- ID:
- ivo://CDS.VizieR/J/ApJ/713/871
- Title:
- Sources in star-forming region Cyg-OB2 from X-ray
- Short Name:
- J/ApJ/713/871
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Cygnus OB2 is the nearest example of a massive star-forming region (SFR), containing over 50 O-type stars and hundreds of B-type stars. We have analyzed the properties of young stars in two fields in Cyg OB2 using the recently published deep catalog of Chandra X-ray point sources with complementary optical and near-IR photometry. Our sample is complete to ~1M_{sun}_ (excluding A- and B-type stars that do not emit X-rays), making this the deepest study of the stellar properties and star formation history in Cyg OB2 to date. Combining our stellar mass function (MF) and an estimate of the radial density profile of the association suggests a total mass of Cyg OB2 of ~3x10^4^M_{sun}_, similar to that of many of our Galaxy's most massive SFRs.
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- ID:
- ivo://CDS.VizieR/J/ApJ/645/676
- Title:
- Spatial distribution of brown dwarfs in Taurus
- Short Name:
- J/ApJ/645/676
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- By combining photometry from the 2MASS Point Source Catalog and the USNO-B1.0 catalog with optical and infrared spectroscopy, I have performed a search for young brown dwarfs in an area of 225deg^2^ encompassing all of the Taurus star-forming region (tau~1Myr). From this work, I have discovered 22 new members of Taurus, 5 of which were independently found by Guieu and coworkers (2006, Cat. <J/A+A/446/485>). Sixteen of these new members have spectral types later than M6 and thus are likely to be brown dwarfs according to the theoretical evolutionary models of Chabrier and Baraffe (2000ApJ...542..464C). After adding these new members to the previously known members of Taurus, I have compared the spatial distributions of stars and brown dwarfs across the entire region. I find no statistically significant difference between these two distributions. Taurus does not contain the large, extended population of brown dwarfs that has been predicted by some embryo ejection models for the formation of brown dwarfs. However, these results are consistent with other ejection models, as well as models in which stars and brown dwarfs share a common formation mechanism.
- ID:
- ivo://CDS.VizieR/J/AJ/146/85
- Title:
- Spectral types of 1576 optical stars in ONC
- Short Name:
- J/AJ/146/85
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We report new spectral types or spectral classification constraints for over 600 stars in the Orion Nebula Cluster (ONC) based on medium resolution (R{approx}1500-2000) red optical spectra acquired using the Palomar 200" and Kitt Peak 3.5m telescopes. Spectral types were initially estimated for F, G, and early K stars from atomic line indices while for late K and M stars, which constitute the majority of our sample, indices involving TiO and VO bands were used. To ensure proper classification, particularly for reddened, veiled, or nebula-contaminated stars, all spectra were then visually examined for type verification or refinement. We provide an updated spectral type table that supersedes previous work, increasing the percentage of optically visible ONC stars with spectral type information from 68% to 90%. However, for many objects, repeated observations have failed to yield spectral types primarily due to the challenges of adequate sky subtraction against a bright and spatially variable nebular background. The scatter between our new and our previously determined spectral types is approximately two spectral sub-classes. We also compare our grating spectroscopy results with classification based on narrow-band TiO filter photometry, finding similar scatter. While the challenges of working in the ONC may explain much of the spread, we highlight several stars showing significant and unexplained bona fide spectral variations in observations taken several years apart; these and similar cases could be due to a combination of accretion and extinction changes. Finally, nearly 20% of ONC stars exhibit obvious CaII triplet emission indicative of strong accretion.
