We present an analysis of the intrinsic colors and temperatures of 5-30 Myr old pre-main-sequence (pre-MS) stars using the F0- through M9-type members of nearby, negligibly reddened groups: the eta ...Cha cluster, the TW Hydra Association, the Beta PicMovingGroup, and the Tucana-Horologium Association. To check the consistency of spectral types from the literature, we estimate new spectral types for 52 nearby pre-MS stars with spectral types F3 through M4 using optical spectra taken with the SMARTS 1.5 m telescope. Lastly, we present (1) a modern Tsubeff, optical/IR color, and BC sequence for O9V-M9V MS stars based on an extensive literature survey, (2) a revised Q-method relation for dereddening UBV photometry of OB-type stars, and (3) introduce two candidate spectral standard stars as representatives of spectral types K8V and K9V.
Abstract
We present a kinematic study of the Scorpius-Centaurus (Sco-Cen) OB association (Sco OB2) using Gaia DR1 parallaxes and proper motions. Our goal is to test the classical theory that OB ...associations are the expanded remnants of dense and compact star clusters disrupted by processes such as residual gas expulsion. Gaia astrometry is available for 258 out of 433 members of the association, with revised Hipparcos astrometry used for the remainder. We use these data to confirm that the three subgroups of Sco-Cen are gravitationally unbound and have non-isotropic velocity dispersions, suggesting that they have not had time to dynamically relax. We also explore the internal kinematics of the subgroups to search for evidence of expansion. We test Blaauw's classical linear model of expansion, search for velocity trends along the Galactic axes, compare the expanding and non-expanding convergence points, perform traceback analysis assuming both linear trajectories and using an epicycle approximation, and assess the evidence for expansion in proper motions corrected for virtual expansion/contraction. None of these methods provide coherent evidence for expansion of the subgroups, with no evidence to suggest that the subgroups had a more compact configuration in the past. We find evidence for kinematic substructure within the subgroups that supports the view that they were not formed by the disruption of individual star clusters. We conclude that Sco-Cen was likely to have been born highly substructured, with multiple small-scale star formation events contributing to the overall OB association, and not as single, monolithic burst of clustered star formation.
A candidate extrasolar planet companion to the young brown dwarf 2MASSW J1207334-393254 (hereafter 2M1207) was recently discovered by Chauvin et al. They find that the temperature and luminosity of ...2M1207b are consistent with a young, 65M sub(J) planet. The 2M1207 system is purported to be a member of the TW Hya association (TWA) and situated 670 pc away. Using a revised space motion vector for TWA and improved proper motion for 2M1207,I use the moving cluster method to estimate the distance to the 2M1207 system and other TWA members. The derived distance for 2M1207 (53 c 6 pc) forces the brown dwarf and planet to be half as luminous as previously thought. The inferred masses for 2M1207A and 2M1207b decrease to 621 and 63-4M sub(J), respectively, with the mass of 2M1207b well below the observed tip of the planetary mass function and the theoretical deuterium-burning limit. After removing probable Lower Centaurus Crux (LCC) members from the TWA sample, as well as the probable nonmember TWA 22, the remaining TWA membership is found to have distances of 49 c 3 (s.e.m.) c 12 (1s) pc and an internal one-dimensional velocity dispersion of 0.8 sub(-0.2) super(+0.3) km s super(-1). There is weak evidence that the TWA is expanding, and the data are consistent with a lower limit on the expansion age of 10 Myr (95% confidence).
On the Age of the TRAPPIST-1 System Burgasser, Adam J.; Mamajek, Eric E.
