- ID:
- ivo://CDS.VizieR/J/ApJ/719/1784
- Title:
- Yellow supergiants in the SMC
- Short Name:
- J/ApJ/719/1784
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The yellow supergiant content of nearby galaxies provides a critical test of massive star evolutionary theory. While these stars are the brightest in a galaxy, they are difficult to identify because a large number of foreground Milky Way stars have similar colors and magnitudes. We previously conducted a census of yellow supergiants within M31 and found that the evolutionary tracks predict a yellow supergiant duration an order of magnitude longer than we observed. Here we turn our attention to the Small Magellanic Cloud (SMC), where the metallicity is 10x lower than that of M31, which is important as metallicity strongly affects massive star evolution. The SMC's large radial velocity (~160km/s) allows us to separate members from foreground stars. Observations of ~500 candidates yielded 176 near-certain SMC supergiants, 16 possible SMC supergiants, along with 306 foreground stars, and provide good relative numbers of yellow supergiants down to 12M_{sun}_. Of the 176 near-certain SMC supergiants, the kinematics predicted by the Besancon model of the Milky Way suggest a foreground contamination of <=4%. After placing the SMC supergiants on the Hertzsprung-Russell diagram (HRD) and comparing our results to the Geneva evolutionary tracks, we find results similar to those of the M31 study: while the locations of the stars on the HRD match the locations of evolutionary tracks well, the models overpredict the yellow supergiant lifetime by a factor of 10. Uncertainties about the mass-loss rates on the main sequence thus cannot be the primary problem with the models.
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- ID:
- ivo://CDS.VizieR/J/ApJ/877/60
- Title:
- YMGs. I. Young binaries & lithium-rich stars
- Short Name:
- J/ApJ/877/60
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Young stars in the solar neighborhood serve as nearby probes of stellar evolution and represent promising targets to directly image self-luminous giant planets. We have carried out an all-sky search for late-type (~K7-M5) stars within 100pc selected primarily on the basis of activity indicators from the Galaxy Evolution Explorer and ROSAT. Approximately 2000 active and potentially young stars are identified, of which we have followed up over 600 with low-resolution optical spectroscopy and over 1000 with diffraction-limited imaging using Robo-AO at the Palomar 1.5m telescope. Strong lithium is present in 58 stars, implying ages spanning ~10-200Myr. Most of these lithium-rich stars are new or previously known members of young moving groups including TWA, {beta}Pic, Tuc-Hor, Carina, Columba, Argus, ABDor, Upper Centaurus Lupus, and Lower Centaurus Crux; the rest appear to be young low-mass stars without connections to established kinematic groups. Over 200 close binaries are identified down to 0.2"-the vast majority of which are new-and will be valuable for dynamical mass measurements of young stars with continued orbit monitoring in the future.
- ID:
- ivo://CDS.VizieR/J/AJ/155/109
- Title:
- Young binaries in Ophiuchus&Upper Centaurus-Lupus
- Short Name:
- J/AJ/155/109
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present measurements of the orbital positions and flux ratios of 17 binary and triple systems in the Ophiuchus star-forming region and the Upper Centaurus-Lupus cluster based on adaptive optics imaging at the Keck Observatory. We report the detection of visual companions in MML 50 and MML 53 for the first time, as well as the possible detection of a third component in WSB 21. For six systems in our sample, our measurements provide a second orbital position following their initial discoveries over a decade ago. For eight systems with sufficient orbital coverage, we analyze the range of orbital solutions that fit the data. Ultimately, these observations will help provide the groundwork toward measuring precise masses for these pre-main-sequence stars and understanding the distribution of orbital parameters in young multiple systems.
- ID:
- ivo://CDS.VizieR/J/ApJ/726/18
- Title:
- Young intermediate-mass stars in W5
- Short Name:
- J/ApJ/726/18
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of a survey of young intermediate-mass stars (age<5Myr, 1.5M_{sun}_<M*<=15M_{sun}_) in the W5 massive star-forming region. We use combined optical, near-infrared, and Spitzer Space Telescope photometry and optical spectroscopy to define a sample of stars of spectral types A and B and examine their infrared excess properties. We find objects with infrared excesses characteristic of optically thick disks, i.e., Herbig AeBe stars. These stars are rare: <1.5% of the entire spectroscopic sample of A and B stars, and absent among stars more massive than 2.4M_{sun}_. 7.5% of the A and B stars possess infrared excesses in a variety of morphologies that suggest their disks are in some transitional phase between an initial, optically thick accretion state and later evolutionary states. We identify four morphological classes based on the wavelength dependence of the observed excess emission above theoretical photospheric levels: (1) the optically thick disks; (2) disks with an optically thin excess over the wavelength range 2-24um, similar to that shown by Classical Be stars; (3) disks that are optically thin in their inner regions based on their infrared excess at 2-8um and optically thick in their outer regions based on the magnitude of the observed excess emission at 24um; (4) disks that exhibit empty inner regions (no excess emission at {lambda}<8um) and some measurable excess emission at 24um. A sub-class of disks exhibit no significant excess emission at {lambda}<=5.8um, have excess emission only in the Spitzer 8um band and no detection at 24um. We discuss these spectral energy distribution types, and suggest physical models for disks exhibiting these emission patterns and additional observations to test these theories.
- ID:
- ivo://CDS.VizieR/J/ApJ/699/649
- Title:
- Young M dwarfs within 25pc. I.
