Gaia Data Release 2 Luri, X.; Brown, A. G. A.; Sarro, L. M. ...
Astronomy and astrophysics (Berlin),
08/2018, Letnik:
616
Journal Article
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Context.
The second
Gaia
data release (
Gaia
DR2) provides precise five-parameter astrometric data (positions, proper motions, and parallaxes) for an unprecedented number of sources (more than 1.3 ...billion, mostly stars). This new wealth of data will enable the undertaking of statistical analysis of many astrophysical problems that were previously infeasible for lack of reliable astrometry, and in particular because of the lack of parallaxes. However, the use of this wealth of astrometric data comes with a specific challenge: how can the astrophysical parameters of interest be properly inferred from these data?
Aims.
The main focus of this paper, but not the only focus, is the issue of the estimation of distances from parallaxes, possibly combined with other information. We start with a critical review of the methods traditionally used to obtain distances from parallaxes and their shortcomings. Then we provide guidelines on how to use parallaxes more efficiently to estimate distances by using Bayesian methods. In particular we also show that negative parallaxes, or parallaxes with relatively large uncertainties still contain valuable information. Finally, we provide examples that show more generally how to use astrometric data for parameter estimation, including the combination of proper motions and parallaxes and the handling of covariances in the uncertainties.
Methods.
The paper contains examples based on simulated
Gaia
data to illustrate the problems and the solutions proposed. Furthermore, the developments and methods proposed in the paper are linked to a set of tutorials included in the
Gaia
archive documentation that provide practical examples and a good starting point for the application of the recommendations to actual problems. In all cases the source code for the analysis methods is provided.
Results.
Our main recommendation is to always treat the derivation of (astro-)physical parameters from astrometric data, in particular when parallaxes are involved, as an inference problem which should preferably be handled with a full Bayesian approach.
Conclusions.
Gaia
will provide fundamental data for many fields of astronomy. Further data releases will provide more data, and more precise data. Nevertheless, to fully use the potential it will always be necessary to pay careful attention to the statistical treatment of parallaxes and proper motions. The purpose of this paper is to help astronomers find the correct approach.
Aims. We take advantage of the second data release of the Gaia space mission and the state-of-the-art astrometry delivered from very long baseline interferometry observations to revisit the structure ...and kinematics of the nearby Taurus star-forming region. Methods. We apply a hierarchical clustering algorithm for partitioning the stars in our sample into groups (i.e., clusters) that are associated with the various molecular clouds of the complex, and derive the distance and spatial velocity of individual stars and their corresponding molecular clouds. Results. We show that the molecular clouds are located at different distances and confirm the existence of important depth effects in this region reported in previous studies. For example, we find that the L 1495 molecular cloud is located at d = 129.9+0.4−0.3 d = 129 . 9 − 0.3 + 0.4 $ d=129.9^{+0.4}_{-0.3} $ pc, while the filamentary structure connected to it (in the plane of the sky) is at d = 160.0+1.2−1.2 d = 160 . 0 − 1.2 + 1.2 $ d=160.0^{+1.2}_{-1.2} $ pc. We report B 215 and L 1558 as the closest (d = 128.5+1.6−1.6 pc) d = 128 . 5 − 1.6 + 1.6 pc $ \left(d=128.5^{+1.6}_{-1.6}\,\mathrm{pc}\right) $ and most remote (d = 198.1+2.5−2.5 pc) d = 198 . 1 − 2.5 + 2.5 pc $ \left(d=198.1^{+2.5}_{-2.5}\,\mathrm{pc}\right) $ substructures of the complex, respectively. The median inter-cloud distance is 25 pc and the relative motion of the subgroups is on the order of a few km s−1. We find no clear evidence for expansion (or contraction) of the Taurus complex, but signs of the potential effects of a global rotation. Finally, we compare the radial velocity of the stars with the velocity of the underlying 13CO molecular gas and report a mean difference of 0.04 ± 0.12 km s−1 (with rms of 0.63 km s−1) confirming that the stars and the gas are tightly coupled.
