.

Research paper thumbnail of SDSS-IV MaNGA: Spatially resolved star formation histories in galaxies as a function of galaxy mass and type

Monthly Notices of the Royal Astronomical Society

We study the internal gradients of stellar population properties within 1.5 R e for a representat... more We study the internal gradients of stellar population properties within 1.5 R e for a representative sample of 721 galaxies with stellar masses ranging between 10 9 M to 10 11.5 M from the SDSS-IV MaNGA IFU survey. Through the use of our full spectral fitting code FIREFLY, we derive light and mass-weighted stellar population properties and their radial gradients, as well as full star formation and metal enrichment histories. We also quanfify the impact that different stellar population models and full spectral fitting routines have on the derived stellar population properties, and the radial gradient measurements. In our analysis, we find that age gradients tend to be shallow for both early-type and late-type galaxies. Massweighted age gradients of early-types are positive (∼ 0.09 dex/R e) pointing to "outside-in" progression of star formation, while late-type galaxies have negative light-weighted age gradients (∼ −0.11 dex/R e), suggesting an "inside-out" formation of discs. We detect negative metallicity gradients in both early and late-type galaxies, but these are significantly steeper in late-types, suggesting that radial dependence of chemical enrichment processes and the effect of gas inflow and metal transport are far more pronounced in discs. Metallicity gradients of both morphological classes correlate with galaxy mass, with negative metallicity gradients becoming steeper with increasing galaxy mass. The correlation with mass is stronger for late-type galaxies, with a slope of d(∇[Z/H])/d(log M) ∼ −0.2 ± 0.05 , compared to d(∇[Z/H])/d(log M) ∼ −0.05 ± 0.05 for early-types. This result suggests that the merger history plays a relatively small role in shaping metallicity gradients of galaxies.

Research paper thumbnail of SDSS-IV MaNGA: stellar population gradients as a function of galaxy environment

Monthly Notices of the Royal Astronomical Society

We study the internal radial gradients of stellar population properties within 1.5 R e and analys... more We study the internal radial gradients of stellar population properties within 1.5 R e and analyse the impact of galaxy environment. We use a representative sample of 721 galaxies with masses ranging between 10 9 M and 10 11.5 M from the SDSS-IV survey MaNGA. We split this sample by morphology into early-type and late-type galaxies. Using the full spectral fitting code FIREFLY, we derive the light and mass-weighted stellar population properties, age and metallicity, and calculate the gradients of these properties. We use three independent methods to quantify galaxy environment, namely the Nth nearest neighbour, the tidal strength parameter Q and distinguish between central and satellite galaxies. In our analysis, we find that early-type galaxies generally exhibit shallow light-weighted age gradients in agreement with the literature and mass-weighted median age gradients tend to be slightly positive. Late-type galaxies, instead, have negative light-weighted age gradients. We detect negative metallicity gradients in both early-and late-type galaxies that correlate with galaxy mass, with the gradients being steeper and the correlation with mass being stronger in late-types. We find, however, that stellar population gradients, for both morphological classifications, have no significant correlation with galaxy environment for all three characterizations of environment. Our results suggest that galaxy mass is the main driver of stellar population gradients in both early and late-type galaxies, and any environmental dependence, if present at all, must be very subtle.

Research paper thumbnail of Discovery of a Metal-Poor Field Giant with a Globular Cluster Second-Generation Abundance Pattern

The Astrophysical Journal

We report on detection, from observations obtained with the APOGEE spectroscopic survey, of a met... more We report on detection, from observations obtained with the APOGEE spectroscopic survey, of a metal-poor ([Fe/H] = −1.3 dex) field giant star with an extreme Mg-Al abundance ratio ([Mg/Fe] = −0.31 dex; [Al/Fe] = 1.49 dex). Such low Mg/Al ratios are seen only among the second-generation population of globular clusters, and are not present among Galactic disk field stars. The light element abundances of this star, 2M16011638-1201525, suggest that it could have been born in a globular cluster. We explore several origin scenarios, in particular studying the orbit of the star to check the probability of it being kinematically related to known globular clusters. We performed simple orbital integrations assuming the estimated distance of 2M16011638-1201525 and the available six-dimensional phase-space coordinates of 63 globular clusters, looking for close encounters in the past with a minimum distance approach within the tidal radius of each cluster. We found a very low probability that 2M16011638-1201525 was ejected from most globular clusters; however, we note that the best progenitor candidate to host this star is globular cluster ω Centauri (NGC 5139). Our dynamical investigation demonstrates that 2M16011638-1201525 reaches a distance |Z max | < 3 kpc from the Galactic plane and a minimum and maximum approach to the Galactic center of R min < 0.62 kpc and R max < 7.26 kpc in an eccentric (e ∼ 0.53) and retrograde orbit. Since the extreme chemical anomaly of 2M16011638-1201525 has also been observed in halo field stars, this object could also be considered a halo contaminant, likely been ejected into the Milky Way disk from the halo. We conclude that, 2M16011638-20152 is also kinematically consistent with the disk but chemically consistent with halo field stars.

