- ID:
- ivo://CDS.VizieR/J/ApJ/793/62
- Title:
- Triangulum-Andromeda stellar properties
- Short Name:
- J/ApJ/793/62
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- As large-scale stellar surveys have become available over the past decade, the ability to detect and characterize substructures in the Galaxy has increased dramatically. These surveys have revealed the Triangulum-Andromeda (TriAnd) region to be rich with substructures in the distance range 20-30 kpc, and the relation of these features to each other, if any, remains unclear. An exploration using Two Micron All Sky Survey (2MASS) photometry reveals not only the faint sequence in M giants detected by Rocha-Pinto et al. (2004ApJ...615..732R) spanning the range 100{deg}<l<160{deg} and -50{deg}<b<-15{deg}, but, in addition, a second, brighter and more densely populated sequence. These sequences are likely associated with the distinct main sequences (MSs) discovered (and labeled TriAnd1 and TriAnd2) by Martin et al. (2007ApJ...668L.123M) in an optical survey in the direction of M31, where TriAnd2 is the optical counterpart of the fainter red giant branch (RGB)/asymptotic giant branch sequence of Rocha-Pinto et al. Here, the age, distance, and metallicity ranges for TriAnd1 and TriAnd2 are estimated by simultaneously fitting isochrones to the 2MASS RGB tracks and the optical MS/MS turn-off features. The two populations are clearly distinct in age and distance: the brighter sequence (TriAnd1) is younger (6-10 Gyr) and closer (distance of ~15-21 kpc), whereas the fainter sequence (TriAnd2) is older (10-12 Gyr) and at an estimated distance of ~24-32 kpc. A comparison with simulations demonstrates that the differences and similarities between TriAnd1 and TriAnd2 can simultaneously be explained if they represent debris originating from the disruption of the same dwarf galaxy, but torn off during two distinct pericentric passages.
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- ID:
- ivo://CDS.VizieR/J/A+A/632/A46
- Title:
- TW Hydrae association with X-shooter
- Short Name:
- J/A+A/632/A46
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Measurements of the protoplanetary disk frequency in young star clusters of different ages indicate disk lifetimes <10Myr. However, our current knowledge of how mass accretion in young stars evolves over the lifespans of disks is subject to many uncertainties, especially at the lower stellar masses. In this study, we investigate ongoing accretion activity in the TW Hydrae association (TWA), the closest association of pre-main sequence stars with active disks. The age (8-10Myr) and the proximity of the TWA render it an ideal target to probe the final stages of disk accretion down to brown dwarf masses. The study is based on homogeneous spectroscopic data from 300nm to 2500nm, obtained synoptically with X-shooter, which allows simultaneous derivation of individual extinction, stellar parameters, and accretion parameters for each star. The continuum excess emission diagnostics is used to estimate the accretion luminosities and mass accretion rates of our disk-bearing targets, and the shape and intensity of permitted and forbidden emission lines are analyzed to probe the physics of the star-disk interaction environment.
- ID:
- ivo://CDS.VizieR/J/MNRAS/334/20
- Title:
- UBV(RI)_c_ photometry of HIP red stars
- Short Name:
- J/MNRAS/334/20
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present homogeneous and standardized UBV(RI)_c_ photometry for nearly 550 M stars selected from the Hipparcos satellite data base (Cat. <I/239>) using the following selection criteria: lack of obvious variability (no Hipparcos variability flag); {delta}<+10{deg}, (V-I)>1.7; and V magnitude fainter than about 7.6. Comparisons are made between the current photometry, other ground-based data sets and Hipparcos photometry. We use linear discriminant analysis to determine a luminosity segregation criterion for late-type stars, and principal component analysis to study the statistical structure of the colour indices and to calibrate absolute magnitude in terms of (V-I) for the dwarf stars. Various methods are used to determine the mean absolute magnitude of the giant stars. We find 10 dwarf stars, apparently previously unrecognized (prior to Hipparcos) as being within 25pc, including five within 20pc.
- ID:
- ivo://CDS.VizieR/J/ApJS/193/1
- Title:
- UBVRIJHK color-temperature calibration
- Short Name:
- J/ApJS/193/1
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- A collection of Johnson/Cousins photometry for stars with known [Fe/H] is used to generate color-color relations that include the abundance dependence. Literature temperature and bolometric correction (BC) dependences are attached to the color relations. The JHK colors are transformed to the Bessell & Brett (1988PASP..100.1134B) homogenized system. The main result of this work is the tabulation of seven colors and the V-band BC as a function of Teff, logg, and [Fe/H] for -1.06<V-K<10.2 and an accompanying interpolation program. Improvements to the present calibration would involve filling photometry gaps, obtaining more accurate and on-system photometry, knowing better logg and [Fe/H] values, improving the statistics for data-impoverished groups of stars such as metal-poor K dwarfs, applying small tweaks in the processing pipeline, and obtaining better empirical temperature and BC relations, especially for supergiants and M stars. A way to estimate dust extinction from M dwarf colors is pointed out.
