- ID:
- ivo://CDS.VizieR/J/ApJ/848/11
- Title:
- Spectroscopic and photometric analysis of WDs
- Short Name:
- J/ApJ/848/11
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a detailed spectroscopic and photometric analysis of 219 DA and DB white dwarfs for which trigonometric parallax measurements are available. Our aim is to compare the physical parameters derived from the spectroscopic and photometric techniques, and then to test the theoretical mass-radius relation for white dwarfs using these results. The agreement between spectroscopic and photometric parameters is found to be excellent, especially for effective temperatures, showing that our model atmospheres and fitting procedures provide an accurate, internally consistent analysis. The values of surface gravity and solid angle obtained, respectively, from spectroscopy and photometry, are combined with parallax measurements in various ways to study the validity of the mass-radius relation from an empirical point of view. After a thorough examination of our results, we find that 73% and 92% of the white dwarfs are consistent within 1{sigma} and 2{sigma} confidence levels, respectively, with the predictions of the mass-radius relation, thus providing strong support to the theory of stellar degeneracy. Our analysis also allows us to identify 15 stars that are better interpreted in terms of unresolved double degenerate binaries. Atmospheric parameters for both components in these binary systems are obtained using a novel approach. We further identify a few white dwarfs that are possibly composed of an iron core rather than a carbon/oxygen core, since they are consistent with Fe-core evolutionary models.
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- 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/AJ/126/3007
- Title:
- Spectroscopy of faint red NLTT dwarfs
- Short Name:
- J/AJ/126/3007
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present low-resolution optical spectroscopy and BVRI photometry of 453 candidate nearby stars drawn from the NLTT (<I/98>) proper-motion catalog. The stars were selected based on optical/near-infrared colors, derived by combining the NLTT photographic data with photometry from the 2MASS Second Incremental Data Release (<II/241>). Based on the derived photometric and spectroscopic parallaxes, we identify 111 stars as lying within 20pc of the Sun, including nine stars with formal distance estimates of less than 10pc. A further 53 stars have distance estimates within 1{sigma} of our 20pc limit. Almost all of those stars are additions to the nearby-star census. In total, our NLTT-based survey has so far identified 496 stars likely to be within 20pc, of which 195 are additions to nearby-star catalogs. Most of the newly identified nearby stars have spectral types between M4 and M8.
- ID:
- ivo://CDS.VizieR/J/ApJ/735/L46
- Title:
- Spectroscopy of 64 K red giants
- Short Name:
- J/ApJ/735/L46
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Based on high-resolution spectra obtained with the MIKE spectrograph on the Magellan telescopes, we present detailed elemental abundances for 20 red giant stars in the outer Galactic disk, located at Galactocentric distances between 9 and 13kpc. The outer disk sample is complemented with samples of red giants from the inner Galactic disk and the solar neighborhood, analyzed using identical methods. For Galactocentric distances beyond 10kpc, we only find chemical patterns associated with the local thin disk, even for stars far above the Galactic plane. Our results show that the relative densities of the thick and thin disks are dramatically different from the solar neighborhood, and we therefore suggest that the radial scale length of the thick disk is much shorter than that of the thin disk. We make a first estimate of the thick disk scale length of L_thick_=2.0kpc, assuming L_thin_=3.8kpc for the thin disk. We suggest that radial migration may explain the lack of radial age, metallicity, and abundance gradients in the thick disk, possibly also explaining the link between the thick disk and the metal-poor bulge.
- ID:
- ivo://CDS.VizieR/J/AJ/123/2828
- Title:
- Spectroscopy of northern NLTT stars
- Short Name:
- J/AJ/123/2828
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present initial results of an all-sky search for late-type dwarfs within 20pc of the Sun using the New Luyten Two-Tenths (NLTT) catalog cross-referenced with the Two Micron All Sky Survey (2MASS) database. The results were obtained with low-resolution optical spectroscopic follow-up of candidate nearby stars as a preliminary test of our methodology. M_J_, derived using spectral indices, and 2MASS J are used to estimate distances. Out of the 70 objects observed, 28 are identified as previously unrecognized objects within 25pc of the Sun, and up to 19 of these are within 20pc. One, LP 647-13, is an M9-type dwarf at 10.5pc, making it one of the four closest M9 dwarfs currently known. We also discuss the chromospheric activity of the observed dwarfs.