Research paper thumbnail of Atypical Mg-poor Milky Way Field Stars with Globular Cluster Second-generation-like Chemical Patterns

The Astrophysical Journal

We report the peculiar chemical abundance patterns of eleven atypical Milky Way (MW) field red gi... more We report the peculiar chemical abundance patterns of eleven atypical Milky Way (MW) field red giant stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). These atypical giants exhibit strong Al and N enhancements accompanied by C and Mg depletions, strikingly similar to those observed in the so-called second-generation (SG) stars of globular clusters (GCs). Remarkably, we find low-Mg abundances ([Mg/Fe]<0.0) together with strong Al and N overabundances in the majority (5/7) of the metal-rich ([Fe/H] −1.0) sample stars, which is at odds with actual observations of SG stars in Galactic CGs of similar metallicities. This chemical pattern is unique and unprecedented among MW stars, posing urgent questions about its origin. These atypical stars could be former SG stars of dissolved GCs formed with intrinsically lower abundances of Mg and enriched Al (subsequently self-polluted by massive AGB stars) or the result of exotic binary systems. We speculate that the stars Mg-deficiency as well as the orbital properties suggest that they could have an extragalactic origin. This discovery should guide future dedicated spectroscopic searches of atypical stellar chemical patterns in our Galaxy; a fundamental step forward to understand the Galactic formation and evolution.

Research paper thumbnail of Optical and near-infrared polarimetric study of the RCW121 Galactic H ii region

Monthly Notices of the Royal Astronomical Society

We present a polarimetric study of the RCW121 star-forming region to derive the orientation of th... more We present a polarimetric study of the RCW121 star-forming region to derive the orientation of the sky-projected magnetic field component traced by the polarization vectors, the morphology of which tends to follow the cloud's structure. Individual polarization-angle values are consistent across the different bands, having a broad distribution towards the RCW121 H II region. We estimate the corresponding magnetic field orientation in the H II region to have a mean value of −12 • ± 7 •. RCW121 shows an elongated shape in the same direction as the magnetic field orientation, which may be evidence that magnetic pressure opposes the H II region expansion. Serkowski's relation was used to determine individual values of the total-to-selective extinction ratio (R V) distribution and a weighted mean value of R V = 3.17 ± 0.05. We derive a foreground component of the polarization degree that is consistent with the literature value for this Galactic region.

Research paper thumbnail of Baade’s window and APOGEE

Astronomy & Astrophysics

Context. Baade's window (BW) is one of the most observed Galactic bulge fields in terms of chemic... more Context. Baade's window (BW) is one of the most observed Galactic bulge fields in terms of chemical abundances. Owing to its low and homogeneous interstellar absorption it is considered the perfect calibration field for Galactic bulge studies. Aims. In the era of large spectroscopic surveys, calibration fields such as BW are necessary for cross calibrating the stellar parameters and individual abundances of the APOGEE survey. Methods. We use the APOGEE BW stars to derive the metallicity distribution function (MDF) and individual abundances for αand iron-peak elements of the APOGEE ASPCAP pipeline (DR13), as well as the age distribution for stars in BW. Results. We determine the MDF of APOGEE stars in BW and find a remarkable agreement with that of the Gaia-ESO survey (GES). Both exhibit a clear bimodal distribution. We also find that the Mg-metallicity planes of the two surveys agree well, except for the metal-rich part ([Fe/H] > 0.1), where APOGEE finds systematically higher Mg abundances with respect to the GES. The ages based on the [C/N] ratio reveal a bimodal age distribution, with a major old population at ∼10 Gyr, with a decreasing tail towards younger stars. A comparison of stellar parameters determined by APOGEE and those determined by other sources reveals detectable systematic offsets, in particular for spectroscopic surface gravity estimates. In general, we find a good agreement between individual abundances of O, Na, Mg, Al, Si, K, Ca, Cr, Mn, Co, and Ni from APOGEE with that of literature values. Conclusions. We have shown that in general APOGEE data show a good agreement in terms of MDF and individual chemical abundances with respect to literature works. Using the [C/N] ratio we found a significant fraction of young stars in BW.

Research paper thumbnail of The Correlation between Mixing Length and Metallicity on the Giant Branch: Implications for Ages in theGaiaEra

The Astrophysical Journal

In the updated APOGEE-Kepler catalog, we have asteroseismic and spectroscopic data for over 3000 ... more In the updated APOGEE-Kepler catalog, we have asteroseismic and spectroscopic data for over 3000 first ascent red giants. Given the size and accuracy of this sample, these data offer an unprecedented test of the accuracy of stellar models on the post-main-sequence. When we compare these data to theoretical predictions, we find a metallicity dependent temperature offset with a slope of around 100 K per dex in metallicity. We find that this effect is present in all model grids tested and that theoretical uncertainties in the models, correlated spectroscopic errors, and shifts in the asteroseismic mass scale are insufficient to explain this effect. Stellar models can be brought into agreement with the data if a metallicity dependent convective mixing length is used, with ∆α ML,YREC ∼ 0.2 per dex in metallicity, a trend inconsistent with the predictions of three dimensional stellar convection simulations. If this effect is not taken into account, isochrone ages for red giants from the Gaia data will be off by as much as a factor of 2 even at modest deviations from solar metallicity ([Fe/H]=−0.5).

