- ID:
- ivo://CDS.VizieR/J/AJ/162/98
- Title:
- Sample of active stars with composite light curves
- Short Name:
- J/AJ/162/98
- Date:
- 14 Mar 2022 07:02:47
- Publisher:
- CDS
- Description:
- Light echoes of flares on active stars offer the opportunity for direct detection of circumstellar dust. We revisit the problem of identifying faint echoes in postflare light curves, focusing on debris disks from ongoing planet formation. Starting with simulations, we develop an algorithm for estimating the radial extent and total mass from disk echo profiles. We apply this algorithm to light curves from over 2100 stars observed by NASA's Kepler mission, selected for multiple short-lived flares in either the long-cadence or short-cadence data sets. While flux uncertainties in light curves from individual stars preclude useful mass limits on circumstellar disks, catalog-averaged light curves yield constraints on disk mass that are comparable to estimates from known debris disks. The average mass in micron- to millimeter-sized dust around the Kepler stars cannot exceed 10% of an Earth mass in exo-Kuiper belts or 10% of a lunar mass in the terrestrial zone. We group stars according to IR excess, based on WISE W1-W3 color, as an indicator for the presence of circumstellar dust. The mass limits are greater for stars with strong IR excess, a hint that echoes are lurking not far beneath the noise in postflare light curves. With increased sensitivity, echo detection will let time-domain astronomy complement spectroscopic and direct-imaging studies in mapping how, when, and where planets form.
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112. S Dor variables
- ID:
- ivo://CDS.VizieR/J/A+A/366/508
- Title:
- S Dor variables
- Short Name:
- J/A+A/366/508
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The goal in writing this paper is five fold: (1) to summarize the scientific achievements in the 20th century on S Dor variables (or LBVs); (2) to present an inventory of these variables in the Galaxy and the Magellanic Clouds with a description of their physical state and instability properties; (3) to emphasize the photometric achievements of the various types of instabilities. Generally this seems to be a neglected item resulting in a number of misunderstandings continuously wandering through literature; (4) to investigate the structure of the S Dor-area on the HR-diagram; (5) to estimate the total numbers of S Dor variables in the three stellar systems.
- ID:
- ivo://CDS.VizieR/J/ApJ/794/60
- Title:
- SDSS RGB stars distances
- Short Name:
- J/ApJ/794/60
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present distance determinations for a large and clean sample of red giant branch stars selected from the ninth data release of the Sloan Digital Sky Survey (Adelman-McCarthy et al. 2012ApJS..203...21A, Cat. V/139). The distances are calculated based on both observational cluster fiducials and theoretical isochrones. Distributions of distances from the two methods are very similar with peaks at about 10 kpc and tails extending to more than 70 kpc. We find that distances from the two methods agree well for the majority of the sample stars; though, on average, distances based on isochrones are 10% higher than those based on fiducials. We test the accuracy of our distance determinations using 332 stars from 10 Galactic globular and open clusters. The average relative deviation from the literature cluster distances is 4% for the fiducial-based distances and 8% for the isochrone-based distances, both of which are within the uncertainties. We find that the effective temperature and surface gravity derived from low-resolution spectra are not accurate enough to essentially improve the performance of distance determinations. However, for stars with significant extinction, effective temperature may help to better constrain their distances to some extent. We make our sample stars and their distances available from an online catalog. The catalog comprises 17941 stars with reasonable distance estimations reaching to more than 70 kpc, which is suitable for the investigation of the formation and evolution of the Galaxy, especially the Galactic halo.
