Abstract
The determination of the astrophysical properties of stars remains challenging and frequently relies on the application of stellar models. Stellar sequences in nearby open clusters provide ...some of the best means to test and calibrate stellar evolutionary models and isochrones and to use these models to assign astrophysical properties consistently to a large sample of stars. We aim at updating the single-star sequence of the members of the Hyades cluster, identifying the best-fitting isochrones, and determining the astrophysical properties of the stars. The Gaia Catalog of Nearby Stars provides a comprehensive sample of high-probability members of the Hyades cluster. We apply a multistep method to flag photometric outliers and to identify bona fide single stars and likely binary and multiple systems. The single stars define a tight sequence, which in the mass range 0.12–2.2
M
⊙
is well fitted by PARSEC isochrones for a supersolar metallicity of M/H = +0.18 ± 0.03 and an age of 775 ± 25 Myr. The isochrones enable us to assign mass, effective temperature, luminosity, and surface gravity to each of the 600 bona fide single main-sequence stars. The observed sequence validates the PARSEC isochrones. The derived stellar properties can serve as benchmarks for atmospheric and evolutionary models and for all-sky catalogs of stellar astrophysical properties. The stellar properties are also relevant for studies of exoplanet properties among Hyades exoplanet hosts.
Single stars in the Hyades open cluster Kopytova, Taisiya G; Brandner, Wolfgang; Tognelli, Emanuele ...
Astronomy and astrophysics (Berlin),
1/2016, Letnik:
585
Journal Article
Recenzirano
Odprti dostop
Age and mass determinations for isolated stellar objects remain model-dependent. While stellar interior and atmospheric theoretical models are rapidly evolving, we need a powerful tool to test them. ...Open clusters are good candidates for this role. We aim to create a fiducial sequence of stellar objects for testing stellar and atmospheric models. We complement previous studies on the Hyades multiplicity by Lucky Imaging observations with the AstraLux Norte camera. This allows us to exclude possible binary and multiple systems with companions outside a 2-7 AU separation and to create a single-star sequence for the Hyades. The sequence encompasses 250 main-sequence stars ranging from A5V to M6V. Using the Tool for Astrophysical Data Analysis (TA-DA), we create various theoretical isochrones applying different combinations of interior and atmospheric models. We compare the isochrones with the observed Hyades single-star sequence on J vs. J-Ks, J vs. J-H, and Ks vs. Within the measurement uncertainties, the current generation of models agree well with the single-star sequence.
As gas giant planets and brown dwarfs radiate away the residual heat from their formation, they cool through a spectral type transition from L to T, which encompasses the dissipation of cloud opacity ...and the appearance of strong methane absorption. While there are hundreds of known T-type brown dwarfs, the first generation of directly imaged exoplanets were all L type. Recently, Kuzuhara et al. announced the discovery of GJ 504 b, the first T dwarf exoplanet. GJ 504 b provides a unique opportunity to study the atmosphere of a new type of exoplanet with a ~500 K temperature that bridges the gap between the first directly imaged planets (~1000 K) and our own solar system's Jupiter (~130 K). Of particular interest, our model fits suggest that GJ 504 b has a superstellar metallicity. Since planet formation can create objects with nonstellar metallicities, while binary star formation cannot, this result suggests that GJ 504 b formed like a planet, not like a binary companion.
We present two epochs of MPG/ESO 2.2 m GROND simultaneous six-band (r'i'z' JHK) photometric monitoring of the closest known L/T transition brown dwarf binary WISE J104915.57-531906.1AB. We report ...here the first resolved variability monitoring of both the T0.5 and L7.5 components. We obtained 4 hr of focused observations on the night of 2013 April 22 (UT), as well as 4 hr of defocused (unresolved) observations on the night of 2013 April 16 (UT). We note a number of robust trends in our light curves. The r' and i' light curves appear to be anti-correlated with z' and H for the T0.5 component and in the unresolved light curve. In the defocused dataset, J appears correlated with z' and H and anti-correlated with r' and i', while in the focused dataset we measure no variability for J at the level of our photometric precision, likely due to evolving weather phenomena. In our focused T0.5 component light curve, the K band light curve displays a significant phase offset relative to both H and z'. We argue that the measured phase offsets are correlated with atmospheric pressure probed at each band, as estimated from one-dimensional atmospheric models. We also report low-amplitude variability in i' and z' intrinsic to the L7.5 component.
Asynchronous calibration could allow opportunistic screening based on routine CT for early osteoporosis detection. In this phantom study, a bone mineral density (BMD) calibration phantom and ...multi-energy CT (MECT) phantom were imaged on eight different CT scanners with multiple tube voltages (80-150 kV
) and image reconstruction settings (e.g. soft/hard kernel). Reference values for asynchronous BMD estimation were calculated from the BMD-phantom and validated with six calcium composite inserts of the MECT-phantom with known ground truth. Relative errors/changes in estimated BMD were calculated and investigated for influence of tube voltage, CT scanner and reconstruction setting. Reference values for 282 acquisitions were determined, resulting in an average relative error between calculated BMD and ground truth of - 9.2% ± 14.0% with a strong correlation (R
= 0.99; p < 0.0001). Tube voltage and CT scanner had a significant effect on calculated BMD (p < 0.0001), with relative differences in BMD of 3.8% ± 28.2% when adapting reference values for tube voltage, - 5.6% ± 9.2% for CT scanner and 0.2% ± 0.2% for reconstruction setting, respectively. Differences in BMD were small when using reference values from a different CT scanner of the same model (0.0% ± 1.4%). Asynchronous phantom-based calibration is feasible for opportunistic BMD assessment based on CT images with reference values adapted for tube voltage and CT scanner model.