Research paper thumbnail of A Curved Magnetic Field in the Ring-like Shell of Bubble N4

The Astrophysical Journal, 2017

Research paper thumbnail of Suppressing star formation in quiescent galaxies with supermassive black hole winds

Nature, May 26, 2016

Quiescent galaxies with little or no ongoing star formation dominate the population of galaxies w... more Quiescent galaxies with little or no ongoing star formation dominate the population of galaxies with masses above 2 × 10(10) times that of the Sun; the number of quiescent galaxies has increased by a factor of about 25 over the past ten billion years (refs 1-4). Once star formation has been shut down, perhaps during the quasar phase of rapid accretion onto a supermassive black hole, an unknown mechanism must remove or heat the gas that is subsequently accreted from either stellar mass loss or mergers and that would otherwise cool to form stars. Energy output from a black hole accreting at a low rate has been proposed, but observational evidence for this in the form of expanding hot gas shells is indirect and limited to radio galaxies at the centres of clusters, which are too rare to explain the vast majority of the quiescent population. Here we report bisymmetric emission features co-aligned with strong ionized-gas velocity gradients from which we infer the presence of centrally dri...

Research paper thumbnail of Now you see it, now you don't: the disappearing central engine of the quasar J1011+5442

Monthly Notices of the Royal Astronomical Society, 2015

We report the discovery of a new "changing-look" quasar, SDSS J101152.98+544206.4, through repeat... more We report the discovery of a new "changing-look" quasar, SDSS J101152.98+544206.4, through repeat spectroscopy from the Time Domain Spectroscopic Survey. This is an addition to a small but growing set of quasars whose blue continua and broad optical emission lines have been observed to decline by a large factor on a time scale of approximately a decade. The 5100Å monochromatic continuum luminosity of this quasar drops by a factor of > 9.8 in a rest-frame time interval of < 9.7 years, while the broad Hα luminosity drops by a factor of 55 in the same amount of time. The width of the broad Hα line increases in the dim state such that the black hole mass derived from the appropriate single-epoch scaling relation agrees between the two epochs within a factor of 3. The fluxes of the narrow emission lines do not appear to change between epochs. The light curve obtained by the Catalina Sky Survey suggests that the transition occurs within a rest-frame time interval of approximately 500 days. We examine three possible mechanisms for this transition suggested in the recent literature. An abrupt change in the reddening towards the central engine is disfavored by the substantial difference between the timescale to obscure the central engine and the observed timescale of the transition. A decaying tidal disruption flare is consistent with the decay rate of the light curve but not with the prolonged bright state preceding the decay; nor can this scenario provide the power required by the luminosities of the emission lines. An abrupt drop in the accretion rate onto the supermassive black hole appears to be the most plausible explanation for the rapid dimming.

Research paper thumbnail of Survey of Young Stellar Objects Associated with Molecular Clouds

Open Issues in Local Star Formation, 2003

ABSTRACT We present the preliminary results of a survey in the near-infrared of young and massive... more ABSTRACT We present the preliminary results of a survey in the near-infrared of young and massive stellar objects embedded in dense molecular clouds. The survey was conducted at the LNA, Brazil, in the J, H, and nbK bands, in the direction of IRAS sources with colors characteristic of compact HII regions that also present strong CS(2-1) line emission. The coordinates of the IRAS sources were improved to 1.9n resolution with the help of the MSX catalog, allowing us to associate them univocally to the detected stars in our sample. Several IR nebulae were detected, indicating also that young stellar clusters are being formed in the core of the molecular clouds. From the total number of 127 observed regions we found that 105 presented stars in the H band inside the IRAS error ellipse, and were associated to a MSX point source. We obtained the integrated bolometric luminosities and main-sequence spectral types of the detected stars, assuming that the bolometric luminosity coincided with the integrated IR luminosity, and using the fact that for massive stars the evolution towards the main sequence proceeds at constant luminosity. The Initial Mass Function of the sample follows a Salpeter’s law with index 1.6 between spectral types B0 and O3.

Research paper thumbnail of The Stellar Population Associated with the IRAS Source 16132-5039

The Astronomical Journal, 2004

We report the discovery of a young massive stellar cluster and infrared nebula in the direction o... more We report the discovery of a young massive stellar cluster and infrared nebula in the direction of the CS molecular cloud associated to the IRAS point source 16132-5039. The analysis of the mid-infrared images from the more accurate MSX catalog, reveled that there are two independent components associated with the IRAS source. The integral of the spectral energy distribution for these components, between 8.28 µm and 100 µm, gave lower limits for the bolometric luminosity of the embedded objects of 8.7 × 10 4 L ⊙ and 9 × 10 3 L ⊙ , which corresponds to ZAMS O8 and B0.5 stars, respectively. The number of Lyman continuum photons expected from the stars that lie along the reddening line for early-type stars is about 1.7 × 10 49 s −1 , enough to produce the detected flux densities at 5 GHz. The NIR spectrum of the nebula increases with frequency, implying that free-free emission cannot be the main source of the extended luminosity, from which we conclude that the observed emission must be mainly dust scattered light. A comparison of the cluster described in this paper with the young stellar cluster associated with the IRAS source 16177-5018, which is located at the same distance and direction, shows that the mean visual absorption of the newly discovered cluster is about 10 magnitudes smaller and it contains less massive stars, suggesting that it was formed from a less massive molecular cloud.