- ID:
- ivo://CDS.VizieR/J/MNRAS/438/426
- Title:
- Spitzer interstellar bubbles
- Short Name:
- J/MNRAS/438/426
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The expansion of interstellar bubbles is suggested to be an important mechanism of triggering material accumulation and star formation. In this work, we investigate the gaseous environment of a large sample of interstellar bubbles identified by the Spitzer space telescope, aiming to explore the possible evidence of triggered gas accumulation and star formation in a statistical sense. By cross-matching 6124 Spitzer interstellar bubbles from the Milky Way Project (MWP) and more than 2500 Galactic HII regions collected by us, we obtain the velocity information for 818 MWP bubbles. To study the gaseous environment of the interstellar bubbles and get rid of the projection effect as much as possible, we constrain the velocity difference between the bubbles and the ^13^CO(1-0) emission extracted from the Galactic Ring Survey (GRS). Three methods: the mean azimuthally averaged radial profile of ^13^CO emission, the surface number density of molecular clumps and the angular cross-correlation function of MWP bubbles and the GRS molecular clumps are adopted. Significant over density of molecular gas is found to be close to the bubble rims. 60 percent of the studied bubbles were found to have associated molecular clumps. By comparing the clump-associated and the clump-unassociated MWP interstellar bubbles, we reveal that the bubbles in associations tend to be larger and thicker in physical sizes. From the different properties shown by the bubble-associated and bubble-unassociated clumps, we speculate that some of the bubble-associated clumps result from the expansion of bubbles. The fraction of the molecular clumps associated with the MWP bubbles is estimated to be about 20 percent after considering the projection effect. For the bubble-clump complexes, we found that the bubbles in the complexes with associated massive young stellar object(s) (MYSO(s)) have larger physical sizes, hence the complexes tend to be older. We propose that an evolutionary sequence might exist between the relatively younger MYSO-unassociated bubble-clump complexes and the MYSO-associated complexes.
- ID:
- ivo://CDS.VizieR/J/ApJ/759/146
- Title:
- Spitzer/IRAC photometry for 37 Galactic Cepheids
- Short Name:
- J/ApJ/759/146
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The Carnegie Hubble Program (CHP) is designed to calibrate the extragalactic distance scale using data from the post-cryogenic era of the Spitzer Space Telescope. The ultimate goal of the CHP is a systematic improvement in the distance scale leading to a determination of the Hubble constant to within an accuracy of 2%. This paper focuses on the measurement and calibration of the Galactic Cepheid period-luminosity (PL, Leavitt) relation using the warm Spitzer/IRAC 1 and 2 bands at 3.6 and 4.5{mu}m. We present photometric measurements covering the period range 4-70 days for 37 Galactic Cepheids. Data at 24 phase points were collected for each star. Three PL relations of the form M=a(log(P)-1)+b are derived. The method adopted here takes the slope a to be -3.31, as determined from the Spitzer Large Magellanic Cloud (LMC) data of Scowcroft et al. (Cat. J/ApJ/743/76). Using the geometric Hubble Space Telescope guide-star distances to 10 Galactic Cepheids, we find a calibrated 3.6{mu}m PL zero point of -5.80+/-0.03. Together with our value for the LMC zero point, we determine a reddening-corrected distance modulus of 18.48+/-0.04mag to the LMC. The mid-IR period-color diagram and the [3.6]-[4.5] color variation with phase are interpreted in terms of CO absorption at 4.5{mu}m. This situation compromises the use of the 4.5{mu}m data for distance determinations.
- ID:
- ivo://CDS.VizieR/J/ApJ/798/87
- Title:
- Spitzer/IRS debris disk catalog. II.
