The spectroscopic study of M8.5-M9.5 stars and brown dwarfs (original) (raw)
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Astronomy and Astrophysics, 2018
Context. Being the most numerous and oldest stars in the galaxy, M dwarfs are objects of great interest for exoplanet searches. The presence of molecules in their atmosphere complicates our understanding of their atmospheric properties. But great advances have recently been made in the modeling of M dwarfs due to the revision of solar abundances. Aims. We aim to determine stellar parameters of M dwarfs using high resolution spectra (R ∼ 90 000) simultaneously in the visible and the near-infrared. The high resolution spectra and broad wavelength coverage provide an unique opportunity to understand the onset of dust and cloud formation at cool temperatures. Furthermore, this study will help in understanding the physical processes which occur in a cool atmospheres, particularly, the redistribution of energy from the optical to the near-infrared. Methods. The stellar parameters of M dwarfs in our sample have been determined by comparing the high resolution spectra both in the optical and in the near-infrared simultaneously observed by CARMENES with the synthetic spectra obtained from the BT-Settl model atmosphere. The detailed spectral synthesis of these observed spectra both in the optical and in the near-infrared helps to understand the missing continuum opacity. Results. For the first time, we derive fundamental stellar parameters of M dwarfs using the high resolution optical and near-infrared spectra simultaneously. We determine T eff , log g and [M/H] for 292 M dwarfs of spectral type M0 to M9, where the formation of dust and clouds are important. The derived T eff for the sample ranges from 2300 to 4000 K, values of log g ranges from 4.5 ≤ log g ≤ 5.5 and the resulting metallicity ranges from −0.5 ≤ [M/H] ≤ +0.5. We have also explored the possible differences in T eff , log g and [M/H] by comparing them with other studies of the same sample of M dwarfs.
New constraints on the formation and settling of dust in the atmospheres of young M and L dwarfs
Astronomy & Astrophysics, 2014
Context. Gravity modifies the spectral features of young brown dwarfs (BDs). A proper characterization of these objects is crucial for the identification of the least massive, and latest-type objects in star-forming regions, and to explain the origin(s) of the peculiar spectro-photometric properties of young directly imaged extrasolar planets and BD companions. Aims. We obtained medium-resolution (R∼1500-1700) near-infrared (1.1-2.5 µm) spectra of seven young M9.5-L3 dwarfs classified at optical wavelengths. We aim to empirically confirm the low surface gravity of the objects in the near-infrared. We also test whether self-consistent atmospheric models correctly represent the formation and the settling of dust clouds in the atmosphere of young late-M and L dwarfs. Methods. We used ISAAC (Infrared Spectrometer And Array Camera) at VLT (Very Large Telescope) to obtain the spectra of the targets. We compared them to those of mature and young BDs, and young late-type companions to nearby stars with known ages, in order to identify and study gravity-sensitive features. We computed spectral indices weakly sensitive to the surface gravity to derive near-infrared spectral types. Finally, we found the best fit between each spectrum and synthetic spectra from the BT-Settl 2010 and 2013 atmospheric models. Using the best fit, we derived the atmospheric parameters of the objects and identify which spectral characteristics the models do not reproduce. Results. We confirmed that our objects are young BDs and we found near-infrared spectral types in agreement with the ones determined at optical wavelengths. The spectrum of the L2γ dwarf 2MASSJ232252.99-615127.5 reproduces well the spectrum of the planetary mass companion 1RXS J160929.1-210524b. BT-Settl models fit the spectra and the 1-5 µm spectral energy distribution of the L0-L3 dwarfs for temperatures between 1600-2000 K. But the models fail to reproduce the shape of the H band, and the nearinfrared slope of some of our targets. This fact, and the best fit solutions found with super-solar metallicity are indicative of a lack of dust, in particular at high altitude, in the cloud models.