Research paper thumbnail of From the bulge to the outer disc: StarHorse stellar parameters, distances, and extinctions for stars in APOGEE DR16 and other spectroscopic surveys

Astronomy & Astrophysics

We combine high-resolution spectroscopic data from APOGEE-2 survey Data Release 16 (DR16) with br... more We combine high-resolution spectroscopic data from APOGEE-2 survey Data Release 16 (DR16) with broad-band photometric data from several sources as well as parallaxes from Gaia Data Release 2 (DR2). Using the Bayesian isochrone-fitting code StarHorse, we derived the distances, extinctions, and astrophysical parameters for around 388 815 APOGEE stars. We achieve typical distance uncertainties of ∼6% for APOGEE giants, ∼2% for APOGEE dwarfs, and extinction uncertainties of ∼0.07 mag, when all photometric information is available, and ∼0.17 mag if optical photometry is missing. StarHorse uncertainties vary with the input spectroscopic catalogue, available photometry, and parallax uncertainties. To illustrate the impact of our results, we show that thanks to Gaia DR2 and the now larger sky coverage of APOGEE-2 (including APOGEE-South), we obtain an extended map of the Galactic plane. We thereby provide an unprecedented coverage of the disc close to the Galactic mid-plane (|ZGal| < 1 k...

Research paper thumbnail of The 16th Data Release of the Sloan Digital Sky Surveys: First Release from the APOGEE-2 Southern Survey and Full Release of eBOSS Spectra

The Astrophysical Journal Supplement Series

This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the ... more This paper documents the 16th data release (DR16) from the Sloan Digital Sky Surveys (SDSS), the fourth and penultimate from the fourth phase (SDSS-IV). This is the first release of data from the Southern Hemisphere survey of the Apache Point Observatory Galactic Evolution Experiment 2 (APOGEE-2); new data from APOGEE-2 North are also included. DR16 is also notable as the final data release for the main cosmological program of the Extended Baryon Oscillation Spectroscopic Survey (eBOSS), and all raw and reduced spectra from that project are released here. DR16 also includes all the data from the Time Domain Spectroscopic Survey and new data from the SPectroscopic IDentification of ERosita Survey programs, both of which were co-observed on eBOSS plates. DR16 has no new data from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey (or the MaNGA Stellar Library "MaStar"). We also preview future SDSS-V operations (due to start in 2020), and summarize plans for the final SDSS-IV data release (DR17).

Research paper thumbnail of NLTE NH3(J,K) = (1,1) observations towards southern hemisphere compact regions

Symposium - International Astronomical Union

We present NH3(J,K) = (1,1) observations toward CS(2,1) sources in the southern hemisphere, obtai... more We present NH3(J,K) = (1,1) observations toward CS(2,1) sources in the southern hemisphere, obtained with the Itapetinga Radio Telescope, that exhibit departure from LTE conditions. The mechanism of selective trapping in the hyperfine transitions of NH3(J, K) =(2,1)-(1,1) is invoked to explain the non-thermal population in the NH3(J, K) = (1,1) hyperfine states. This effect is relevant only when the width of the hyperfine lines lie between 0.3 and 0.6 km s−1. Due to this restriction, the assumption that the molecular cloud is formed by clumps, which produce spectra within this line-width interval seems to be a natural explanation for the non-thermal population. The observed spectra can be the result of the superposition of individual clump spectra with different central velocities. This model was applied to determine the physical conditions of the observed regions, providing satisfactory results for most of them. However, for some sources the model is not adequate to reproduce the o...

Research paper thumbnail of Massive Stars in the SDSS-IV/APOGEE SURVEY. I. OB Stars

The Astrophysical Journal

In this work, we make use of DR14 APOGEE spectroscopic data to study a sample of 92 known OB star... more In this work, we make use of DR14 APOGEE spectroscopic data to study a sample of 92 known OB stars. We developed a near-infrared semi-empirical spectral classification method that was successfully used in case of four new exemplars, previously classified as later B-type stars. Our results agree well with those determined independently from ECHELLE optical spectra, being in line with the spectral types derived from the "canonical" MK blue optical system. This confirms that the APOGEE spectrograph can also be used as a powerful tool in surveys aiming to unveil and study a large number of moderately and highly obscured OB stars still hidden in the Galaxy.

