Maria Rosa Zapatero Osorio - Academia.edu (original) (raw)

Papers by Maria Rosa Zapatero Osorio

Astronomy & Astrophysics, 2022

The strong intervening absorption system at redshift 1.15 towards the very bright quasar HE 0515−... more The strong intervening absorption system at redshift 1.15 towards the very bright quasar HE 0515−4414 is the most studied absorber for measuring possible cosmological variations in the fine-structure constant, α. We observed HE 0515−4414 for 16.1 h with the Very Large Telescope and present here the first constraint on relative variations in α with parts-per-million (ppm) precision from the new ESPRESSO spectrograph: Δα/α = 1.3 ± 1.3stat ± 0.4sys ppm. The statistical uncertainty (1σ) is similar to the ensemble precision of previous large samples of absorbers and derives from the high signal-to-noise ratio achieved (≈105 per 0.4 km s−1 pixel). ESPRESSO’s design, and the calibration of our observations with its laser frequency comb, effectively removed wavelength calibration errors from our measurement. The high resolving power of our ESPRESSO spectrum (R = 145 000) enabled the identification of very narrow components within the absorption profile, allowing a more robust analysis of Δα...

arXiv: Instrumentation and Methods for Astrophysics, 2020

HIRES will be the high-resolution spectrograph of the European Extremely Large Telescope at optic... more HIRES will be the high-resolution spectrograph of the European Extremely Large Telescope at optical and near-infrared wavelengths. It consists of three fibre-fed spectrographs providing a wavelength coverage of 0.4-1.8 mic (goal 0.35-1.8 mic) at a spectral resolution of ~100,000. The fibre-feeding allows HIRES to have several, interchangeable observing modes including a SCAO module and a small diffraction-limited IFU in the NIR. Therefore, it will be able to operate both in seeing and diffraction-limited modes. ELT-HIRES has a wide range of science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Some of the top science cases will be the detection of bio signatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars (PopIII), tests on the stability of Nature's fundamental couplings, and the direct detection of the cosmic acceleration. The HIRES consortium is composed of more than 30 institutes from 14 cou...

Astronomy & Astrophysics, 2021

Observations of metal absorption systems in the spectra of distant quasars allow one to constrain... more Observations of metal absorption systems in the spectra of distant quasars allow one to constrain a possible variation of the fine-structure constant throughout the history of the Universe. Such a test poses utmost demands on the wavelength accuracy and previous studies were limited by systematics in the spectrograph wavelength calibration. A substantial advance in the field is therefore expected from the new ultra-stable high-resolution spectrograph ESPRESSO, which was recently installed at the VLT. In preparation of the fundamental physics related part of the ESPRESSO GTO program, we present a thorough assessment of the ESPRESSO wavelength accuracy and identify possible systematics at each of the different steps involved in the wavelength calibration process. Most importantly, we compare the default wavelength solution, which is based on the combination of Thorium-Argon arc lamp spectra and a Fabry-Pérot interferometer, to the fully independent calibration obtained from a laser fr...

Astronomy & Astrophysics, 2020

Aims. Our objective is to identify analogues of gas giant planets, but located as companions at w... more Aims. Our objective is to identify analogues of gas giant planets, but located as companions at wide separations of very young stars. The main purpose is to characterise the binarity frequency and the properties of these substellar objects, and to elucidate their early evolutionary stages. Methods. To identify these objects, we cross correlated the Visible and Infrared Survey Telescope for Astronomy Hemisphere Survey and the United Kingdom Infrared Telescope Infrared Deep Sky Survey Galactic Clusters Survey catalogues to search for common proper motion companions to 1195 already known members of Upper Scorpius (USco; age ~5–10 Myr, distance ~145 pc). We present the discovery and spectroscopic characterisation of two very wide substellar companions of two early-M stars in Upper Scorpius: USco1621 B and USco1556 B. We obtained optical and near-infrared low-resolution spectroscopy of the candidates to characterise their spectral energy distribution and confirm their youth and membershi...

The Astrophysical Journal, 2001

We present K-band photometry and low-resolution near-infrared spectroscopy from 1.44 to 2.45 µm o... more We present K-band photometry and low-resolution near-infrared spectroscopy from 1.44 to 2.45 µm of isolated planetary-mass candidate members in the σ Orionis cluster found by Zapatero Osorio et al. The new data have been obtained with NIRC at the Keck I telescope. All of our targets, except for one, are confirmed as likely cluster members. Hence, we also confirm that the planetary-mass domain in the cluster is well populated. Using our deep K-band images we searched for companions to the targets in the separation range 0.3 ′′ to 10 ′′ up to a maximum faint limit of K=19.5 mag. One suspected companion seems to be an extremely red galaxy. The near-infrared colors of the σ Orionis substellar members indicate that dust grains condense and settle in their atmospheres. We estimate that the surface temperatures range from 2500 K down to 1500 K. The spectroscopic sequence covers the full range of L subclasses, and the faintest object is tentatively classified as T0. These targets provide a sequence of substellar objects of known age, distance and metallicity, which can be used as benchmark for understanding the spectral properties of ultracool dwarfs.

VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, ha... more VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, have been used to establish the membership and the properties of a sample of low-mass candidate members of the sigma Orionis cluster with masses spanning from 1 Msun down to about 0.013 Msun (i.e., deuterium-burning mass limit). We have observed K-band infrared excess and remarkably intense H(alpha) emission in

The Astrophysical Journal, 2008

We present the discovery of a companion near the deuterium burning mass limit located at a very w... more We present the discovery of a companion near the deuterium burning mass limit located at a very wide distance, at an angular separation of 4.6±0.1 (projected distance of ∼ 670 AU) from UScoC-TIO 108, a brown dwarf of the very young Upper Scorpius association. Optical and near-infrared photometry and spectroscopy confirm the cool nature of both objects, with spectral types of M7 and M9.5, respectively, and that they are bona fide members of the association, showing low gravity and features of youth. Their masses, estimated from the comparison of their bolometric luminosities and theoretical models for the age range of the association, are 60 ± 20 and 14 +2 −8 M Jup , respectively. The existence of this object around a brown dwarf at this wide orbit suggests that the companion is unlikely to have formed in a disk based on current planet formation models. Because this system is rather weakly bound, they did not probably form through dynamical ejection of stellar embryos.

The Astrophysical Journal, 1999

The Astrophysical Journal, 1999

We present near-infrared photometry (J-band) and low-resolution optical spectroscopy (600-1000 nm... more We present near-infrared photometry (J-band) and low-resolution optical spectroscopy (600-1000 nm) for one of the faintest substellar member candidates in the young σ Orionis cluster, S Ori 47 (I = 20.53, Béjar et al. 1999). Its very red (I − J) = 3.3±0.1 color and its optical spectrum allow us to classify S Ori 47 as an L1.5-type object which fits the low-luminosity end of the cluster photometric and spectroscopic sequences. It also displays atmospheric features indicative of low gravity such as weak alkaline lines and hydride and oxide bands, consistent with the expectation for a very young object still undergoing gravitational collapse. Our data lead us to conclude that S Ori 47 is a true substellar member of the σ Orionis cluster. Additionally, we present the detection of Li i in its atmosphere which provides an independent confirmation of youth and substellarity. Using current theoretical evolutionary tracks and adopting an age interval of 1-5 Myr for the σ Orionis cluster, we estimate the mass of S Ori 47 at 0.015±0.005 M , i.e. at the minimum mass for deuterium burning, which has been proposed as a definition for the boundary between brown dwarfs and giant planets. S Ori 47 could well be the result of a natural extension of the process of cloud fragmentation down to the deuterium burning mass limit; a less likely alternative is that it has originated from a protoplanetary disc around a more massive cluster member and later ejected from its orbit due to interacting effects within this rather sparse (∼12 objects pc −3) young cluster. The study of this object serves as a guide for future deep searches for free-floating objects with planetary masses.

The Astrophysical Journal, 2001

We report CCD photometric monitoring of the nonemission ultracool dwarf BRI 0021−0214 (M9.5) obta... more We report CCD photometric monitoring of the nonemission ultracool dwarf BRI 0021−0214 (M9.5) obtained during 10 nights in 1995 November and 4 nights in 1996 August, with CCD cameras at 1 m class telescopes on the observatories of the Canary Islands. We present differential photometry of BRI 0021−0214, and we report significant variability in the I-band light curve obtained in 1995. A periodogram analysis finds a strong peak at a period of 0.84 day. This modulation appears to be transient because it is present in the 1995 data but not in the 1996 data. We also find a possible period of 0.20 day, which appears to be present in both the 1995 and 1996 datasets. However, we do not find any periodicity close to the rotation period expected from the spectroscopic rotational broadening (≤0.14 day). BRI 0021−0214 is a very inactive object, with extremely low levels of H α and X-ray emission. Thus, it is unlikely that magnetically induced cool spots can account for the photometric variability. The photometric variability of BRI 0021−0214 could be explained by the presence of an active meteorology that leads to inhomogeneous clouds on the surface. The lack of photometric modulation at the expected rotational period suggests that the pattern of surface features may be more complicated than previously anticipated.

The Astrophysical Journal, 1996

Based on observations made with the William Herschel Telescope, operated on the island of La Palm... more Based on observations made with the William Herschel Telescope, operated on the island of La Palma by the Royal Greenwich Observatory in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, and on data collected at the 3.5m telescope of Calar Alto German-Spanish Observatory.

The Astrophysical Journal, 1998

We present optical and infrared observations of RPr 1, a faint (I = 21.01) and very red object (I... more We present optical and infrared observations of RPr 1, a faint (I = 21.01) and very red object (I − K = 4.57) discovered in a deep CCD survey, covering an area of 800 square arcmin of the Praesepe open cluster. A low resolution spectrum shows that RPr 1 is a very late object, the latest object in Praesepe for which a spectrum has been taken to date. Our estimates give a mass between 0.063 and 0.084 M ⊙ , and indicate that RPr 1 may turn out to be the first brown dwarf in this cluster.

