- ID:
- ivo://CDS.VizieR/J/AJ/154/31
- Title:
- {lambda} Bootis stars: the southern survey I.
- Short Name:
- J/AJ/154/31
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The {lambda} Boo stars are a class of chemically peculiar Population I A-type stars characterized by under-abundances of the refractory elements, but near-solar abundances of carbon, nitrogen, oxygen, and sulfur. There is some evidence that {lambda} Boo stars have higher frequencies of "bright" debris disks than normal A-type stars. The discovery of four exoplanets orbiting HR8799, a {lambda} Boo star with a resolved debris disk, suggests that the {lambda} Boo phenomenon may be related to the presence of a dynamic debris disk, perhaps perturbed by migrating planets. However, only 64 {lambda} Boo stars are known, and those stars were discovered with different techniques, making it problematic to use that sample for statistical purposes, including determining the frequency of debris disks. The purpose of this paper is to derive a new sample of {lambda} Boo stars using a technique that does not lead to biases with respect to the presence of infrared excesses. Through spectroscopic observations in the southern hemisphere, we have discovered 33 {lambda} Boo stars and have confirmed 12 others. As a step toward determining the proportion of {lambda} Boo stars with infrared excesses, we have used WISE data to examine the infrared properties of this sample out to 22{mu}m. On this basis, we cannot conclude that {lambda} Boo stars have a greater tendency than normal A-type stars to show infrared excesses. However, observing this sample at longer wavelengths may change that conclusion, as many {lambda} Boo debris disks are cool and do not radiate strongly at 22{mu}m.
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- ID:
- ivo://CDS.VizieR/J/AJ/151/13
- Title:
- LAMOST-Kepler MKCLASS spectral classification
- Short Name:
- J/AJ/151/13
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The LAMOST-Kepler project was designed to obtain high-quality, low-resolution spectra of many of the stars in the Kepler field with the Large Sky Area Multi Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic telescope. To date 101086 spectra of 80447 objects over the entire Kepler field have been acquired. Physical parameters, radial velocities, and rotational velocities of these stars will be reported in other papers. In this paper we present MK spectral classifications for these spectra determined with the automatic classification code MKCLASS. We discuss the quality and reliability of the spectral types and present histograms showing the frequency of the spectral types in the main table organized according to luminosity class. Finally, as examples of the use of this spectral database, we compute the proportion of A-type stars that are Am stars, and identify 32 new barium dwarf candidates.
- ID:
- ivo://CDS.VizieR/J/ApJ/788/40
- Title:
- Late-type targets in Taurus, Cha I, and Upper Sco
- Short Name:
- J/ApJ/788/40
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have used WFPC2 on board the Hubble Space Telescope to obtain images of 47 members of the Taurus and Chamaeleon I star-forming regions that have spectral types of M6-L0 (M~0.01-0.1 M_{sun}_). An additional late-type member of Taurus, FU Tau (M7.25+M9.25), was also observed with adaptive optics at Keck Observatory. In these images, we have identified promising candidate companions to 2MASS J04414489+2301513 ({rho}=0.105"/15 AU), 2MASS J04221332+1934392 ({rho}=0.05"/7 AU), and ISO 217 ({rho}=0.03"/5 AU). We reported the first candidate in a previous study, showing that it has a similar proper motion as the primary in images from WFPC2 and Gemini adaptive optics. We have collected an additional epoch of data with Gemini that further supports that result. By combining our survey with previous high-resolution imaging in Taurus, Chamaeleon I, and Upper Sco ({tau}~10 Myr), we measure binary fractions of 14/93=0.15_-0.03_^+0.05^ for M4-M6 (M~0.1-0.3 M_{sun}_) and 4/108=0.04_-0.01_^+0.03^ for >M6 (M<~0.1 M_{sun}_) at separations of >10 AU. Given the youth and low density of these regions, the lower binary fraction at later types is probably primordial rather than due to dynamical interactions among association members. The widest low-mass binaries (>100 AU) also appear to be more common in Taurus and Chamaeleon I than in the field, which suggests that the widest low-mass binaries are disrupted by dynamical interactions at >10 Myr, or that field brown dwarfs have been born predominantly in denser clusters where wide systems are disrupted or inhibited from forming.