Research paper thumbnail of The APOGEE-2 Survey of the Orion Star-forming Complex. I. Target Selection and Validation with Early Observations

The Astrophysical Journal Supplement Series

The Orion Star Forming Complex (OSFC) is a central target for the APOGEE-2 Young Cluster Survey. ... more The Orion Star Forming Complex (OSFC) is a central target for the APOGEE-2 Young Cluster Survey. Existing membership catalogs span limited portions of the OSFC, reflecting the difficulty of selecting targets homogeneously across this extended, highly structured region. We have used data from wide field photometric surveys to produce a less biased parent sample of young stellar objects (YSOs) with infrared (IR) excesses indicative of warm circumstellar material or photometric variability at optical wavelengths across the full 420 square degrees extent of the OSFC. When restricted to YSO candidates with H < 12.4, to ensure S/N ∼ 100 for a six visit source, this uniformly selected sample includes 1307 IR excess sources selected using criteria vetted by Koenig & Liesawitz and 990 optical variables identified in the Pan-STARRS1 3π survey: 319 sources exhibit both optical variability and evidence of circumstellar disks through IR excess. Objects from this uniformly selected sample received the highest priority for targeting, but required fewer than half of the fibers on each APOGEE-2 plate. We fill the remaining fibers with previously confirmed

Research paper thumbnail of A Chemical and Kinematical Analysis of the Intermediate-age Open Cluster IC 166 from APOGEE and Gaia DR2

The Astronomical Journal

IC 166 is an intermediate-age open cluster (OC) (∼1 Gyr) that lies in the transition zone of the ... more IC 166 is an intermediate-age open cluster (OC) (∼1 Gyr) that lies in the transition zone of the metallicity gradient in the outer disk. Its location, combined with our very limited knowledge of its salient features, make it an interesting object of study. We present the first high-resolution spectroscopic and precise kinematical analysis of IC 166, which lies in the outer disk with R GC ∼12.7 kpc. High-resolution H-band spectra were analyzed using observations from the SDSS-IV Apache Point Observatory Galactic Evolution Experiment survey. We made use of the Brussels Automatic Stellar Parameter code to provide chemical abundances based on a line-by-line approach for up to eight chemical elements (Mg, Si, Ca, Ti, Al, K, Mn, and Fe). The α-element (Mg, Si, Ca, and whenever available Ti) abundances, and their trends with Fe abundances have been analyzed for a total of 13 high-likelihood cluster members. No significant abundance scatter was found in any of the chemical species studied. Combining the positional, heliocentric distance, and kinematic information, we derive, for the first time, the probable orbit of IC 166 within a Galactic model including a rotating boxy bar, and found that it is likely that IC 166 formed in the Galactic disk, supporting its nature as an unremarkable Galactic OC with an orbit bound to the Galactic plane.

Research paper thumbnail of StarHorse: a Bayesian tool for determining stellar masses, ages, distances, and extinctions for field stars

Monthly Notices of the Royal Astronomical Society

Understanding the formation and evolution of our Galaxy requires accurate distances, ages and che... more Understanding the formation and evolution of our Galaxy requires accurate distances, ages and chemistry for large populations of field stars. Here we present several updates to our spectro-photometric distance code, that can now also be used to estimate ages, masses, and extinctions for individual stars. Given a set of measured spectrophotometric parameters, we calculate the posterior probability distribution over a given grid of stellar evolutionary models, using flexible Galactic stellar-population priors. The code (called StarHorse) can acommodate different observational datasets, prior options, partially missing data, and the inclusion of parallax information into the estimated probabilities. We validate the code using a variety of simulated stars as well as real stars with parameters determined from asteroseismology, eclipsing binaries, and isochrone fits to star clusters. Our main goal in this validation process is to test the applicability of the code to field stars with known Gaia-like parallaxes. The typical internal precision (obtained from realistic simulations of an APOGEE+Gaia-like sample) are 8% in distance, 20% in age, 6% in mass, and 0.04 mag in A V. The median external precision (derived from comparisons with earlier work for real stars) varies with the sample used, but lies in the range of [0, 2]% for distances, [12, 31]% for ages, [4, 12]% for masses, and 0.07 mag for A V. We provide StarHorse distances and extinctions for the APOGEE DR14, RAVE DR5, GES DR3 and GALAH DR1 catalogues.

Research paper thumbnail of The 13th Data Release of the Sloan Digital Sky Survey: First Spectroscopic Data from the SDSS-IV Survey Mapping Nearby Galaxies at Apache Point Observatory

The Astrophysical Journal Supplement Series

Where a licence is displayed above, please note the terms and conditions of the licence govern yo... more Where a licence is displayed above, please note the terms and conditions of the licence govern your use of this document. When citing, please reference the published version. Take down policy While the University of Birmingham exercises care and attention in making items available there are rare occasions when an item has been uploaded in error or has been deemed to be commercially or otherwise sensitive.