- ID:
- ivo://CDS.VizieR/J/ApJ/815/63
- Title:
- SDSS wide double white dwarfs spectroscopy
- Short Name:
- J/ApJ/815/63
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present observational constraints on the initial-final mass relation (IFMR) using wide double white dwarfs (DWDs). We identify 65 new candidate wide DWDs within the Sloan Digital Sky Survey, bringing the number of candidate wide DWDs to 142. We then engage in a spectroscopic follow-up campaign and collect existing spectra for these objects; using these spectra, we derive masses and cooling ages for 54 hydrogen (DA) WDs in DWDs. We also identify one new DA/DB pair, four candidate DA/DC pairs, four candidate DA/DAH pairs, and one new candidate triple degenerate system. Because wide DWDs are co-eval and evolve independently, the difference in the pre-WD lifetimes should equal the difference in the WD cooling ages. We use this to develop a Bayesian hierarchical framework and construct a likelihood function to determine the probability that any particular IFMR fits a sample of wide DWDs. We then define a parametric model for the IFMR and find the best parameters indicated by our sample of DWDs. We place robust constraints on the IFMR for initial masses of 2-4M_{sun}_. The WD masses produced by our model for stars within this mass range differ from those predicted by semi-empirical fits to open cluster WDs. Within this mass range, where there are few constraining open cluster WDs and disagreements in the cluster ages, wide DWDs may provide more reliable constraints on the IFMR. Expanding this method to the many wide DWDs expected to be discovered by Gaia may transform our understanding of the IFMR.
- ID:
- ivo://CDS.VizieR/J/AJ/158/68
- Title:
- Short-period variables in young open cluster Stock 8
- Short Name:
- J/AJ/158/68
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present time-series photometry in the field of Stock 8 and identified 130 short-period variable stars. Twenty-eight main-sequence and 23 pre-main-sequence variables are found to be part of cluster Stock 8. The main-sequence variables are classified as slow pulsators of the B-type, {beta} Cep, and {delta} Scuti stars. Fourteen main-sequence stars could be new class variables as discussed by Mowlavi et al. (2013, J/A+A/554/A108) and Lata et al. (2011MNRAS.418.1346L; 2012MNRAS.427.1449L; 2014, J/MNRAS/442/273; 2016MNRAS.456.2505L). The age and mass of pre-main-sequence variables are found to be ~<5 Myr and in the mass range of 0.5-2.8 M_{sun}_, respectively. These pre-main-sequence stars could be T-Tauri variables. We have found 5 and 2 of 23 pre-main-sequence variables as classical T-Tauri stars and Herbig Ae/Be stars, respectively, whereas 16 pre-main-sequence stars are classified as weak-line T-Tauri stars.
- ID:
- ivo://CDS.VizieR/J/AJ/147/94
- Title:
- Solar neighborhood. XXXII. L and M dwarfs
- Short Name:
- J/AJ/147/94
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We construct a Hertzsprung-Russell diagram for the stellar/substellar boundary based on a sample of 63 objects ranging in spectral type from M6V to L4. We report newly observed VRI photometry for all 63 objects and new trigonometric parallaxes for 37 objects. The remaining 26 objects have trigonometric parallaxes from the literature. We combine our optical photometry and trigonometric parallaxes with 2MASS and WISE photometry and employ a novel spectral energy distribution fitting algorithm to determine effective temperatures, bolometric luminosities, and radii. Our uncertainties range from ~20K to ~150K in temperature, ~0.01 to ~0.06 in log (L/L_{sun}_) and ~3% to ~10% in radius. We check our methodology by comparing our calculated radii to radii directly measured via long baseline optical interferometry. We find evidence for the local minimum in the radius-temperature and radius-luminosity trends that signals the end of the stellar main sequence and the start of the brown dwarf sequence at T_eff_~2075K, log(L/L_{sun}_)~-3.9, and (R/R_{sun}_)~0.086. The existence of this local minimum is predicted by evolutionary models, but at temperatures ~400K cooler. The minimum radius happens near the locus of 2MASS J0523-1403, an L2.5 dwarf with V-K=9.42. We make qualitative arguments as to why the effects of the recent revision in solar abundances accounts for the discrepancy between our findings and the evolutionary models. We also report new color-absolute magnitude relations for optical and infrared colors which are useful for estimating photometric distances. We study the optical variability of all 63 targets and find an overall variability fraction of 36_-7_^+9^% at a threshold of 15mmag in the I band, which is in agreement with previous studies.