Stellar parameters of early-M dwarfs from ratios of spectral features at optical wavelengths
Astronomy & Astrophysics, 2015
Context. Low-mass stars have been recognised as promising targets in the search for rocky, small planets with the potential of supporting life. As a consequence, Doppler search programmes using high-resolution spectrographs like HARPS or HARPS-N are providing huge quantities of optical spectra of M dwarfs. However, determining the stellar parameters of M dwarfs using optical spectra has proven to be challenging. Aims. We aim to calibrate empirical relationships to determine accurate stellar parameters for early M dwarfs (spectral types M0-M4.5) using the same spectra that are used for the radial velocity determinations, without the necessity of acquiring IR spectra or relying on atmospheric models and/or photometric calibrations. Methods. Our methodology consists in the use of ratios of pseudo equivalent widths of spectral features as a temperature diagnostic, a technique largely used in solar-type stars. Stars with effective temperatures obtained from interferometric estimates of their radii are used as calibrators. Empirical calibrations for the spectral type are also provided. Combinations of features and ratios of features are used to derive calibrations for the stellar metallicity. Our methods are then applied to a large sample of M dwarfs that are currently being observed in the framework of the HARPS GTO search for extrasolar planets. The derived temperatures and metallicities are used together with photometric estimates of mass, radius, and surface gravity to calibrate empirical relationships for these parameters. Results. A large list of spectral features in the optical spectra of early M dwarfs was identified. From this list the pseudo equivalent width of roughly 43% of the features shows a strong anticorrelation with the effective temperature. The correlation with the stellar metallicity is weaker. A total of 112 temperature sensitive ratios have been identified and calibrated over the range 3100-3950 K, providing effective temperatures with typical uncertainties of the order of 70 K. Eighty-two ratios of pseudo equivalent widths of features were calibrated to derive spectral types within 0.5 subtypes for stars with spectral types between K7V and M4.5V. Regarding stellar metallicity, 696 combinations of pseudo equivalent widths of individual features and temperature-sensitive ratios have been calibrated, over the metallicity range from -0.54 to +0.24 dex, with estimated uncertainties in the range of 0.07-0.10 dex. We provide our own empirical calibrations for stellar mass, radius, and surface gravity. These parameters are found to show a dependence on the stellar metallicity. For a given effective temperature, lower metallicities predict lower masses and radii, as well as larger gravities. Conclusions.
Spectral Properties of Brown Dwarfs and Hot Jupiters
2005
Brown dwarfs bridge the gap between the stellar and planetary mass regimes. Evolving from conditions very similar to the lowest-mass stars, the atmospheres of older brown dwarfs closely resemble those expected in close-in extrasolar giant planets, and with cooler BDs still being discovered, more and more approach the properties of gas giants at wider separation. Interpreting the spectra of BDs is therefore a crucial step towards understanding and predicting the spectral and thermal properties of EGPs.
The effective temperature scale of M dwarfs
Astronomy & Astrophysics, 2013
Context. Despite their large number in the Galaxy, M dwarfs remain elusive objects and the modeling of their photospheres has long remained a challenge (molecular opacities, dust cloud formation). Aims. Our objectives are to validate the BT-Settl model atmospheres, update the M dwarf T eff -spectral type relation, and find the atmospheric parameters of the stars in our sample. Methods. We compare two samples of optical spectra covering the whole M dwarf sequence with the most recent BT-Settl synthetic spectra and use a χ 2 minimization technique to determine T eff . The first sample consists of 97 low-resolution spectra obtained with NTT at La Silla Observatory. The second sample contains 55 mid-resolution spectra obtained at the Siding Spring Observatory (SSO). The spectral typing is realized by comparison with already classified M dwarfs. Results. We show that the BT-Settl synthetic spectra reproduce the slope of the spectral energy distribution and most of its features. Only the CaOH band at 5570Å and AlH and NaH hydrides in the blue part of the spectra are still missing in the models. The T eff -scale obtained with the higher resolved SSO 2.3 m spectra is consistent with that obtained with the NTT spectra. We compare our T effscale with those of other authors and to published isochrones using the BT-Settl colors. We also present relations between effective temperature, spectral type and colors of the M dwarfs.