Astrophysical journal/The Astrophysical journal,
08/2017, Letnik:
845, Številka:
2
Journal Article
Recenzirano
Odprti dostop
The nearby (d = 12 pc) M8 dwarf star TRAPPIST-1 (2MASS J23062928−0502285) hosts a compact system of at least seven exoplanets with sizes similar to Earth. Given its importance for testing planet ...formation and evolution theories, and for assessing the prospects for habitability among Earth-size exoplanets orbiting the most common type of star in the Galaxy, we present a comprehensive assessment of the age of this system. We collate empirical age constraints based on the color-absolute magnitude diagram, average density, lithium absorption, surface gravity features, metallicity, kinematics, rotation, and magnetic activity; and conclude that TRAPPIST-1 is a transitional thin/thick disk star with an age of 7.6 2.2 Gyr. The star's color-magnitude position indicates that it is slightly metal-rich, which is consistent with the previously reported near-infrared spectroscopic metallicity; and it has a radius ) that is larger by 8%-14% than the predictions of solar-metallicity evolutionary models. We discuss some implications of the old age of this system with regard to the stability and habitability of its planets.
We present results of a spectroscopic survey for new K- and M-type members of Scorpius-Centaurus (Sco-Cen), the nearest OB Association (~100-200 pc). Using an X-ray, proper motion and ...colour-magnitude selected sample, we obtained spectra for 361 stars, for which we report spectral classifications and Li and Ha equivalent widths. We identified 156 new members of Sco-Cen, and recovered 51 previously published members. We have combined these with previously known members to form a sample of 493 solar-mass (~0.7-1.3 M...) members of Sco-Cen. We investigated the star formation history of this sample, and re-assessed the ages of the massive main-sequence turn-off and the G-type members in all three subgroups. We performed a census for circumstellar discs in our sample using WISE infrared data and find a protoplanetary disc fraction for K-type stars of 4.4... per cent for Upper Centaurus-Lupus and Lower Centaurus-Crux at ~16 Myr and 9.0... per cent for Upper Scorpius at ~10 Myr. These data are consistent with a protoplanetary disc e-folding time-scale of ~4-5 Myr for ~1 M... stars, twice that previously quoted, but consistent with the Bell et al. revised age scale of young clusters. Finally, we construct an age map of Scorpius-Centaurus which clearly reveals substructure consisting of concentrations of younger and older stars. We find evidence for strong age gradients within all three subgroups. None of the subgroups are consistent with being simple, coeval populations which formed in single bursts, but likely represents a multitude of smaller star formation episodes of hundreds to tens of stars each. (ProQuest: ... denotes formulae/symbols omitted.)
We present a census of circumstellar disks in the Chamaeleon I star- forming region. Using the Infrared Array Camera and the Multiband Imaging Photometer on board the Spitzer Space Telescope, we have ...obtained images of Chamaeleon I at 3.6, 4.5, 5.8, 8.0, and 24 mum. To search for new disk- bearing members of the cluster, we have performed spectroscopy on objects that have red colors in these data. Through this work, we have discovered four new members of Chamaeleon I with spectral types of M4, M6, M7.5, and L0. The first three objects are highly embedded and reside near known protostars, indicating that they may be among the youngest low-mass sources in the cluster. The L0 source is the coolest known member of Chamaeleon I. Its luminosity implies a mass of 0.004-0.01 M sub(image), making it the least massive brown dwarf for which a circumstellar disk has been reliably detected. To characterize the disk population in Chamaeleon I, we have classified the infrared spectral energy distributions of the 203 known members that are encompassed by the Spitzer images. Through these classifications, we find that the disk fraction in Chamaeleon I is roughly constant at image50% from 0.01 to 0.3 M sub(image). These data are similar to the disk fraction of IC 348, which is a denser cluster at the same age as Chamaeleon I. However, the disk fraction at image is significantly higher in Chamaeleon I than in IC 348 (65% vs. 20%), indicating longer disk lifetimes in Chamaeleon I for this mass range. Thus, low-density star-forming regions like Chamaeleon I may offer more time for planet formation around solar-type stars than denser clusters.