- ID:
- ivo://CDS.VizieR/J/AJ/154/32
- Title:
- Solar neighborhood. XXXIX. Nearby white dwarfs
- Short Name:
- J/AJ/154/32
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present 114 trigonometric parallaxes for 107 nearby white dwarf (WD) systems from both the Cerro Tololo Inter-American Observatory Parallax Investigation (CTIOPI) and the U. S. Naval Observatory Flagstaff Station (NOFS) parallax programs. Of these, 76 parallaxes for 69 systems were measured by the CTIOPI program and 38 parallaxes for as many systems were measured by the NOFS program. A total of 50 systems are confirmed to be within the 25-pc horizon of interest. Coupled with a spectroscopic confirmation of a common proper-motion companion to a Hipparcos star within 25pc as well as confirmation parallax determinations for two WD systems included in the recently released Tycho Gaia Astrometric Solution catalog, we add 53 new systems to the 25-pc WD sample-a 42% increase. Our sample presented here includes four strong candidate halo systems, a new metal-rich DAZ WD, a confirmation of a recently discovered nearby short-period (P=2.85hr) double degenerate, a WD with a new astrometric perturbation (long period, unconstrained with our data), and a new triple system where the WD companion main-sequence star has an astrometric perturbation (P~1.6year).
- ID:
- ivo://CDS.VizieR/J/AJ/162/75
- Title:
- Speckle observations TESS exoplanet host stars. II.
- Short Name:
- J/AJ/162/75
- Date:
- 18 Mar 2022 09:30:45
- Publisher:
- CDS
- Description:
- We present high-angular-resolution imaging observations of 517 host stars of TESS exoplanet candidates using the 'Alopeke and Zorro speckle cameras at Gemini North and South. The sample consists mainly of bright F, G, K stars at distances of less than 500pc. Our speckle observations span angular resolutions of ~20mas out to 1.2", yielding spatial resolutions of <10-500au for most stars, and our contrast limits can detect companion stars 5-9mag fainter than the primary at optical wavelengths. We detect 102 close stellar companions and determine the separation, magnitude difference, mass ratio, and estimated orbital period for each system. Our observations of exoplanet host star binaries reveal that they have wider separations than field binaries, with a mean orbital semimajor axis near 100 au. Other imaging studies have suggested this dearth of very closely separated binaries in systems which host exoplanets, but incompleteness at small separations makes it difficult to disentangle unobserved companions from a true lack of companions. With our improved angular resolution and sensitivity, we confirm that this lack of close exoplanet host binaries is indeed real. We also search for a correlation between planetary orbital radii versus binary star separation; but, given the very short orbital periods of the TESS planets, we do not find any clear trend. We do note that in exoplanet systems containing binary host stars, there is an observational bias against detecting Earth-size planet transits due to transit depth dilution caused by the companion star.
- ID:
- ivo://CDS.VizieR/J/A+A/546/A9
- Title:
- Spectral classification of stars in NGC 6530
- Short Name:
- J/A+A/546/A9
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Mechanisms involved in the star formation process and in particular the duration of the different phases of the cloud contraction are not yet fully understood. Photometric data alone suggest that objects coexist in the young cluster NGC 6530 with ages from ~1Myr up to 10Myrs. We want to derive accurate stellar parameters and, in particular, stellar ages to be able to constrain a possible age spread in the star-forming region NGC6530.
- ID:
- ivo://CDS.VizieR/J/AJ/135/1239
- Title:
- Spectral distances to DA white dwarfs
- Short Name:
- J/AJ/135/1239
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Using recent photometric calibrations, we develop greatly improved distance estimates for DA white dwarfs using multi-band synthetic photometry based on spectroscopic temperatures and gravities. Very good correlations are shown to exist between our spectroscopically based photometric distance estimates and those derived from trigonometric parallaxes. We investigate the uncertainties involved in our distance estimates, as well as discuss the circumstances where such techniques are most likely to fail. We apply our techniques to the large sample of Sloan Digital Sky Survey DA white dwarfs where automated fitting of HI Balmer profiles yields spectrometric temperatures and gravities. We determine simple empirical corrections to these temperatures and gravities with respect to published slit spectroscopy. After applying these T_eff_-logg corrections as well as appropriate interstellar extinction corrections, where necessary, we derive spectroscopically based photometric distances for 7062 DA stars from this sample.