Research paper thumbnail of SDSS-IV MaNGA: Spatially resolved star formation histories in galaxies as a function of galaxy mass and type

Monthly Notices of the Royal Astronomical Society

We study the internal gradients of stellar population properties within 1.5 R e for a representat... more We study the internal gradients of stellar population properties within 1.5 R e for a representative sample of 721 galaxies with stellar masses ranging between 10 9 M to 10 11.5 M from the SDSS-IV MaNGA IFU survey. Through the use of our full spectral fitting code FIREFLY, we derive light and mass-weighted stellar population properties and their radial gradients, as well as full star formation and metal enrichment histories. We also quanfify the impact that different stellar population models and full spectral fitting routines have on the derived stellar population properties, and the radial gradient measurements. In our analysis, we find that age gradients tend to be shallow for both early-type and late-type galaxies. Massweighted age gradients of early-types are positive (∼ 0.09 dex/R e) pointing to "outside-in" progression of star formation, while late-type galaxies have negative light-weighted age gradients (∼ −0.11 dex/R e), suggesting an "inside-out" formation of discs. We detect negative metallicity gradients in both early and late-type galaxies, but these are significantly steeper in late-types, suggesting that radial dependence of chemical enrichment processes and the effect of gas inflow and metal transport are far more pronounced in discs. Metallicity gradients of both morphological classes correlate with galaxy mass, with negative metallicity gradients becoming steeper with increasing galaxy mass. The correlation with mass is stronger for late-type galaxies, with a slope of d(∇[Z/H])/d(log M) ∼ −0.2 ± 0.05 , compared to d(∇[Z/H])/d(log M) ∼ −0.05 ± 0.05 for early-types. This result suggests that the merger history plays a relatively small role in shaping metallicity gradients of galaxies.

Research paper thumbnail of SDSS-IV MaNGA: stellar population gradients as a function of galaxy environment

Monthly Notices of the Royal Astronomical Society

We study the internal radial gradients of stellar population properties within 1.5 R e and analys... more We study the internal radial gradients of stellar population properties within 1.5 R e and analyse the impact of galaxy environment. We use a representative sample of 721 galaxies with masses ranging between 10 9 M and 10 11.5 M from the SDSS-IV survey MaNGA. We split this sample by morphology into early-type and late-type galaxies. Using the full spectral fitting code FIREFLY, we derive the light and mass-weighted stellar population properties, age and metallicity, and calculate the gradients of these properties. We use three independent methods to quantify galaxy environment, namely the Nth nearest neighbour, the tidal strength parameter Q and distinguish between central and satellite galaxies. In our analysis, we find that early-type galaxies generally exhibit shallow light-weighted age gradients in agreement with the literature and mass-weighted median age gradients tend to be slightly positive. Late-type galaxies, instead, have negative light-weighted age gradients. We detect negative metallicity gradients in both early-and late-type galaxies that correlate with galaxy mass, with the gradients being steeper and the correlation with mass being stronger in late-types. We find, however, that stellar population gradients, for both morphological classifications, have no significant correlation with galaxy environment for all three characterizations of environment. Our results suggest that galaxy mass is the main driver of stellar population gradients in both early and late-type galaxies, and any environmental dependence, if present at all, must be very subtle.

Research paper thumbnail of Discovery of a Metal-Poor Field Giant with a Globular Cluster Second-Generation Abundance Pattern

The Astrophysical Journal

We report on detection, from observations obtained with the APOGEE spectroscopic survey, of a met... more We report on detection, from observations obtained with the APOGEE spectroscopic survey, of a metal-poor ([Fe/H] = −1.3 dex) field giant star with an extreme Mg-Al abundance ratio ([Mg/Fe] = −0.31 dex; [Al/Fe] = 1.49 dex). Such low Mg/Al ratios are seen only among the second-generation population of globular clusters, and are not present among Galactic disk field stars. The light element abundances of this star, 2M16011638-1201525, suggest that it could have been born in a globular cluster. We explore several origin scenarios, in particular studying the orbit of the star to check the probability of it being kinematically related to known globular clusters. We performed simple orbital integrations assuming the estimated distance of 2M16011638-1201525 and the available six-dimensional phase-space coordinates of 63 globular clusters, looking for close encounters in the past with a minimum distance approach within the tidal radius of each cluster. We found a very low probability that 2M16011638-1201525 was ejected from most globular clusters; however, we note that the best progenitor candidate to host this star is globular cluster ω Centauri (NGC 5139). Our dynamical investigation demonstrates that 2M16011638-1201525 reaches a distance |Z max | < 3 kpc from the Galactic plane and a minimum and maximum approach to the Galactic center of R min < 0.62 kpc and R max < 7.26 kpc in an eccentric (e ∼ 0.53) and retrograde orbit. Since the extreme chemical anomaly of 2M16011638-1201525 has also been observed in halo field stars, this object could also be considered a halo contaminant, likely been ejected into the Milky Way disk from the halo. We conclude that, 2M16011638-20152 is also kinematically consistent with the disk but chemically consistent with halo field stars.