- ID:
- ivo://CDS.VizieR/J/MNRAS/451/4368
- Title:
- Lithium abundance of solar-like stars
- Short Name:
- J/MNRAS/451/4368
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report on the determination of the lithium abundance [A(Li)] of 52 solar-like stars. For 41 objects the A(Li) here presented corresponds to the first measurement. We have measured the equivalent widths of the 6708 {AA} lithium feature in high-resolution spectroscopic images (R~80000), obtained at the Observatorio Astrofisico Guillermo Haro (Sonora, Mexico), as part of the first scientific observations of the revitalized Lunar and Planetary Laboratory (LPL) Echelle Spectrograph, now known as the Cananea High-resolution Spectrograph (CanHiS). Lithium abundances were derived with the Fortran code MOOG, using as fundamental input a set of atmospheric parameters recently obtained by our group. With the help of an additional small sample with previous A(Li) determinations, we demonstrate that our lithium abundances are in agreement, to within uncertainties, with other works. Two target objects stand out from the rest of the sample. The star BD+47 3218 (T_eff_=6050+/-52 K, A(Li)=1.86+/-0.07 dex) lies inside the so-called lithium desert in the A(Li)-T_eff_ plane. The other object, BD+28 4515, has an A(Li)=3.05+/-0.07 dex, which is the highest of our sample and compatible with the expected abundances of relatively young stars.
- ID:
- ivo://CDS.VizieR/J/ApJ/689/1295
- Title:
- Lithium test implications for brown dwarfs
- Short Name:
- J/ApJ/689/1295
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using a large sample of optical spectra of late-type dwarfs, we identify a subset of late-M through L field dwarfs that, because of the presence of low-gravity features in their spectra, are believed to be unusually young. From a combined sample of 303 field L dwarfs, we find observationally that 7.6+/-1.6% are younger than 100Myr. This percentage is in agreement with theoretical predictions once observing biases are taken into account. We find that these young L dwarfs tend to fall in the southern hemisphere (DEC<0{deg}) and may be previously unrecognized, low-mass members of nearby, young associations like Tucana-Horologium, TW Hydrae, {beta} Pictoris, and AB Doradus. We use a homogeneously observed sample of ~150 optical spectra to examine lithium strength as a function of L/T spectral type and further corroborate the trends noted by Kirkpatrick and coworkers. We use our low-gravity spectra to investigate lithium strength as a function of age. The data weakly suggest that for early- to mid-L dwarfs the line strength reaches a maximum for a fewx100Myr, whereas for much older (few Gyr) and much younger (<100Myr) L dwarfs the line is weaker or undetectable.
- ID:
- ivo://CDS.VizieR/J/AJ/154/102
- Title:
- LMC blue supergiants spectroscopic observations
- Short Name:
- J/AJ/154/102
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- High-quality spectra of 90 blue supergiant stars in the Large Magellanic Cloud are analyzed with respect to effective temperature, gravity, metallicity, reddening, extinction, and extinction law. An average metallicity, based on Fe and Mg abundances, relative to the Sun of [Z]=-0.35+/-0.09 dex is obtained. The reddening distribution peaks at E(B-V)=0.08 mag, but significantly larger values are also encountered. A wide distribution of the ratio of extinction to reddening is found ranging from R_V_=2 to 6. The results are used to investigate the blue supergiant relationship between flux-weighted gravity, g_F_=g/T_eff_^4^, and absolute bolometric magnitude M_bol_. The existence of a tight relationship, the Flux-weighted Gravity-Luminosity Relationship (FGLR), is confirmed. However, in contrast to previous work, the observations reveal that the FGLR is divided into two parts with a different slope. For flux-weighted gravities larger than 1.30 dex, the slope is similar to that found in previous work, but the relationship becomes significantly steeper for smaller values of the flux-weighted gravity. A new calibration of the FGLR for extragalactic distance determinations is provided.