WISE J085510.83-071442.5 (hereafter, WISE 0855-07) is the coolest Y dwarf known to date and is located at a distance of 2.31 + or - 0.08 pc, giving it the fourth largest parallax of any known star or ...brown dwarf system. We report deep z-band observations of WISE 0855-07 using FORS2 on UT1/Very Large Telescope. We do not detect any counterpart to WISE 0855-07 in our z-band images and estimate a brightness upper limit of AB mag > 24.8 (F sub(v) < 0.45 mu Jy) at 910 + or - 65 nm with 3sigma confidence. We combine our z-band upper limit with previous near-and mid-infrared photometry to place constraints on the atmospheric properties of WISE 0855-07 via comparison to models which implement water clouds in the atmospheres of T sub(eff) < 300 K substellar objects. We find that none of the available models that implement water clouds can completely reproduce the observed spectral energy distribution of WISE 0855-07. Every model significantly disagrees with the (3.6 mu m/4.5 mu m) flux ratio and at least one other bandpass. Since methane is predicted to be the dominant absorber at 3-4 mu m, these mismatches might point to an incorrect or incomplete treatment of methane in current models. We conclude that (a) WISE0855-07 has T sub(eff) ~ 200-250 K, (b) < 80% of its surface is covered by clouds, and (c) deeper observations, and improved models of substellar evolution, atmospheres, clouds, and opacities will be necessary to better characterize this object.
Abstract
Based on GAIA Early Data Release 3 (EDR3), we revisit and update our sample of bonafide single stars in the Hyades open cluster. The small observational uncertainties in parallax and ...photometry of EDR3 result in a tightly defined stellar sequence, which is ideal for the testing and calibration of theoretical stellar evolutionary tracks and isochrones. We benchmark the solar-scaled mesa evolutionary models against the single star sequence. We find that the non-rotating mesa models for Fe/H = +0.25 provide a good fit for stars with masses above 0.85, and very low mass stars below 0.25 M⊙. For stars with masses between 0.25 and 0.85 M⊙, the models systematically under predict the observed stellar luminosity. One potential limitation of the models for partially convective stars more massive than 0.35 M⊙ is the prescription of (superadiabatic) convection with the mixing-length theory parameter αML tuned to match the Solar model. Below 0.35 M⊙, the increased scatter in the stellar sequence might be a manifestation of the convective kissing instability, which is driven by variations in the 3He nuclear energy production rate due to instabilities at the convective core to envelope boundary. For a Hyades-like stellar population, the application of solar-scaled models to subsolar mass stars could result in a significant underestimate of the age, or an overestimate of the metallicity. We suggest that future grids of solar-scaled evolutionary stellar models could be complemented by Hyades-scaled models in the mass range 0.25 to 0.85 M⊙.
Context.
The
Gaia
astrophysical parameters inference system (Apsis) provides astrophysical parameter estimates for several to hundreds of millions of stars.
Aims.
We aim to benchmark
Gaia
DR3 Apsis.
...Methods.
We compiled approximately 1500 bona fide single stars in the Hyades and Pleiades open clusters for validation of PARSEC isochrones, and for comparison with Apsis estimates. PARSEC stellar isochrones in the
Gaia
photometric system enable us to assign average ages and metallicities to the clusters, and mass, effective temperature, luminosity, and surface gravity to the individual stars.
Results.
Apsis does not recover the single-age, single-metallicity characteristic of the cluster populations. Ages assigned to cluster members seemingly follow the input template for Galactic populations, with earlier-type stars being systematically assigned younger ages than later-type stars. Cluster metallicities are underestimated by 0.10–0.2 dex. Effective temperature estimates are in general reliable. Surface gravity estimates reveal strong systematic errors for specific ranges of the
Gaia
BP
−
RP
colours.
Conclusions.
We caution that
Gaia
DR3 Apsis estimates can be subject to significant systematic uncertainties. Some of the Apsis estimates, such as metallicity, might only be meaningful for statistical studies of the time-averaged Galactic stellar population, but are not recommended to be used for individual stars.
ABSTRACT
Only a small number of exoplanets have been identified in stellar cluster environments. We initiated a high angular resolution direct imaging search using the Hubble Space Telescope (HST) ...and its Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) instrument for self-luminous giant planets in orbit around seven white dwarfs in the 625 Myr old nearby (≈45 pc) Hyades cluster. The observations were obtained with Near-Infrared Camera 1 (NIC1) in the F110W and F160W filters, and encompass two HST roll angles to facilitate angular differential imaging. The difference images were searched for companion candidates, and radially averaged contrast curves were computed. Though we achieve the lowest mass detection limits yet for angular separations ≥0.5 arcsec, no planetary mass companion to any of the seven white dwarfs, whose initial main-sequence masses were >2.8 M⊙, was found. Comparison with evolutionary models yields detection limits of ≈5–7 Jupiter masses (MJup) according to one model, and between 9 and ≈12 MJup according to another model, at physical separations corresponding to initial semimajor axis of ≥5–8 au (i.e. before the mass-loss events associated with the red and asymptotic giant branch phase of the host star). The study provides further evidence that initially dense cluster environments, which included O- and B-type stars, might not be highly conducive to the formation of massive circumstellar discs, and their transformation into giant planets (with m ≥ 6 MJup and a ≥6 au). This is in agreement with radial velocity surveys for exoplanets around G- and K-type giants, which did not find any planets around stars more massive than ≈3 M⊙.
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