- ID:
- ivo://CDS.VizieR/J/A+A/641/A170
- Title:
- Ultracool dwarf K2 light curves
- Short Name:
- J/A+A/641/A170
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- With the discovery of a planetary system around the ultracool dwarf TRAPPIST-1, there has been a surge of interest in such stars as potential planet hosts. Planetary systems around ultracool dwarfs represent our best chance of characterising temperate rocky-planet atmospheres with the James Webb Space Telescope. However, TRAPPIST-1 remains the only known system of its kind and the occurrence rate of planets around ultracool dwarfs is still poorly constrained. We seek to perform a complete transit search on the ultracool dwarfs observed by NASA's K2 mission, and use the results to constrain the occurrence rate of planets around these stars. We filter and characterise the sample of ultracool dwarfs observed by K2 by fitting their spectral energy distributions and using parallaxes from Gaia. We build an automatic pipeline to perform photometry, detrend the light curves, and search for transit signals. Using extensive injection-recovery tests of our pipeline, we compute the detection sensitivity of our search, and thus the completeness of our sample. We infer the planetary occurrence rates within a hierarchical Bayesian model (HBM) to treat uncertain planetary parameters.With the occurrence rate parametrised by a step-wise function, we present a convenient way to directly marginalise over the second level of our HBM (the planetary parameters). Our method is applicable generally and can greatly speed up inference with larger catalogues of detected planets. We detect one planet in our sample of 702 ultracool dwarfs: a previously validated mini-Neptune. We thus infer a mini-Neptune (2-4R_{Earth}_) occurrence rate of {eta}=0.20^+0.16^_0.11_ within orbital periods of 1-20 days. For super-Earths (1-2R_{Earth}_) and ice or gas giants (4-6R_{Earth}_) within 1-20 days, we place 95% credible intervals of {eta}<1.14 and {eta}<0.29, respectively. If TRAPPIST-1-like systems were ubiquitous, we would have a 96% chance of finding at least one.
- ID:
- ivo://CDS.VizieR/J/AJ/160/19
- Title:
- 827 ultracool dwarfs with K2
- Short Name:
- J/AJ/160/19
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The occurrence of planets orbiting ultracool dwarfs is poorly constrained. We present results from a guest observer program on NASA's K2 spacecraft to search for transiting planets orbiting a sample of 827 ultracool dwarfs. Having found no transiting planets in our sample, we determined an upper limit on the occurrence of planets. We simulated planets orbiting our sample for a range of orbital periods and sizes. For the simulated planets that transit their host, we injected the transit light curve into the real K2 light curves, then attempted to recover the injected planets. For a given occurrence rate, we calculated the probability of seeing no planets, and use the results to place an upper limit on planet occurrence as a function of planet radius and orbital period. We find that short-period, mini-Neptune and Jupiter-sized planets are rare around ultracool dwarfs, consistent with results for early and mid-type M dwarf stars. We constrain the occurrence rate {eta} for planets between 0.5 and 10 R{earth} with orbital periods between 1 and 26.3days.
- ID:
- ivo://CDS.VizieR/J/MNRAS/431/2063
- Title:
- UV/X-ray activity of M dwarfs within 10pc
- Short Name:
- J/MNRAS/431/2063
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- M dwarfs are the most numerous stars in the Galaxy. They are characterized by strong magnetic activity. The ensuing high-energy emission is crucial for the evolution of their planets and the eventual presence of life on them. We systematically study the X-ray and ultraviolet emission of a subsample of M dwarfs from a recent proper-motion survey, selecting all M dwarfs within 10pc to obtain a nearly volume-limited sample (~90 percent completeness). Archival ROSAT, XMM-Newton and GALEX data are combined with published spectroscopic studies of H{alpha} emission and rotation to obtain a broad picture of stellar activity on M dwarfs. We make use of synthetic model spectra to determine the relative contributions of photospheric and chromospheric emission to the ultraviolet flux. We also analyse the same diagnostics for a comparison sample of young M dwarfs in the TW Hya association (~10Myr). We find that generally the emission in the GALEX bands is dominated by the chromosphere but the photospheric component is not negligible in early-M field dwarfs. The surface fluxes for the H{alpha}, near-ultraviolet, far-ultraviolet and X-ray emission are connected via a power-law dependence. We present here for the first time such flux-flux relations involving broad-band ultraviolet emission for M dwarfs. Activity indices are defined as flux ratio between the activity diagnostic and the bolometric flux of the star in analogy to the CaII R'HK index. For given spectral type, these indices display a spread of 2-3dex which is largest for M4 stars. Strikingly, at mid-M spectral types, the spread of rotation rates is also at its highest level. The mean activity index for fast rotators, likely representing the saturation level, decreases from X-rays over the FUV to the NUV band and H{alpha}, i.e. the fractional radiation output increases with atmospheric height. The comparison to the ultraviolet and X-ray properties of TWHya members shows a drop of nearly three orders of magnitude for the luminosity in these bands between ~10Myr and few Gyr age. A few young field dwarfs (<1Gyr) in the 10-pc sample bridge the gap indicating that the drop in magnetic activity with age is a continuous process. The slope of the age decay is steeper for the X-ray than for the UV luminosity.