The Astrophysical Journal, 2005

We report on the linear polarimetric observations in the Johnson I-band filter of 44 ultracool dw... more We report on the linear polarimetric observations in the Johnson I-band filter of 44 ultracool dwarfs with spectral types between M6 and L7.5, corresponding to effective temperatures in the range 2800-1400 K, and one M4.5-type star. Based on our measurements of polarization (P) and their associated error bars (P), 11 (10 L and 1 M) dwarfs appear to have significant linear polarization (P= P ! 3). For these, the polarization degrees we have measured are in the interval P ¼ 0:2% 2:5%. Because of the typical average uncertainty of our data, we can easily confirm polarization of ultracool dwarfs that show degree of linear polarization greater than 0.4%. We have compared the two populations in our sample, the M and L type dwarfs, and have found evidence for a larger frequency of high I-band polarization in the coolest objects, supporting the presence of significant amounts of dust in L-type atmospheres. The probable mechanism polarizing the far-red optical photons of ultracool dwarfs is related to the presence of heterogeneous dust clouds nonuniformly distributed across the visible photospheres and the asymmetric shape of the objects (rapid rotations impose deviations from sphericity). In some young ultracool dwarfs, surrounding dusty disks (or shells) may also yield polarization. For polarimetric detections, a trend for slightly larger polarization from L0 to L6.5 may be present in our data, suggesting changes in the distribution of the grain properties and in the vertical height of the cloud layer. Faster rotations and important differences in metallicity and age within our sample could also account for this trend. One of the targets is the peculiar brown dwarf 2MASS J22443167+2043433 (L6.5), for which we have determined the largest I-band polarization degree in our study. We discuss that the origin of such large polarization may lie in a surrounding dusty disk (or shell) and /or rather large photospheric dust grains. Two of the likely polarized dwarfs (CFHT-BD-Tau 4, a very young, M7-type brown dwarf of the Taurus star-forming region, and 2MASS J00361617+1821104, an L3.5 field dwarf) were also observed in the Johnson R-band filter, allowing us to discuss qualitatively the size of the grains responsible for the polarization. Our data support the presence of a circum(sub)stellar disk around the young accreting brown dwarf CFHT-BD-Tau 4. The higher degree of polarization in the R band than in the I band indicates that the grain growth lies in the submicron regime in the visible photosphere of 2MASS J00361617+1821104 (effective temperature of about 1900 K). Our polarimetric data does not obviously correlate with activity (H and radio emission) or projected rotational velocity. Three polarized early-to mid-L dwarfs have been photometrically monitored in the I band , displaying light curves with amplitudes below 10 mmag.

Astronomy & Astrophysics, 2003

VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, ha... more VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, have been used to establish the membership and the properties of a sample of low-mass candidate members of the σ Orionis cluster with masses spanning from 1 M down to about 0.013 M (i.e., deuterium-burning mass limit). We have observed K-band infrared excess and remarkably intense Hα emission in various cluster members, which, in addition to the previously detected forbidden emision lines and the presence of Li  in absorption at 6708 Å, have allowed us to tentatively classify σ Orionis members as classical or weak-line T Tauri stars and substellar analogs. Variability of the Hα line has been investigated and detected in some objects. Based on the K-band infrared excesses and the intensity of Hα emission, we estimate that the minimum disk frequency of the σ Orionis low-mass population is in the range 5-12%.

Astronomy & Astrophysics, 2001

We have obtained low resolution optical spectra of 15 isolated planetary mass objects (IPMOs) in ... more We have obtained low resolution optical spectra of 15 isolated planetary mass objects (IPMOs) in the σ Orionis cluster, and derived spectral types by comparison with nearby M and L dwarfs. The spectral types are in the range late M-mid L, in agreement with our expectations based on colors and magnitudes for bona fide members. Therefore, most of these objects have masses below the deuterium burning limit. About 2/3 show Hα in emission at our spectral resolution. From our spectroscopic and photometric data, we infer that three IPMOs in this sample may be binaries with components of similar masses. These results confirm that the substellar mass function of the σ Orionis cluster, in the form dN/dM , keeps rising in the planetary domain.

Astronomy & Astrophysics, 2004

We have carried out multi-epoch, time-series differential I-band photometry of a large sample of ... more We have carried out multi-epoch, time-series differential I-band photometry of a large sample of objects in the southeast region of the young (∼3 Myr), nearby (∼350 pc) σ Orionis open cluster. A field of ∼1000 arcmin 2 was monitored during four nights over a period of two years. Using this dataset, we have studied the photometric variability of twenty-eight brown dwarf cluster candidates with masses ranging from the stellar-substellar boundary down to the planetary-mass domain. We have found that about 50% of the sample show photometric variability on timescales from less than one hour to several days and years. The amplitudes of the I-band light curves range from less than 0.01 up to ∼0.4 mag. A correlation between the near-infrared excess in the K s band, strong Hα emission and large-amplitude photometric variation is observed. We briefly discuss how these results may fit the different scenarios proposed to explain the variability of cool and ultracool dwarfs (i.e. magnetic spots, patchy obscuration by dust clouds, surrounding accretion discs and binarity). Additionally, we have determined tentative rotational periods in the range 3 to 40 h for three objects with masses around 60 M Jup , and the rotational velocity of 14 ± 4 km s −1 for one of them.