Far‐Infrared Properties of M Dwarfs
The Astrophysical Journal, 2007
We report the mid-and far-infrared properties of nearby M dwarfs. Spitzer/MIPS measurements were obtained for a sample of 62 stars at 24 µm, with subsamples of 41 and 20 stars observed at 70 µm and 160 µm respectively. We compare the results with current models of M star photospheres and look for indications of circumstellar dust in the form of significant deviations of K-[24 µm] colors and 70 µm / 24 µm flux ratios from the average M star values. At 24 µm, all 62 of the targets were detected; 70 µm detections were achieved for 20 targets in the subsample observed; and no detections were seen in the 160 µm subsample. No clear far-infrared excesses were detected in our sample. The average far infrared excess relative to the photospheric emission of the M stars is at least four times smaller than the similar average for a sample of solar-type stars. However, this limit allows the average fractional infrared luminosity in the M-star sample to be similar to that for more massive stars. We have also set low limits (10 −4 to 10 −9 M Earth depending on location) for the maximum mass of dust possible around our stars.
A library of near-infrared integral field spectra of young M–L dwarfs
Astronomy & Astrophysics, 2014
Context. At young ages, low surface gravity affects the atmospheric properties of ultracool dwarfs. The impact on medium-resolution near-infrared (NIR) spectra has only been slightly investigated at the M-L transition sofar. Aims. We present a library of near-infrared (1.1-2.45 µm) medium-resolution (R∼1500-2000) integral field spectra of 15 young M6-L0 dwarfs. We aim at deriving updated NIR spectral type, luminosity and physical parameters (T eff , log g , M, L/L ) of each source. This work also aims at testing the latest generation of BT-SETTL atmospheric models. Methods. We estimated spectral types using spectral indices and spectra of reference young objects classified in the optical. We used the 2010 and 2012 releases of the BT-SETTL synthetic spectral grid and cross-checked the results with the DRIFT-PHOENIX models to derive the atmospheric properties of the sources. Results. We do not evidence significant differences between the spectra of young companions and of reference young isolated browndwarfs in the same spectral type range. We derive infrared spectral types L0±1, L0±1, M9.5±0.5, M9.5±0.5, M9.25±0.25, M8 +0.5 −0.75 , and M8.5±0.5 for AB Pic b, Cha J110913-773444, USco CTIO 108B, GSC 08047-00232 B, DH Tau B, CT Cha b, and HR7329B respectively. BT-SETTL and DRIFT-PHOENIX models yield close T eff and log g estimates for each sources. The models seem to evidence a 600 +600 −300 K drop of the effective temperature at the M-L transition. Assuming the former temperatures are correct, we derive new mass estimates which confirm that DH Tau B, USco CTIO 108B, AB Pic b, KPNO Tau 4, OTS 44, and Cha1109 lies inside or at the boundary of the planetary mass range. We combine the empirical luminosities of the M9.5-L0 sources to the T eff to derive semi-empirical radii estimates that do not match "hot-start" evolutionary models predictions at 1-3 Myr. We use complementary data to demonstrate that atmospheric models are able to reproduce the combined optical and infrared spectral energy distribution, together with the near-infrared spectra of these sources simultaneously. But the models still fail to represent the dominant features in the optical. This issue rises doubts on the ability of these models to predict effective temperatures from near-infrared spectra alone. Conclusions. The library provides templates for the characterization of other young and late type objects. The study advocates the use of photometric and spectroscopic informations over a broad range of wavelengths to study the properties of very low mass young companions to be detected with the planet imagers (Subaru/SCExAO, LBT/LMIRCam, Gemini/GPI, VLT/SPHERE).