- ID:
- ivo://CDS.VizieR/J/A+A/654/A118
- Title:
- Vanadium measurements for 135 M dwarfs
- Short Name:
- J/A+A/654/A118
- Date:
- 22 Feb 2022
- Publisher:
- CDS
- Description:
- M-dwarf spectra are complex and notoriously difficult to model, posing challenges to understanding their photospheric properties and compositions in depth. Vanadium (V) is an iron-group element whose abundance supposedly closely tracks that of iron, but has origins that are not completely understood. Our aim is to characterize a series of neutral vanadium atomic absorption lines in the 800-910nm wavelength region of high signal-to-noise, high-resolution, telluric-corrected M-dwarf spectra from the CARMENES survey. Many of these lines are prominent and exhibit a distinctive broad and flat-bottom shape -- a result of hyperfine splitting (HFS). We investigate the potential and implications of these HFS-split lines for abundance analysis of cool stars. With standard spectral synthesis routines, as provided by the spectroscopy software iSpec and the latest atomic data (including HFS) available from the VALD3 database, we model these striking line profiles. We use them to measure V abundances of cool dwarfs. We determine V abundances for 135 early-M dwarfs (M0.0V to M3.5V) in the CARMENES guaranteed time observations sample. They exhibit a [V/Fe]-[Fe/H] trend consistent with that derived from nearby FG dwarfs. The tight (+/-0.1dex) correlation between [V/H] and [Fe/H] suggests the potential application of V as an alternative metallicity indicator in M dwarfs. We also show hints that a neglect to model HFS could partially explain the temperature correlation in V abundance measurements observed in previous studies of samples involving dwarf stars with Teff<=5300K. Our work suggests that HFS can impact certain absorption lines in cool photospheres more severely than in Sun-like ones. Therefore, we advocate that HFS should be carefully treated in abundance studies in stars cooler than ~5000 K. On the other hand, strong HFS split lines in high-resolution spectra present an opportunity for precision chemical analyses of large samples of cool stars. The V-to-Fe trends exhibited by the local M dwarfs continue to challenge theoretical models of V production in the Galaxy.
- ID:
- ivo://CDS.VizieR/J/A+A/614/A12
- Title:
- VLTI/PIONIER observations of CE Tauri
- Short Name:
- J/A+A/614/A12
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Red supergiant stars represent one of the latest stages of the evolution of massive stars. Their photospheric convection may play an important role in the launching mechanism of their mass loss. Yet, its characteristics and dynamics are poorly constrained. By observing red supergiant stars with near infrared interferometry at different epochs, we expect to unveil the evolution of bright convective features on their stellar surface. We observed the M2Iab-Ib red supergiant star CE Tau with the VLTI/PIONIER instrument in the H band at two different epochs separated by one month. We derive the angular diameter of the star, basic stellar parameters and reconstruct two reliable images of its H band photosphere. The contrast of the convective pattern of the reconstructed images is 5+/-1% and 6+/-1 % for our two epochs of observation. The stellar photosphere shows few changes between the two epochs. The contrast of the convective pattern is below the average contrast variations obtained on 30 randomly chosen snapshots of the best matching 3D radiative hydrodynamics simulation: 23+/-1% for the original simulation images, and 16+/-1% for the maps degraded to the reconstruction resolution. We offer two hypotheses to explain this observation: CE Tau may be experiencing a quiet convective activity episode or it could be a consequence of its warmer effective temperature (hence its smaller radius) compared to the simulation.
- ID:
- ivo://CDS.VizieR/J/ApJ/705/1416
- Title:
- Volume-limited sample of M7-M9.5 dwarfs <20pc
- Short Name:
- J/ApJ/705/1416
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- In a volume-limited sample of 63 ultracool dwarfs of spectral type M7-M9.5, we have obtained high-resolution spectroscopy with UVES at the Very Large Telescope and HIRES at Keck Observatory. In this first paper we introduce our volume-complete sample from DENIS and 2MASS targets, and we derive radial velocities and space motion. Kinematics of our sample are consistent with the stars being predominantly members of the young disk. The kinematic age of the sample is 3.1Gyr. We find that six of our targets show strong Li lines implying that they are brown dwarfs younger than several hundred million years. Five of the young brown dwarfs were unrecognized before. Comparing the fraction of Li detections to later spectral types, we see a hint of an unexpected local maximum of this fraction at spectral type M9. It is not yet clear whether this maximum is due to insufficient statistics, or to a combination of physical effects including spectral appearance of young brown dwarfs, Li line formation, and the star formation rate at low masses.