Research paper thumbnail of Atypical Mg-poor Milky Way Field Stars with Globular Cluster Second-generation-like Chemical Patterns

The Astrophysical Journal

We report the peculiar chemical abundance patterns of eleven atypical Milky Way (MW) field red gi... more We report the peculiar chemical abundance patterns of eleven atypical Milky Way (MW) field red giant stars observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE). These atypical giants exhibit strong Al and N enhancements accompanied by C and Mg depletions, strikingly similar to those observed in the so-called second-generation (SG) stars of globular clusters (GCs). Remarkably, we find low-Mg abundances ([Mg/Fe]<0.0) together with strong Al and N overabundances in the majority (5/7) of the metal-rich ([Fe/H] −1.0) sample stars, which is at odds with actual observations of SG stars in Galactic CGs of similar metallicities. This chemical pattern is unique and unprecedented among MW stars, posing urgent questions about its origin. These atypical stars could be former SG stars of dissolved GCs formed with intrinsically lower abundances of Mg and enriched Al (subsequently self-polluted by massive AGB stars) or the result of exotic binary systems. We speculate that the stars Mg-deficiency as well as the orbital properties suggest that they could have an extragalactic origin. This discovery should guide future dedicated spectroscopic searches of atypical stellar chemical patterns in our Galaxy; a fundamental step forward to understand the Galactic formation and evolution.

Research paper thumbnail of Optical and near-infrared polarimetric study of the RCW121 Galactic H ii region

Monthly Notices of the Royal Astronomical Society

We present a polarimetric study of the RCW121 star-forming region to derive the orientation of th... more We present a polarimetric study of the RCW121 star-forming region to derive the orientation of the sky-projected magnetic field component traced by the polarization vectors, the morphology of which tends to follow the cloud's structure. Individual polarization-angle values are consistent across the different bands, having a broad distribution towards the RCW121 H II region. We estimate the corresponding magnetic field orientation in the H II region to have a mean value of −12 • ± 7 •. RCW121 shows an elongated shape in the same direction as the magnetic field orientation, which may be evidence that magnetic pressure opposes the H II region expansion. Serkowski's relation was used to determine individual values of the total-to-selective extinction ratio (R V) distribution and a weighted mean value of R V = 3.17 ± 0.05. We derive a foreground component of the polarization degree that is consistent with the literature value for this Galactic region.

Research paper thumbnail of Baade’s window and APOGEE

Astronomy & Astrophysics

Context. Baade's window (BW) is one of the most observed Galactic bulge fields in terms of chemic... more Context. Baade's window (BW) is one of the most observed Galactic bulge fields in terms of chemical abundances. Owing to its low and homogeneous interstellar absorption it is considered the perfect calibration field for Galactic bulge studies. Aims. In the era of large spectroscopic surveys, calibration fields such as BW are necessary for cross calibrating the stellar parameters and individual abundances of the APOGEE survey. Methods. We use the APOGEE BW stars to derive the metallicity distribution function (MDF) and individual abundances for αand iron-peak elements of the APOGEE ASPCAP pipeline (DR13), as well as the age distribution for stars in BW. Results. We determine the MDF of APOGEE stars in BW and find a remarkable agreement with that of the Gaia-ESO survey (GES). Both exhibit a clear bimodal distribution. We also find that the Mg-metallicity planes of the two surveys agree well, except for the metal-rich part ([Fe/H] > 0.1), where APOGEE finds systematically higher Mg abundances with respect to the GES. The ages based on the [C/N] ratio reveal a bimodal age distribution, with a major old population at ∼10 Gyr, with a decreasing tail towards younger stars. A comparison of stellar parameters determined by APOGEE and those determined by other sources reveals detectable systematic offsets, in particular for spectroscopic surface gravity estimates. In general, we find a good agreement between individual abundances of O, Na, Mg, Al, Si, K, Ca, Cr, Mn, Co, and Ni from APOGEE with that of literature values. Conclusions. We have shown that in general APOGEE data show a good agreement in terms of MDF and individual chemical abundances with respect to literature works. Using the [C/N] ratio we found a significant fraction of young stars in BW.

Research paper thumbnail of The Correlation between Mixing Length and Metallicity on the Giant Branch: Implications for Ages in theGaiaEra

The Astrophysical Journal

In the updated APOGEE-Kepler catalog, we have asteroseismic and spectroscopic data for over 3000 ... more In the updated APOGEE-Kepler catalog, we have asteroseismic and spectroscopic data for over 3000 first ascent red giants. Given the size and accuracy of this sample, these data offer an unprecedented test of the accuracy of stellar models on the post-main-sequence. When we compare these data to theoretical predictions, we find a metallicity dependent temperature offset with a slope of around 100 K per dex in metallicity. We find that this effect is present in all model grids tested and that theoretical uncertainties in the models, correlated spectroscopic errors, and shifts in the asteroseismic mass scale are insufficient to explain this effect. Stellar models can be brought into agreement with the data if a metallicity dependent convective mixing length is used, with ∆α ML,YREC ∼ 0.2 per dex in metallicity, a trend inconsistent with the predictions of three dimensional stellar convection simulations. If this effect is not taken into account, isochrone ages for red giants from the Gaia data will be off by as much as a factor of 2 even at modest deviations from solar metallicity ([Fe/H]=−0.5).