Astronomy & Astrophysics, 2004

We explored the regions within a radius of 25 around 473 nearby, low-metallicity G-to M-type star... more We explored the regions within a radius of 25 around 473 nearby, low-metallicity G-to M-type stars using (VR)I optical filters and small-aperture telescopes. About 10% of the sample was searched up to angular separations of 90. We applied photometric and astrometric techniques to detect true physical companions to the targets. The great majority of the sample stars was drawn from the Carney-Latham surveys; their metallicities range from roughly solar to [Fe/H] = −3.5 dex. Our I-band photometric survey detected objects that are between 0 and 5 mag fainter (completeness) than the target stars; the maximum dynamical range of our exploration is 9 mag. We also investigated the literature, and inspected images from the Digitized Sky Surveys to complete our search. By combining photometric and proper motion measurements, we retrieved 29 previously known companions, and identified 13 new proper motion companions. Near-infrared 2MASS photometry is provided for the great majority of them. Low-resolution optical spectroscopy (386-1000 nm) was obtained for eight of the new companion stars. These spectroscopic data confirm them as cool, late-type, metal-depleted dwarfs, with spectral classes from esdK7 to sdM3. After comparison with low-metallicity evolutionary models, we estimate the masses of the proper motion companion stars to be in the range 0.5-0.1 M. They are moving around their primary stars at projected separations between ∼32 and ∼57 000 AU. These orbital sizes are very similar to those of solar-metallicity stars of the same spectral types. Our results indicate that about 15% of the metal-poor stars have stellar companions in wide orbits, which is in agreement with the binary fraction observed among main sequence G-to M-type stars and T Tauri stars.

Astronomy and Astrophysics, 2009

Context. There is now growing evidence that some brown dwarfs (BDs) have very strong magnetic fie... more Context. There is now growing evidence that some brown dwarfs (BDs) have very strong magnetic fields, and yet their surface temperatures are so low that the coupling is expected to be small between the matter and the magnetic field in the atmosphere. In the deeper layers, however, the coupling is expected to be much stronger. Aims. This raises the question of whether the magnetic field still leads to the formation of structures in the photosphere and of a solar-like chromosphere and corona. Methods. We carried out a spectroscopic monitoring campaign in which we observed ultracool dwarfs that have strong magnetic fields: the BD LP944-20 and 2MASSW J0036159+182110. The objects were monitored over several rotation periods spectroscopically. LP944-20 was observed simultaneously in the optical and in the near infrared regime, 2MASSW J0036159+182110 only in the infrared. From the spectra, we determined the temperature of the objects in each spectrum, and measured the equivalent width in a number of diagnostically important lines. Temperature variations would indicate the presence of warm and cold regions, variations in the equivalent widths of photospheric lines are sensitive to the structure of cloud layers, and H α is a diagnostic for chromospheric structures. Results. Both dwarfs turned out to be remarkably constant. In the case of LP944-20, the T eff-variations are ≤50 K, and the rmsvariations in the equivalent widths of H α small. We also find that the equivalent widths of photospheric lines are remarkably constant. We did not find any significant variations in the case of 2MASSW J0036159+182110 either. Thus the most important result is that no significant variability was found at the time of our observations. We find that Hα-line is in emission but the equivalent width is only −4.4 ± 0.3 Å. When comparing our spectra with spectra taken over the past 11 years, we recognize significant changes during this time. Conclusions. We interpret these results as evidence that the photosphere of these objects are remarkably homogeneous, with only little structure in them, and despite the strong magnetic fields. Thus, unlike active stars, there are no prominent spots on these objects.

Astronomy & Astrophysics, 2002

We report on the finding of the strongest Hα emission-pseudoequivalent width of 705 Å-known so fa... more We report on the finding of the strongest Hα emission-pseudoequivalent width of 705 Å-known so far in a young, late type dwarf. This object, named as S Ori 71, is a substellar candidate member of the 1-8 Myr star cluster σ Orionis. Due to its overluminous location in color-magnitude diagrams, S Ori 71 might be younger than other cluster members, or a binary of similar components. Its mass is in the range 0.021-0.012 M , depending on evolutionary models and possible binarity. The broad Hα line of S Ori 71 appears asymmetric, indicative of high velocity mass motions in the Hα forming region. The origin of this emission is unclear at the present time. We discuss three possible scenarios: accretion from a disk, mass exchange between the components of a binary system, and emission from a chromosphere.