Dust in brown dwarfs and extra-solar planets
Astronomy and Astrophysics, 2009
Aims. Substellar objects have extremely long life-spans. The cosmological consequence for older objects are low abundances of heavy elements, which results in a wide distribution of objects over metallicity, hence over age. Within their cool atmosphere, dust clouds become a dominant feature, affecting the opacity and the remaining gas phase abundance of heavy elements. We investigate the influence of the stellar metallicity on the dust formation in substellar atmospheres and on the dust cloud structure and its feedback on the atmosphere. This work has implications for the general question of star formation and of dust formation in the early universe. Methods. We utilize numerical simulations in which we solve a set of moment equations in order to determine the quasi-static dust cloud structure (Drift). These equations model the nucleation, the kinetic growth of composite particles, their evaporation and the gravitational settling as a stationary dust formation process. Element conservation equations augment this system of equations including the element replenishment by convective overshooting. The integration with an atmosphere code (Phoenix) allows to determine a consistent (T, p, v conv )-structure (T -local temperature, p -local pressure, v conv -convective velocity), and, hence, also to calculate synthetic spectra. Results. A grid of Drift-Phoenix model atmospheres was calculated for a wide range of metallicity, [M/H] ∈ [+0.5,-0.0,-0.5,...,-6.0], to allow for a systematic study of atmospheric cloud structures throughout the evolution of the universe. We find dust clouds in even the most metal-poor ([M/H]=-6.0) atmosphere of brown dwarfs. Only the most massive among the youngest brown dwarfs and giant gas planets can resist dust formation. For very low heavy element abundances, a temperature inversion develops which has a drastic impact on the dust cloud structure.
The Astrophysical Journal, 2009
We model the mineralogy and distribution of dust around the white dwarf G29-39 using the infrared spectrum from 1-35 µm. The spectral model for G29-38 dust combines a wide range of materials based on spectral studies of comets and debris disks. In order of their contribution to the mid-infrared emission, the most abundant minerals around G29-38 are amorphous carbon (λ < 8 µm), amorphous and crystalline silicates (5-40 µm), water ice (10-15 and 23-35 µm), and metal sulfides (18-28 µm). The amorphous C can be equivalently replaced by other materials (like metallic Fe) with featureless infrared spectra. The bestfitting crystalline silicate is Fe-rich pyroxene. In order to absorb enough starlight to power the observed emission, the disk must either be much thinner than the stellar radius (so that it can be heated from above and below) or it must have an opening angle wider than 2 • . A 'moderately optically thick' torus model fits well if the dust extends inward to 50 times the white dwarf radius, all grains hotter than 1100 K are vaporized, the optical depth from the star through the disk is τ = 5, and the radial density profile ∝ r −2.7 ; the total mass of this model disk is 2 × 10 19 g. A physically thin (less than the white dwarf radius) and optically -2thick disk can contribute to the near-infrared continuum only; such a disk cannot explain the longer-wavelength continuum or strong emission features. The combination of a physically thin, optically-thick inner disk and an outer, physically thick and moderately optically thin cloud or disk produces a reasonably good fit to the spectrum and requires only silicates in the outer cloud. We discuss the mineralogical results in comparison to planetary materials. The silicate composition contains minerals found from cometary spectra and meteorites, but Fe-rich pyroxene is more abundant than enstatite (Mg-rich pyroxene) or forsterite (Mg-rich olivine) in G29-38 dust, in contrast to what is found in most comet or meteorite mineralogies. Enstatite meteorites may be the most similar solar system materials to the G29-38 dust. Finally, we suggest the surviving core of a 'hot jupiter' as an alternative (neither cometary nor asteroidal) origin for the debris, though further theoretical work is needed to determine if this hypothesis is viable.
Observation and modelling of dusty, low gravity L, and M dwarfs
2008
Abstract: Observational facilities allow now the detection of optical and IR spectra of young M-and L-dwarfs. This enables empirical comparisons with old M-and L-dwarfs, and detailed studies in comparison with synthetic spectra. While classical stellar atmosphere physics seems perfectly appropriate for old M-dwarfs, more physical and chemical processes, cloud formation in particular, needs to be modelled in the substellar regime to allow a detailed spectral interpretation.