Research paper thumbnail of A Curved Magnetic Field in the Ring-like Shell of Bubble N4

The Astrophysical Journal, 2017

Research paper thumbnail of Suppressing star formation in quiescent galaxies with supermassive black hole winds

Nature, May 26, 2016

Quiescent galaxies with little or no ongoing star formation dominate the population of galaxies w... more Quiescent galaxies with little or no ongoing star formation dominate the population of galaxies with masses above 2 × 10(10) times that of the Sun; the number of quiescent galaxies has increased by a factor of about 25 over the past ten billion years (refs 1-4). Once star formation has been shut down, perhaps during the quasar phase of rapid accretion onto a supermassive black hole, an unknown mechanism must remove or heat the gas that is subsequently accreted from either stellar mass loss or mergers and that would otherwise cool to form stars. Energy output from a black hole accreting at a low rate has been proposed, but observational evidence for this in the form of expanding hot gas shells is indirect and limited to radio galaxies at the centres of clusters, which are too rare to explain the vast majority of the quiescent population. Here we report bisymmetric emission features co-aligned with strong ionized-gas velocity gradients from which we infer the presence of centrally dri...

Research paper thumbnail of Now you see it, now you don't: the disappearing central engine of the quasar J1011+5442

Monthly Notices of the Royal Astronomical Society, 2015

We report the discovery of a new "changing-look" quasar, SDSS J101152.98+544206.4, through repeat... more We report the discovery of a new "changing-look" quasar, SDSS J101152.98+544206.4, through repeat spectroscopy from the Time Domain Spectroscopic Survey. This is an addition to a small but growing set of quasars whose blue continua and broad optical emission lines have been observed to decline by a large factor on a time scale of approximately a decade. The 5100Å monochromatic continuum luminosity of this quasar drops by a factor of > 9.8 in a rest-frame time interval of < 9.7 years, while the broad Hα luminosity drops by a factor of 55 in the same amount of time. The width of the broad Hα line increases in the dim state such that the black hole mass derived from the appropriate single-epoch scaling relation agrees between the two epochs within a factor of 3. The fluxes of the narrow emission lines do not appear to change between epochs. The light curve obtained by the Catalina Sky Survey suggests that the transition occurs within a rest-frame time interval of approximately 500 days. We examine three possible mechanisms for this transition suggested in the recent literature. An abrupt change in the reddening towards the central engine is disfavored by the substantial difference between the timescale to obscure the central engine and the observed timescale of the transition. A decaying tidal disruption flare is consistent with the decay rate of the light curve but not with the prolonged bright state preceding the decay; nor can this scenario provide the power required by the luminosities of the emission lines. An abrupt drop in the accretion rate onto the supermassive black hole appears to be the most plausible explanation for the rapid dimming.

Research paper thumbnail of Survey of Young Stellar Objects Associated with Molecular Clouds

Open Issues in Local Star Formation, 2003

ABSTRACT We present the preliminary results of a survey in the near-infrared of young and massive... more ABSTRACT We present the preliminary results of a survey in the near-infrared of young and massive stellar objects embedded in dense molecular clouds. The survey was conducted at the LNA, Brazil, in the J, H, and nbK bands, in the direction of IRAS sources with colors characteristic of compact HII regions that also present strong CS(2-1) line emission. The coordinates of the IRAS sources were improved to 1.9n resolution with the help of the MSX catalog, allowing us to associate them univocally to the detected stars in our sample. Several IR nebulae were detected, indicating also that young stellar clusters are being formed in the core of the molecular clouds. From the total number of 127 observed regions we found that 105 presented stars in the H band inside the IRAS error ellipse, and were associated to a MSX point source. We obtained the integrated bolometric luminosities and main-sequence spectral types of the detected stars, assuming that the bolometric luminosity coincided with the integrated IR luminosity, and using the fact that for massive stars the evolution towards the main sequence proceeds at constant luminosity. The Initial Mass Function of the sample follows a Salpeter’s law with index 1.6 between spectral types B0 and O3.

Research paper thumbnail of The Stellar Population Associated with the IRAS Source 16132-5039

The Astronomical Journal, 2004

We report the discovery of a young massive stellar cluster and infrared nebula in the direction o... more We report the discovery of a young massive stellar cluster and infrared nebula in the direction of the CS molecular cloud associated to the IRAS point source 16132-5039. The analysis of the mid-infrared images from the more accurate MSX catalog, reveled that there are two independent components associated with the IRAS source. The integral of the spectral energy distribution for these components, between 8.28 µm and 100 µm, gave lower limits for the bolometric luminosity of the embedded objects of 8.7 × 10 4 L ⊙ and 9 × 10 3 L ⊙ , which corresponds to ZAMS O8 and B0.5 stars, respectively. The number of Lyman continuum photons expected from the stars that lie along the reddening line for early-type stars is about 1.7 × 10 49 s −1 , enough to produce the detected flux densities at 5 GHz. The NIR spectrum of the nebula increases with frequency, implying that free-free emission cannot be the main source of the extended luminosity, from which we conclude that the observed emission must be mainly dust scattered light. A comparison of the cluster described in this paper with the young stellar cluster associated with the IRAS source 16177-5018, which is located at the same distance and direction, shows that the mean visual absorption of the newly discovered cluster is about 10 magnitudes smaller and it contains less massive stars, suggesting that it was formed from a less massive molecular cloud.