Astronomy and Astrophysics, 2008

Astronomy & Astrophysics, 2022

The strong intervening absorption system at redshift 1.15 towards the very bright quasar HE 0515−... more The strong intervening absorption system at redshift 1.15 towards the very bright quasar HE 0515−4414 is the most studied absorber for measuring possible cosmological variations in the fine-structure constant, α. We observed HE 0515−4414 for 16.1 h with the Very Large Telescope and present here the first constraint on relative variations in α with parts-per-million (ppm) precision from the new ESPRESSO spectrograph: Δα/α = 1.3 ± 1.3stat ± 0.4sys ppm. The statistical uncertainty (1σ) is similar to the ensemble precision of previous large samples of absorbers and derives from the high signal-to-noise ratio achieved (≈105 per 0.4 km s−1 pixel). ESPRESSO’s design, and the calibration of our observations with its laser frequency comb, effectively removed wavelength calibration errors from our measurement. The high resolving power of our ESPRESSO spectrum (R = 145 000) enabled the identification of very narrow components within the absorption profile, allowing a more robust analysis of Δα...

arXiv: Instrumentation and Methods for Astrophysics, 2020

HIRES will be the high-resolution spectrograph of the European Extremely Large Telescope at optic... more HIRES will be the high-resolution spectrograph of the European Extremely Large Telescope at optical and near-infrared wavelengths. It consists of three fibre-fed spectrographs providing a wavelength coverage of 0.4-1.8 mic (goal 0.35-1.8 mic) at a spectral resolution of ~100,000. The fibre-feeding allows HIRES to have several, interchangeable observing modes including a SCAO module and a small diffraction-limited IFU in the NIR. Therefore, it will be able to operate both in seeing and diffraction-limited modes. ELT-HIRES has a wide range of science cases spanning nearly all areas of research in astrophysics and even fundamental physics. Some of the top science cases will be the detection of bio signatures from exoplanet atmospheres, finding the fingerprints of the first generation of stars (PopIII), tests on the stability of Nature's fundamental couplings, and the direct detection of the cosmic acceleration. The HIRES consortium is composed of more than 30 institutes from 14 cou...

Astronomy & Astrophysics, 2021

Observations of metal absorption systems in the spectra of distant quasars allow one to constrain... more Observations of metal absorption systems in the spectra of distant quasars allow one to constrain a possible variation of the fine-structure constant throughout the history of the Universe. Such a test poses utmost demands on the wavelength accuracy and previous studies were limited by systematics in the spectrograph wavelength calibration. A substantial advance in the field is therefore expected from the new ultra-stable high-resolution spectrograph ESPRESSO, which was recently installed at the VLT. In preparation of the fundamental physics related part of the ESPRESSO GTO program, we present a thorough assessment of the ESPRESSO wavelength accuracy and identify possible systematics at each of the different steps involved in the wavelength calibration process. Most importantly, we compare the default wavelength solution, which is based on the combination of Thorium-Argon arc lamp spectra and a Fabry-Pérot interferometer, to the fully independent calibration obtained from a laser fr...

Astronomy & Astrophysics, 2020

Aims. Our objective is to identify analogues of gas giant planets, but located as companions at w... more Aims. Our objective is to identify analogues of gas giant planets, but located as companions at wide separations of very young stars. The main purpose is to characterise the binarity frequency and the properties of these substellar objects, and to elucidate their early evolutionary stages. Methods. To identify these objects, we cross correlated the Visible and Infrared Survey Telescope for Astronomy Hemisphere Survey and the United Kingdom Infrared Telescope Infrared Deep Sky Survey Galactic Clusters Survey catalogues to search for common proper motion companions to 1195 already known members of Upper Scorpius (USco; age ~5–10 Myr, distance ~145 pc). We present the discovery and spectroscopic characterisation of two very wide substellar companions of two early-M stars in Upper Scorpius: USco1621 B and USco1556 B. We obtained optical and near-infrared low-resolution spectroscopy of the candidates to characterise their spectral energy distribution and confirm their youth and membershi...

The Astrophysical Journal, 2001

We present K-band photometry and low-resolution near-infrared spectroscopy from 1.44 to 2.45 µm o... more We present K-band photometry and low-resolution near-infrared spectroscopy from 1.44 to 2.45 µm of isolated planetary-mass candidate members in the σ Orionis cluster found by Zapatero Osorio et al. The new data have been obtained with NIRC at the Keck I telescope. All of our targets, except for one, are confirmed as likely cluster members. Hence, we also confirm that the planetary-mass domain in the cluster is well populated. Using our deep K-band images we searched for companions to the targets in the separation range 0.3 ′′ to 10 ′′ up to a maximum faint limit of K=19.5 mag. One suspected companion seems to be an extremely red galaxy. The near-infrared colors of the σ Orionis substellar members indicate that dust grains condense and settle in their atmospheres. We estimate that the surface temperatures range from 2500 K down to 1500 K. The spectroscopic sequence covers the full range of L subclasses, and the faintest object is tentatively classified as T0. These targets provide a sequence of substellar objects of known age, distance and metallicity, which can be used as benchmark for understanding the spectral properties of ultracool dwarfs.

VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, ha... more VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, have been used to establish the membership and the properties of a sample of low-mass candidate members of the sigma Orionis cluster with masses spanning from 1 Msun down to about 0.013 Msun (i.e., deuterium-burning mass limit). We have observed K-band infrared excess and remarkably intense H(alpha) emission in

The Astrophysical Journal, 2008

We present the discovery of a companion near the deuterium burning mass limit located at a very w... more We present the discovery of a companion near the deuterium burning mass limit located at a very wide distance, at an angular separation of 4.6±0.1 (projected distance of ∼ 670 AU) from UScoC-TIO 108, a brown dwarf of the very young Upper Scorpius association. Optical and near-infrared photometry and spectroscopy confirm the cool nature of both objects, with spectral types of M7 and M9.5, respectively, and that they are bona fide members of the association, showing low gravity and features of youth. Their masses, estimated from the comparison of their bolometric luminosities and theoretical models for the age range of the association, are 60 ± 20 and 14 +2 −8 M Jup , respectively. The existence of this object around a brown dwarf at this wide orbit suggests that the companion is unlikely to have formed in a disk based on current planet formation models. Because this system is rather weakly bound, they did not probably form through dynamical ejection of stellar embryos.

The Astrophysical Journal, 1999

The Astrophysical Journal, 1999

We present near-infrared photometry (J-band) and low-resolution optical spectroscopy (600-1000 nm... more We present near-infrared photometry (J-band) and low-resolution optical spectroscopy (600-1000 nm) for one of the faintest substellar member candidates in the young σ Orionis cluster, S Ori 47 (I = 20.53, Béjar et al. 1999). Its very red (I − J) = 3.3±0.1 color and its optical spectrum allow us to classify S Ori 47 as an L1.5-type object which fits the low-luminosity end of the cluster photometric and spectroscopic sequences. It also displays atmospheric features indicative of low gravity such as weak alkaline lines and hydride and oxide bands, consistent with the expectation for a very young object still undergoing gravitational collapse. Our data lead us to conclude that S Ori 47 is a true substellar member of the σ Orionis cluster. Additionally, we present the detection of Li i in its atmosphere which provides an independent confirmation of youth and substellarity. Using current theoretical evolutionary tracks and adopting an age interval of 1-5 Myr for the σ Orionis cluster, we estimate the mass of S Ori 47 at 0.015±0.005 M , i.e. at the minimum mass for deuterium burning, which has been proposed as a definition for the boundary between brown dwarfs and giant planets. S Ori 47 could well be the result of a natural extension of the process of cloud fragmentation down to the deuterium burning mass limit; a less likely alternative is that it has originated from a protoplanetary disc around a more massive cluster member and later ejected from its orbit due to interacting effects within this rather sparse (∼12 objects pc −3) young cluster. The study of this object serves as a guide for future deep searches for free-floating objects with planetary masses.

The Astrophysical Journal, 2001

We report CCD photometric monitoring of the nonemission ultracool dwarf BRI 0021−0214 (M9.5) obta... more We report CCD photometric monitoring of the nonemission ultracool dwarf BRI 0021−0214 (M9.5) obtained during 10 nights in 1995 November and 4 nights in 1996 August, with CCD cameras at 1 m class telescopes on the observatories of the Canary Islands. We present differential photometry of BRI 0021−0214, and we report significant variability in the I-band light curve obtained in 1995. A periodogram analysis finds a strong peak at a period of 0.84 day. This modulation appears to be transient because it is present in the 1995 data but not in the 1996 data. We also find a possible period of 0.20 day, which appears to be present in both the 1995 and 1996 datasets. However, we do not find any periodicity close to the rotation period expected from the spectroscopic rotational broadening (≤0.14 day). BRI 0021−0214 is a very inactive object, with extremely low levels of H α and X-ray emission. Thus, it is unlikely that magnetically induced cool spots can account for the photometric variability. The photometric variability of BRI 0021−0214 could be explained by the presence of an active meteorology that leads to inhomogeneous clouds on the surface. The lack of photometric modulation at the expected rotational period suggests that the pattern of surface features may be more complicated than previously anticipated.

The Astrophysical Journal, 1996

Based on observations made with the William Herschel Telescope, operated on the island of La Palm... more Based on observations made with the William Herschel Telescope, operated on the island of La Palma by the Royal Greenwich Observatory in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, and on data collected at the 3.5m telescope of Calar Alto German-Spanish Observatory.

The Astrophysical Journal, 1998

We present optical and infrared observations of RPr 1, a faint (I = 21.01) and very red object (I... more We present optical and infrared observations of RPr 1, a faint (I = 21.01) and very red object (I − K = 4.57) discovered in a deep CCD survey, covering an area of 800 square arcmin of the Praesepe open cluster. A low resolution spectrum shows that RPr 1 is a very late object, the latest object in Praesepe for which a spectrum has been taken to date. Our estimates give a mass between 0.063 and 0.084 M ⊙ , and indicate that RPr 1 may turn out to be the first brown dwarf in this cluster.