The nearby (d = 7.7 pc) A3V star Fomalhaut is orbited by a resolved dusty debris disk and a controversial candidate extrasolar planet. The commonly cited age for the system (200 + or - 100 Myr) from ...Barrado y Navascues et al. relied on a combination of isochronal age plus youth indicators for the K4V common proper-motion system TW PsA. TW PsA is 1degrees.96 away from Fomalhaut and was first proposed as a companion by Luyten, but the physicality of the binarity is worth testing with modern data. I demonstrate that TW PsA is unequivocally a physical stellar companion to Fomalhaut, with true separation (ProQuest: Formulae and/or non-USASCII text omitted) pc ((ProQuest: Formulae and/or non-USASCII text omitted) kAU) and sharing velocities within 0.1 + or - 0.5 km s super(-1)-consistent with being a bound system. Hence, TW PsA should be considered "Fomalhaut B." Combining modern H-R diagram constraints with four sets of evolutionary tracks, and assuming the star was born with protosolar composition, I estimate a new isochronal age for Fomalhaut of 450 + or - 40 Myr and mass of 1.92 + or - 0.02 M sub(middot in circle). Various stellar youth diagnostics are re-examined for TW PsA. The star's rotation, X-ray emission, and Li abundances are consistent with approximate ages of 410, 380, and 360 Myr, respectively, yielding a weighted mean age of 400 + or - 70 Myr. Combining the independent ages, I estimate a mean age for the Fomalhaut-TW PsA binary of 440 + or - 40 Myr. The older age implies that substellar companions of a given mass are approximately 1 mag fainter at IR wavelengths than previously assumed.
Abstract
BANYAN Σ is a new Bayesian algorithm to identify members of young stellar associations within 150 pc of the Sun. It includes 27 young associations with ages in the range ∼1–800 Myr, modeled ...with multivariate Gaussians in six-dimensional (6D)
XYZUVW
space. It is the first such multi-association classification tool to include the nearest sub-groups of the Sco-Cen OB star-forming region, the IC 2602, IC 2391, Pleiades and Platais 8 clusters, and the
ρ
Ophiuchi, Corona Australis, and Taurus star formation regions. A model of field stars is built from a mixture of multivariate Gaussians based on the Besançon Galactic model. The algorithm can derive membership probabilities for objects with only sky coordinates and proper motion, but can also include parallax and radial velocity measurements, as well as spectrophotometric distance constraints from sequences in color–magnitude or spectral type–magnitude diagrams. BANYAN Σ benefits from an analytical solution to the Bayesian marginalization integrals over unknown radial velocities and distances that makes it more accurate and significantly faster than its predecessor BANYAN II. A contamination versus hit rate analysis is presented and demonstrates that BANYAN Σ achieves a better classification performance than other moving group tools available in the literature, especially in terms of cross-contamination between young associations. An updated list of bona fide members in the 27 young associations, augmented by the Gaia-DR1 release, as well as all parameters for the 6D multivariate Gaussian models for each association and the Galactic field neighborhood within 300 pc are presented. This new tool will make it possible to analyze large data sets such as the upcoming Gaia-DR2 to identify new young stars. IDL and Python versions of BANYAN Σ are made available with this publication, and a more limited online web tool is available at
http://www.exoplanetes.umontreal.ca/banyan/banyansigma.php
.
Abstract
We have performed a survey for new members of the Upper Sco association that have circumstellar disks using mid-infrared photometry from the
Wide-field Infrared Survey Explorer
(
WISE
). ...Through optical and near-infrared spectroscopy, we have confirmed 185 candidates as likely members of Upper Sco with spectral types ranging from mid-K to M9. They comprise ∼36% of the known disk-bearing members of the association. We also have compiled all available mid-infrared photometry from
WISE
and the
Spitzer Space Telescope
for the known members of Upper Sco, resulting in a catalog of data for 1608 objects. We have used these data to identify the members that exhibit excess emission from disks and we have classified the evolutionary stages of those disks with criteria similar to those applied in our previous studies of Taurus and Upper Sco. Among 484 members with excesses in at least one band (excluding five Be stars), we classify 296 disks as full, 66 as evolved, 19 as transitional, 22 as evolved or transitional, and 81 as evolved transitional or debris. Many of these disks have not been previously reported, including 129 full disks and 50 disks that are at more advanced evolutionary stages.
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