- ID:
- ivo://CDS.VizieR/J/A+A/650/A43
- Title:
- Spectra of NGC 1333 Class I-Class II stars
- Short Name:
- J/A+A/650/A43
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The mass accretion rate (dM/dt_acc_) is the fundamental parameter to understand the process of mass assembly that results in the formation of a low-mass star. This parameter has been largely studied in Classical T Tauri stars in star-forming regions with ages of ~1-10Myr. However, little is known about the accretion properties of young stellar objects (YSOs) in younger regions and early stages of star formation, such as in the Class 0/I phases. We present new near-infrared spectra of 17 Class I/Flat and 35 Class II sources located in the young (<1Myr) NGC 1333 cluster, acquired with the KMOS instrument at the Very Large Telescope. Our goal is to study whether the mass accretion rate evolves with age, as suggested by the widely adopted viscous evolution model, by comparing the properties of the NGC 1333 members with samples of older regions. For the Class II sources in our sample, we measured the stellar parameters (SpT, AV, and L*) through a comparison of the IR spectra with a grid of non-accreting Class III stellar templates. We then computed the accretion luminosity by using the known correlation between Lacc and the luminosity of HI lines (Pa{beta} and Br{gamma}). For the Class I sample, where the presence of a large IR excess makes it impossible to use the same spectral typing method, we applied a procedure that allowed us to measure the stellar and accretion luminosity in a self-consistent way. Mass accretion rates dM/dt_acc_ were then measured once masses and radii were estimated adopting suitable evolutionary tracks. The NGC 1333 Class II sources of our sample have L_acc_~10^-4^-1L_{sun}_ and dM/dt_acc_~10^-11^-10^-7^M_{sun}_/yr. We find a correlation between accretion and stellar luminosity in the form of log L_acc_=(1.5+/-0.2)logL*+(-1.0+/-0.1), and a correlation between the mass accretion rate and stellar mass in the form of log(dM/dt_acc_)=(2.6+/-0.9)logM*+(-7.3+/-0.7). Both correlations are compatible within the errors with the older Lupus star-forming region, while only the latter is consistent with results from Chamaeleon I. The Class I sample shows larger accretion luminosities (~10^-2^-10^2^L_{sun}_) and mass accretion rates (~10^-9^-10^-6^M_{sun}_/yr) with respect to the Class II stars of the same cloud. However, the derived mass accretion rates are not sufficiently high to build up the inferred stellar masses, assuming steady accretion during the Class I lifetime. This suggests that the sources are not in their main accretion phase and that most of their mass has already been accumulated during a previous stage and/or that the accretion is an episodic phenomenon. We show that some of the targets originally classified as Class I through Spitzer photometry are in fact evolved or low accreting objects. This evidence can have implications for the estimated protostellar phase lifetimes. The accretion rates of our sample are larger in more embedded and early stage YSOs. Further observations of larger samples in young star-forming regions are needed to determine if this is a general result. In addition, we highlight the importance of spectroscopic surveys of YSOs to confirm their classification and perform a more correct estimate of their lifetime.
- ID:
- ivo://CDS.VizieR/J/AJ/133/2524
- Title:
- Spectra of young nearby stars
- Short Name:
- J/AJ/133/2524
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present high-dispersion (R~16000) optical (3900-8700{AA}) spectra of 390 stars obtained with the Palomar 60inch telescope. The majority of stars observed are part of the Spitzer Legacy Science Program "The Formation and Evolution of Planetary Systems." Through detailed analysis we determine stellar properties for this sample, including radial and rotational velocities, LiI{lambda}6708 and H{alpha} equivalent widths, the chromospheric activity index R'_HK_, and temperature- and gravity-sensitive line ratios. Several spectroscopic binaries are also identified. From our tabulations, we illustrate basic age- and rotation-related correlations among measured indices. One novel result is that CaII chromospheric emission appears to saturate at vsini values above ~30km/s, similar to the well-established saturation of X-rays that originate in the spatially separate coronal region.
- ID:
- ivo://CDS.VizieR/J/A+A/442/211
- Title:
- Spectroscopic distances of 322 NLTT stars
- Short Name:
- J/A+A/442/211
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Distance estimates based on low-resolution spectroscopy and Two Micron All Sky Survey (2MASS, Cat. <II/246>) J magnitudes are presented for 322 nearby candidates from Luyten's NLTT catalogue (<I/98>). Mainly relatively bright (typically 7<Ks<11) and red high proper motion stars have been selected according to their 2MASS magnitudes and optical-to-infrared colours (+1<R-Ks<+7). Some LHS stars previously lacking spectroscopy have also been included. We have classified the majority of the objects as early-M dwarfs (M2-M5). More than 70% of our targets turned out to lie within the 25pc horizon of the catalogue of nearby stars, with 50 objects placed within 15pc and 8 objects being closer than 10pc. Three objects in the 10pc sample have no previously published spectral type: LP 876-10 (M4), LP 870-65 (M4.5), and LP 869-26 (M5). A large fraction of the objects in our sample (57%) ave independent distance estimates, mainly by the recent efforts of Reid and collaborators. Our distance determinations are generally in good agreement with theirs. 11 rather distant (d>100pc) objects have also been identified, including a probable halo, but relatively hot (Teff=13000K) white dwarf (LHS 1200) and 10 red dwarfs with extremely large tangential velocities (250<v_t_<1150km/s). Altogether, there are 11 red dwarfs (including one within 70pc) with tangential velocities larger than about 250km/s. All these objects are suspected to be in fact subdwarfs, if so, their distances would be only about half of our original estimates. The three most extreme objects in that respect are the K and early M dwarfs LP 323-168, LHS 5343 and LP 552-21 with corrected distances between 180pc and 400pc and resulting tangential velocities still larger than about 400km/s.