The Astrophysical Journal, 2005

We report on the linear polarimetric observations in the Johnson I-band filter of 44 ultracool dw... more We report on the linear polarimetric observations in the Johnson I-band filter of 44 ultracool dwarfs with spectral types between M6 and L7.5, corresponding to effective temperatures in the range 2800-1400 K, and one M4.5-type star. Based on our measurements of polarization (P) and their associated error bars (P), 11 (10 L and 1 M) dwarfs appear to have significant linear polarization (P= P ! 3). For these, the polarization degrees we have measured are in the interval P ¼ 0:2% 2:5%. Because of the typical average uncertainty of our data, we can easily confirm polarization of ultracool dwarfs that show degree of linear polarization greater than 0.4%. We have compared the two populations in our sample, the M and L type dwarfs, and have found evidence for a larger frequency of high I-band polarization in the coolest objects, supporting the presence of significant amounts of dust in L-type atmospheres. The probable mechanism polarizing the far-red optical photons of ultracool dwarfs is related to the presence of heterogeneous dust clouds nonuniformly distributed across the visible photospheres and the asymmetric shape of the objects (rapid rotations impose deviations from sphericity). In some young ultracool dwarfs, surrounding dusty disks (or shells) may also yield polarization. For polarimetric detections, a trend for slightly larger polarization from L0 to L6.5 may be present in our data, suggesting changes in the distribution of the grain properties and in the vertical height of the cloud layer. Faster rotations and important differences in metallicity and age within our sample could also account for this trend. One of the targets is the peculiar brown dwarf 2MASS J22443167+2043433 (L6.5), for which we have determined the largest I-band polarization degree in our study. We discuss that the origin of such large polarization may lie in a surrounding dusty disk (or shell) and /or rather large photospheric dust grains. Two of the likely polarized dwarfs (CFHT-BD-Tau 4, a very young, M7-type brown dwarf of the Taurus star-forming region, and 2MASS J00361617+1821104, an L3.5 field dwarf) were also observed in the Johnson R-band filter, allowing us to discuss qualitatively the size of the grains responsible for the polarization. Our data support the presence of a circum(sub)stellar disk around the young accreting brown dwarf CFHT-BD-Tau 4. The higher degree of polarization in the R band than in the I band indicates that the grain growth lies in the submicron regime in the visible photosphere of 2MASS J00361617+1821104 (effective temperature of about 1900 K). Our polarimetric data does not obviously correlate with activity (H and radio emission) or projected rotational velocity. Three polarized early-to mid-L dwarfs have been photometrically monitored in the I band , displaying light curves with amplitudes below 10 mmag.

Astronomy & Astrophysics, 2003

VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, ha... more VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, have been used to establish the membership and the properties of a sample of low-mass candidate members of the σ Orionis cluster with masses spanning from 1 M down to about 0.013 M (i.e., deuterium-burning mass limit). We have observed K-band infrared excess and remarkably intense Hα emission in various cluster members, which, in addition to the previously detected forbidden emision lines and the presence of Li  in absorption at 6708 Å, have allowed us to tentatively classify σ Orionis members as classical or weak-line T Tauri stars and substellar analogs. Variability of the Hα line has been investigated and detected in some objects. Based on the K-band infrared excesses and the intensity of Hα emission, we estimate that the minimum disk frequency of the σ Orionis low-mass population is in the range 5-12%.

Astronomy & Astrophysics, 2001

We have obtained low resolution optical spectra of 15 isolated planetary mass objects (IPMOs) in ... more We have obtained low resolution optical spectra of 15 isolated planetary mass objects (IPMOs) in the σ Orionis cluster, and derived spectral types by comparison with nearby M and L dwarfs. The spectral types are in the range late M-mid L, in agreement with our expectations based on colors and magnitudes for bona fide members. Therefore, most of these objects have masses below the deuterium burning limit. About 2/3 show Hα in emission at our spectral resolution. From our spectroscopic and photometric data, we infer that three IPMOs in this sample may be binaries with components of similar masses. These results confirm that the substellar mass function of the σ Orionis cluster, in the form dN/dM , keeps rising in the planetary domain.

Astronomy & Astrophysics, 2004

We have carried out multi-epoch, time-series differential I-band photometry of a large sample of ... more We have carried out multi-epoch, time-series differential I-band photometry of a large sample of objects in the southeast region of the young (∼3 Myr), nearby (∼350 pc) σ Orionis open cluster. A field of ∼1000 arcmin 2 was monitored during four nights over a period of two years. Using this dataset, we have studied the photometric variability of twenty-eight brown dwarf cluster candidates with masses ranging from the stellar-substellar boundary down to the planetary-mass domain. We have found that about 50% of the sample show photometric variability on timescales from less than one hour to several days and years. The amplitudes of the I-band light curves range from less than 0.01 up to ∼0.4 mag. A correlation between the near-infrared excess in the K s band, strong Hα emission and large-amplitude photometric variation is observed. We briefly discuss how these results may fit the different scenarios proposed to explain the variability of cool and ultracool dwarfs (i.e. magnetic spots, patchy obscuration by dust clouds, surrounding accretion discs and binarity). Additionally, we have determined tentative rotational periods in the range 3 to 40 h for three objects with masses around 60 M Jup , and the rotational velocity of 14 ± 4 km s −1 for one of them.