- ID:
- ivo://CDS.VizieR/J/MNRAS/447/1267
- Title:
- Low-mass members of the Octans association
- Short Name:
- J/MNRAS/447/1267
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Octans association is one of several young stellar moving groups recently discovered in the Solar neighbourhood and hence a valuable laboratory for studies of stellar, circumstellar disc and planetary evolution. However, a lack of low-mass members or any members with trigonometric parallaxes means the age, distance and space motion of the group are poorly constrained. To better determine its membership and age, we present the first spectroscopic survey for new K- and M-type Octans members, resulting in the discovery of 29 UV-bright K5-M4 stars with kinematics, photometry and distances consistent with existing members. Nine new members possess strong LiI 6708 absorption, which allow us to estimate a lithium age of 30-40Myr, similar to that of the Tucana-Horologium association and bracketed by the firm lithium depletion boundary ages of the beta Pictoris (20Myr) and Argus/IC 2391 (50Myr) associations. Several stars also show hints in our medium-resolution spectra of fast rotation or spectroscopic binarity. More so than other nearby associations, Octans is much larger than its age and internal velocity dispersion imply. It may be the dispersing remnant of a sparse, extended structure which includes some younger members of the foreground Octans-Near association recently proposed by Zuckerman and collaborators.
- ID:
- ivo://CDS.VizieR/J/ApJ/752/59
- Title:
- Low-mass population in L1641
- Short Name:
- J/ApJ/752/59
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present results from an optical photometric and spectroscopic survey of the young stellar population in L1641, the low-density star-forming region of the Orion A cloud south of the Orion Nebula Cluster (ONC). Our goal is to determine whether L1641 has a large enough low-mass population to make the known lack of high-mass stars a statistically significant demonstration of environmental dependence of the upper mass stellar initial mass function (IMF). Our spectroscopic sample consists of IR-excess objects selected from the Spitzer/IRAC survey and non-excess objects selected from optical photometry. We have spectral confirmation of 864 members, with another 98 probable members; of the confirmed members, 406 have infrared excesses and 458 do not. Assuming the same ratio of stars with and without IR excesses in the highly extincted regions, L1641 may contain as many as ~1600 stars down to ~0.1M_{sun}_, comparable within a factor of two to the ONC. Compared to the standard models of the IMF, L1641 is deficient in O and early B stars to a 3{sigma}-4{sigma} significance level, assuming that we know of all the massive stars in L1641.
- ID:
- ivo://CDS.VizieR/J/AJ/154/14
- Title:
- Low-mass stars in 25 Ori group and Orion OB1a
- Short Name:
- J/AJ/154/14
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Orion OB1a sub-association is a rich low-mass star (LMS) region. Previous spectroscopic studies have confirmed 160 LMSs in the 25 Orionis stellar group (25 Ori), which is the most prominent overdensity of Orion OB1a. Nonetheless, the current census of the 25 Ori members is estimated to be lower than 50% complete, leaving a large number of members to be still confirmed. We retrieved 172 low-resolution stellar spectra in Orion OB1a observed as ancillary science in the SDSS-III/BOSS survey, for which we classified their spectral types and determined physical parameters. To determine memberships, we analyzed the H{alpha} emission, LiI{lambda}6708 absorption, and NaI{lambda}{lambda}8183,8195 absorption as youth indicators in stars classified as M type. We report 50 new LMSs spread across the 25 Orionis, ASCC18, and ASCC20 stellar groups with spectral types from M0 to M6, corresponding to a mass range of 0.10{<=}m/M_{Sun}_{<=}0.58. This represents an increase of 50% in the number of known LMSs in the area and a net increase of 20% in the number of 25 Ori members in this mass range. Using parallax values from the Gaia DR1 catalog, we estimated the distances to these three stellar groups and found that they are all co-distant, at 338+/-66pc. We analyzed the spectral energy distributions of these LMSs and classified their disks into evolutionary classes. Using H-R diagrams, we found a suggestion that 25 Ori could be slightly older than the other two observed groups in Orion OB1a.
- ID:
- ivo://CDS.VizieR/J/AJ/128/805
- Title:
- Low-mass stars in the Cepheus OB2 region
- Short Name:
- J/AJ/128/805
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the first identification of low-mass (spectral types K-M) stars in the young clusters Tr37 and NGC7160, members of the CepOB2 association. This is part of a program to follow the evolution of protoplanetary accretion disks through the ages thought to be crucial to understanding disk dissipation and planet formation (~3-10Myr). Combining optical photometry and optical spectroscopy, we have identified ~40 members in Tr37 and ~15 in NGC7160, using several independent tests for determining the membership (optical colors, optical variability, H{alpha} emission, and Li{lambda}6707 absorption).