- ID:
- ivo://CDS.VizieR/J/ApJ/653/657
- Title:
- Spectroscopic observations of Herbig Ae/Be stars
- Short Name:
- J/ApJ/653/657
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present optical spectra of 45 intermediate-mass Herbig Ae/Be stars. Together with the multiepoch spectroscopic and photometric data compiled for a large sample of these stars and ages estimated for individual stars by using pre-main-sequence evolutionary tracks, we have studied the evolution of emission-line activity in them. We find that, on average, the H{alpha} emission line strength decreases with increasing stellar age in Herbig Ae/Be stars, indicating that the accretion activity gradually declines during the pre-main-sequence phase.
- ID:
- ivo://CDS.VizieR/J/A+A/504/829
- Title:
- Spectroscopic survey of youngest field stars
- Short Name:
- J/A+A/504/829
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the first results of an ambitious ground-based observation programme conducted on 1-4m class telescopes. Our sample consists of 1097 active and presumably young stars, all of them being optical (Tycho Catalogue) counterparts of ROSAT All- Sky Survey X-ray sources in the northern hemisphere. In this paper, we concentrate on the optically brightest (VT<=9.5m) candidates (704 objects).We acquired high-resolution optical spectroscopy in the H{alpha} and/or lithium spectral regions for 426 of such stars without relevant data in the literature. We describe the star sample and the observations and we start to discuss the physical properties of the investigated stars.
- ID:
- ivo://CDS.VizieR/J/A+A/612/A96
- Title:
- Spectroscopic survey of youngest field stars II.
- Short Name:
- J/A+A/612/A96
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Star formation in the solar neighborhood is mainly traced by young stars in open clusters, associations, and in the field, which can be identified, for example, by their X-ray emission. The determination of stellar parameters for the optical counterparts of X-ray sources is crucial for a full characterization of these stars. This work extends the spectroscopic study of the RasTyc sample, obtained by the cross-correlation of the TYCHO and ROSAT All-Sky Survey catalogs, to stars fainter than V=9.5mag and aims to identify sparse populations of young stars in the solar neighborhood. We acquired 625 high-resolution spectra for 443 presumably young stars with four different instruments in the northern hemisphere. The radial and rotational velocity (vsini) of our targets were measured by means of the cross-correlation technique, which is also helpful to discover single-lined (SB1), double-lined spectroscopic binaries (SB2), and multiple systems. We used the code ROTFIT to perform an MK spectral classification and to determine the atmospheric parameters (Teff, logg, [Fe/H]) and and vsini of the single stars and SB1 systems. For these objects, we used the spectral subtraction of slowly rotating templates to measure the equivalent widths of the H{alpha} and LiI-6708A lines, which enabled us to derive their chromospheric activity level and lithium abundance. We made use of Gaia DR1 parallaxes and proper motions to locate the targets in the HR diagram and to compute the space velocity components of the youngest objects. We find a remarkable percentage (at least 35%) of binaries and multiple systems. On the basis of the lithium abundance, the sample of single stars and SB1 systems appears to be mostly (about 60%) composed of stars younger than the members of the UMa cluster. The remaining sources are in the age range between the UMa and Hyades clusters (about 20%) or older (20%). In total, we identify 42 very young (PMS-like) stars, which lie above or very close to the Pleiades upper envelope of the lithium abundance. A significant percentage (about 12%) of evolved stars (giants and subgiants) is also present in our sample. Some of these stars are also lithium rich (A(Li)>1.4).
- ID:
- ivo://CDS.VizieR/J/MNRAS/385/2210
- Title:
- Spectroscopy around lam Ori and sig Ori
- Short Name:
- J/MNRAS/385/2210
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have obtained multi-epoch, high-resolution spectroscopy of 218 candidate low-mass stars and brown dwarfs (BDs) in the young clusters around {lambda} Ori and {sigma} Ori. We find that 196 targets are cluster members based on their radial velocity, the equivalent width of their NaI 8200 lines and the spectral type from their TiO band strength. We have identified 11 new binary stars among the cluster members based on their variable radial velocity and an additional binary from the variation in its linewidth and shape. Of these, six are double-lined spectroscopic binaries (SB2) where the components of the binary are of comparable brightness. The others are single-lined binaries (SB1) in which the companion is faint or the spectra of the stars are blended. There are three narrow-lined SB1 binaries in our sample for which the companion is more than 2.5mag fainter than the primary.