Astronomy & Astrophysics, 2004

We explored the regions within a radius of 25 around 473 nearby, low-metallicity G-to M-type star... more We explored the regions within a radius of 25 around 473 nearby, low-metallicity G-to M-type stars using (VR)I optical filters and small-aperture telescopes. About 10% of the sample was searched up to angular separations of 90. We applied photometric and astrometric techniques to detect true physical companions to the targets. The great majority of the sample stars was drawn from the Carney-Latham surveys; their metallicities range from roughly solar to [Fe/H] = −3.5 dex. Our I-band photometric survey detected objects that are between 0 and 5 mag fainter (completeness) than the target stars; the maximum dynamical range of our exploration is 9 mag. We also investigated the literature, and inspected images from the Digitized Sky Surveys to complete our search. By combining photometric and proper motion measurements, we retrieved 29 previously known companions, and identified 13 new proper motion companions. Near-infrared 2MASS photometry is provided for the great majority of them. Low-resolution optical spectroscopy (386-1000 nm) was obtained for eight of the new companion stars. These spectroscopic data confirm them as cool, late-type, metal-depleted dwarfs, with spectral classes from esdK7 to sdM3. After comparison with low-metallicity evolutionary models, we estimate the masses of the proper motion companion stars to be in the range 0.5-0.1 M. They are moving around their primary stars at projected separations between ∼32 and ∼57 000 AU. These orbital sizes are very similar to those of solar-metallicity stars of the same spectral types. Our results indicate that about 15% of the metal-poor stars have stellar companions in wide orbits, which is in agreement with the binary fraction observed among main sequence G-to M-type stars and T Tauri stars.

Astronomy and Astrophysics, 2009

Context. There is now growing evidence that some brown dwarfs (BDs) have very strong magnetic fie... more Context. There is now growing evidence that some brown dwarfs (BDs) have very strong magnetic fields, and yet their surface temperatures are so low that the coupling is expected to be small between the matter and the magnetic field in the atmosphere. In the deeper layers, however, the coupling is expected to be much stronger. Aims. This raises the question of whether the magnetic field still leads to the formation of structures in the photosphere and of a solar-like chromosphere and corona. Methods. We carried out a spectroscopic monitoring campaign in which we observed ultracool dwarfs that have strong magnetic fields: the BD LP944-20 and 2MASSW J0036159+182110. The objects were monitored over several rotation periods spectroscopically. LP944-20 was observed simultaneously in the optical and in the near infrared regime, 2MASSW J0036159+182110 only in the infrared. From the spectra, we determined the temperature of the objects in each spectrum, and measured the equivalent width in a number of diagnostically important lines. Temperature variations would indicate the presence of warm and cold regions, variations in the equivalent widths of photospheric lines are sensitive to the structure of cloud layers, and H α is a diagnostic for chromospheric structures. Results. Both dwarfs turned out to be remarkably constant. In the case of LP944-20, the T eff-variations are ≤50 K, and the rmsvariations in the equivalent widths of H α small. We also find that the equivalent widths of photospheric lines are remarkably constant. We did not find any significant variations in the case of 2MASSW J0036159+182110 either. Thus the most important result is that no significant variability was found at the time of our observations. We find that Hα-line is in emission but the equivalent width is only −4.4 ± 0.3 Å. When comparing our spectra with spectra taken over the past 11 years, we recognize significant changes during this time. Conclusions. We interpret these results as evidence that the photosphere of these objects are remarkably homogeneous, with only little structure in them, and despite the strong magnetic fields. Thus, unlike active stars, there are no prominent spots on these objects.

Astronomy & Astrophysics, 2002

We report on the finding of the strongest Hα emission-pseudoequivalent width of 705 Å-known so fa... more We report on the finding of the strongest Hα emission-pseudoequivalent width of 705 Å-known so far in a young, late type dwarf. This object, named as S Ori 71, is a substellar candidate member of the 1-8 Myr star cluster σ Orionis. Due to its overluminous location in color-magnitude diagrams, S Ori 71 might be younger than other cluster members, or a binary of similar components. Its mass is in the range 0.021-0.012 M , depending on evolutionary models and possible binarity. The broad Hα line of S Ori 71 appears asymmetric, indicative of high velocity mass motions in the Hα forming region. The origin of this emission is unclear at the present time. We discuss three possible scenarios: accretion from a disk, mass exchange between the components of a binary system, and emission from a chromosphere.

Astronomy and Astrophysics, 2008