- Short Name:
- J/ApJ/798/87
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- During the Spitzer Space Telescope cryogenic mission, astronomers obtained Infrared Spectrograph (IRS) observations of hundreds of debris disk candidates that have been compiled in the Spitzer IRS Debris Disk Catalog (Chen et al. 2014, J/ApJS/211/25). We have discovered 10 and/or 20 {mu}m silicate emission features toward 120 targets in the catalog and modeled the IRS spectra of these sources, consistent with MIPS 70 {mu}m observations, assuming that the grains are composed of silicates (olivine, pyroxene, forsterite, and enstatite) and are located either in a continuous disk with power-law size and surface density distributions or thin rings that are well-characterized using two separate dust grain temperatures. For systems better fit by the continuous disk model, we find that (1) the dust size distribution power-law index is consistent with that expected from a collisional cascade, q=3.5-4.0, with a large number of values outside this range, and (2) the minimum grain size, a_min_, increases with stellar luminosity, L_*_, but the dependence of a_min_ on L_*_ is weaker than expected from radiation pressure alone. In addition, we also find that (3) the crystalline fraction of dust in debris disks evolves as a function of time with a large dispersion in crystalline fractions for stars of any particular stellar age or mass, (4) the disk inner edge is correlated with host star mass, and (5) there exists substantial variation in the properties of coeval disks in Sco-Cen, indicating that the observed variation is probably due to stochasticity and diversity in planet formation.
- ID:
- ivo://CDS.VizieR/J/ApJS/226/8
- Title:
- Spitzer/IRS survey of Class II objects in Orion A. I.
- Short Name:
- J/ApJS/226/8
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present our investigation of 319 Class II objects in Orion A observed by Spitzer/IRS. We also present the follow-up observations of 120 of these Class II objects in Orion A from the Infrared Telescope Facility/SpeX. We measure continuum spectral indices, equivalent widths, and integrated fluxes that pertain to disk structure and dust composition from IRS spectra of Class II objects in Orion A. We estimate mass accretion rates using hydrogen recombination lines in the SpeX spectra of our targets. Utilizing these properties, we compare the distributions of the disk and dust properties of Orion A disks with those of Taurus disks with respect to position within Orion A (Orion Nebular Cluster [ONC] and L1641) and with the subgroups by the inferred radial structures, such as transitional disks (TDs) versus radially continuous full disks (FDs). Our main findings are as follows. (1) Inner disks evolve faster than the outer disks. (2) The mass accretion rates of TDs and those of radially continuous FDs are statistically significantly displaced from each other. The median mass accretion rate of radially continuous disks in the ONC and L1641 is not very different from that in Taurus. (3) Less grain processing has occurred in the disks in the ONC compared to those in Taurus, based on analysis of the shape index of the 10{mu}m silicate feature (F_11.3_/F_9.8_). (4) The 20-31{mu}m continuum spectral index tracks the projected distance from the most luminous Trapezium star, {theta}^1^ Ori C. A possible explanation is UV ablation of the outer parts of disks.
- ID:
- ivo://CDS.VizieR/J/MNRAS/436/1465
- Title:
- Star clusters distances and extinctions
- Short Name:
- J/MNRAS/436/1465
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Determining star cluster distances is essential to analyse their properties and distribution in the Galaxy. In particular, it is desirable to have a reliable, purely photometric distance estimation method for large samples of newly discovered cluster candidates e.g. from the Two Micron All Sky Survey, the UK Infrared Deep Sky Survey Galactic Plane Survey and VVV. Here, we establish an automatic method to estimate distances and reddening from near-infrared photometry alone, without the use of isochrone fitting. We employ a decontamination procedure of JHK photometry to determine the density of stars foreground to clusters and a galactic model to estimate distances. We then calibrate the method using clusters with known properties. This allows us to establish distance estimates with better than 40 percent accuracy. We apply our method to determine the extinction and distance values to 378 known open clusters and 397 cluster candidates from the list of Froebrich, Scholz & Raftery (2007MNRAS.374..399F, Cat. J/MNRAS/374/399). We find that the sample is biased towards clusters of a distance of approximately 3kpc, with typical distances between 2 and 6kpc. Using the cluster distances and extinction values, we investigate how the average extinction per kiloparsec distance changes as a function of the Galactic longitude. We find a systematic dependence that can be approximated by A_H_(l)[mag/kpc]=0.10+0.001x|l-180{deg}|/{deg} for regions more than 60{deg} from the Galactic Centre.