- Short Name:
- J/ApJ/699/649
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have completed a high-resolution (R~60000) optical spectroscopic survey of 185 nearby M dwarfs identified using ROSAT data to select active, young objects with fractional X-ray luminosities comparable to or greater than Pleiades members. Our targets are drawn from the "NStars" 20pc census and the "Moving-M" sample with distances determined from parallaxes or spectrophotometric relations. We limited our sample to 25pc from the Sun, prior to correcting for pre-main-sequence overluminosity or binarity. Nearly half of the resulting M dwarfs are not present in the Gliese catalog and have no previously published spectral types. We identified 30 spectroscopic binaries (SBs) from the sample, which have strong X-ray emission due to tidal spin-up rather than youth.
- ID:
- ivo://CDS.VizieR/J/AJ/130/188
- Title:
- Young stars in Trumpler 37 and NGC 7160
- Short Name:
- J/AJ/130/188
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the results of MMT observations of young stars for our study of protoplanetary disks at ages 1-10Myr in two young clusters located in the Cepheus OB2 region: Trumpler 37 (embedded in the HII region IC 1396) and NGC 7160. Using low-resolution optical spectra from the Hectospec multifiber spectrograph, we have tripled the number of known low-mass cluster members, identifying 130 new members in Tr 37 and 30 in NGC 7160. We use indicators of youth (Li absorption at 6707{AA}) and accretion/chromospheric activity (H{alpha} emission) to identify and classify the low-mass cluster members. We derive spectral types for all the low-mass candidates and calculate the individual extinctions and the average over the clusters.
- ID:
- ivo://CDS.VizieR/J/ApJ/840/87
- Title:
- Young star systems observed with SALT
- Short Name:
- J/ApJ/840/87
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a spectroscopic and kinematic analysis of 79 nearby M dwarfs in 77 systems. All of these dwarfs are low-proper-motion southern hemisphere objects and were identified in a nearby star survey with a demonstrated sensitivity to young stars. Using low-resolution optical spectroscopy from the Red Side Spectrograph on the South African Large Telescope, we have determined radial velocities, H-alpha, lithium 6708{AA}, and potassium 7699{AA} equivalent widths linked to age and activity, and spectral types for all of our targets. Combined with astrometric information from literature sources, we identify 44 young stars. Eighteen are previously known members of moving groups within 100pc of the Sun. Twelve are new members, including one member of the TW Hydra moving group, one member of the 32 Orionis moving group, 9 members of Tucana- Horologium, one member of Argus, and two new members of AB Doradus. We also find 14 young star systems that are not members of any known groups. The remaining 33 star systems do not appear to be young. This appears to be evidence of a new population of nearby young stars not related to the known nearby young moving groups.
- ID:
- ivo://CDS.VizieR/J/ApJ/727/64
- Title:
- Young stellar groups and their most massive stars
- Short Name:
- J/ApJ/727/64
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We analyze the masses and spatial distributions of 14 young stellar groups in Taurus, Lupus3, ChaI, and IC348. These nearby groups, which typically contain 20-40 members, have membership catalogs complete to ~0.02M_{sun}_, and are sufficiently young that their locations should be similar to where they formed. These groups show five properties seen in clusters having many more stars and much greater surface density of stars: (1) a broad range of masses, (2) a concentration of the most massive star toward the center of the group, (3) an association of the most massive star with a high surface density of lower mass stars, (4) a correlation of the mass of the most massive star with the total mass of the group, and (5) the distribution of a large fraction of the mass in a small fraction of the stars.
- ID:
- ivo://CDS.VizieR/J/AJ/159/200
- Title:
- Young stellar objects in Lupus star-forming region
- Short Name:
- J/AJ/159/200
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The identification and characterization of stellar members within a star-forming region are critical to many aspects of star formation, including formalization of the initial mass function, circumstellar disk evolution, and star formation history. Previous surveys of the Lupus star-forming region have identified members through infrared excess and accretion signatures. We use machine learning to identify new candidate members of Lupus based on surveys from two space-based observatories: ESA's Gaia and NASA's Spitzer. Astrometric measurements from Gaia's Data Release 2 and astrometric and photometric data from the Infrared Array Camera on the Spitzer Space Telescope, as well as from other surveys, are compiled into a catalog for the random forest (RF) classifier. The RF classifiers are tested to find the best features, membership list, non-membership identification scheme, imputation method, training set class weighting, and method of dealing with class imbalance within the data. We list 27 candidate members of the Lupus star-forming region for spectroscopic follow-up. Most of the candidates lie in Clouds V and VI, where only one confirmed member of Lupus was previously known. These clouds likely represent a slightly older population of star formation.
- ID:
- ivo://CDS.VizieR/J/ApJ/878/7
- Title:
- 2yr obs. of JHK variability of stars in Tr37
- Short Name:
- J/ApJ/878/7
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have monitored nearly a square degree in IC 1396A/Tr 37 over 21 epochs extending over 2014-2016 for sources variable in the JHK bands. In our data, 65%+/-8% of previously identified cluster members show variations, compared with <=0.3% of field stars. We identify 119 members of Tr 37 on the basis of variability, forming an unbiased sample down to the brown dwarf regime. The K-band luminosity function in Tr 37 is similar to that of IC 348 but shifted to somewhat brighter values, implying that the K- and M-type members of Tr 37 are younger than those in IC 348. We introduce methods to classify the causes of variability, based on behavior in the color-color and color-magnitude diagrams. Accretion hot spots cause larger variations at J than at K with substantial scatter in the diagrams; there are at least a dozen, with the most active resembling EXors. Eleven sources are probably dominated by intervention of dust clumps in their circumstellar disks, with color behavior indicating the presence of grains larger than for interstellar dust, presumably due to grain growth in their disks. Thirteen sources have larger variations at K than at J or H. For 11 of them, the temperature fitted to the variable component is very close to 2000K, suggesting that the changes in output are caused by turbulence at the inner rim of the circumstellar disk exposing previously protected populations of grains.