We present an evaluation of the performance of an automated classification of the Hipparcos periodic variable stars into 26 types. The sub-sample with the most reliable variability types available in ...the literature is used to train supervised algorithms to characterize the type dependencies on a number of attributes. The most useful attributes evaluated with the random forest methodology include, in decreasing order of importance, the period, the amplitude, the V−I colour index, the absolute magnitude, the residual around the folded light-curve model, the magnitude distribution skewness and the amplitude of the second harmonic of the Fourier series model relative to that of the fundamental frequency. Random forests and a multi-stage scheme involving Bayesian network and Gaussian mixture methods lead to statistically equivalent results. In standard 10-fold cross-validation (CV) experiments, the rate of correct classification is between 90 and 100 per cent, depending on the variability type. The main mis-classification cases, up to a rate of about 10 per cent, arise due to confusion between SPB and ACV blue variables and between eclipsing binaries, ellipsoidal variables and other variability types. Our training set and the predicted types for the other Hipparcos periodic stars are available online.
ABSTRACT
We identify and investigate known late M, L, and T dwarfs in the Gaia second data release. This sample is being used as a training set in the Gaia data processing chain of the ultracool ...dwarfs work package. We find 695 objects in the optical spectral range M8–T6 with accurate Gaia coordinates, proper motions, and parallaxes which we combine with published spectral types and photometry from large area optical and infrared sky surveys. We find that 100 objects are in 47 multiple systems, of which 27 systems are published and 20 are new. These will be useful benchmark systems and we discuss the requirements to produce a complete catalogue of multiple systems with an ultracool dwarf component. We examine the magnitudes in the Gaia passbands and find that the GBP magnitudes are unreliable and should not be used for these objects. We examine progressively redder colour–magnitude diagrams and see a notable increase in the main-sequence scatter and a bivariate main sequence for old and young objects. We provide an absolute magnitude – spectral subtype calibration for G and GRP passbands along with linear fits over the range M8–L8 for other passbands.
Context.
The high-precision parallax data of the
Gaia
mission allows for significant improvements in the distance determination to stellar clusters and their stars. In order to obtain accurate and ...precise distance determinations, systematics such as parallax spatial correlations need to be accounted for, especially with regard to stars in small sky regions.
Aims.
Our aim is to provide the astrophysical community with a free and open code designed to simultaneously infer cluster parameters (i.e., distance and size) and distances to the cluster stars using
Gaia
parallax measurements. The code includes cluster-oriented prior families and it is specifically designed to deal with the
Gaia
parallax spatial correlations.
Methods.
A Bayesian hierarchical model is created to allow for the inference of both the cluster parameters and distances to its stars.
Results.
Using synthetic data that mimics
Gaia
parallax uncertainties and spatial correlations, we observe that our cluster-oriented prior families result in distance estimates with smaller errors than those obtained with an exponentially decreasing space density prior. In addition, the treatment of the parallax spatial correlations minimizes errors in the estimated cluster size and stellar distances, and avoids the underestimation of uncertainties. Although neglecting the parallax spatial correlations has no impact on the accuracy of cluster distance determinations, it underestimates the uncertainties and may result in measurements that are incompatible with the true value (i.e., falling beyond the 2
σ
uncertainties).
Conclusions.
The combination of prior knowledge with the treatment of
Gaia
parallax spatial correlations produces accurate (error < 10%) and trustworthy estimates (i.e., true values contained within the 2
σ
uncertainties) of cluster distances for clusters up to ∼5 kpc, along with cluster sizes for clusters up to ∼1 kpc.
Human heart tissues grown as three-dimensional spheroids and consisting of different cardiac cell types derived from pluripotent stem cells (hiPSCs) recapitulate aspects of human physiology better ...than standard two-dimensional models in vitro. They typically consist of less than 5000 cells and are used to measure contraction kinetics although not contraction force. By contrast, engineered heart tissues (EHTs) formed around two flexible pillars, can measure contraction force but conventional EHTs often require between 0.5 and 2 million cells. This makes large-scale screening of many EHTs costly. Our goals here were (i) to create a physiologically relevant model that required fewer cells than standard EHTs making them less expensive, and (ii) to ensure that this miniaturized model retained correct functionality. We demonstrated that fully functional EHTs could be generated from physiologically relevant combinations of hiPSC-derived cardiomyocytes (70%), cardiac fibroblasts (15%) and cardiac endothelial cells (15%), using as few as 1.6 × 104 cells. Our results showed that these EHTs were viable and functional up to 14 days after formation. The EHTs could be electrically paced in the frequency range between 0.6 and 3 Hz, with the optimum between 0.6 and 2 Hz. This was consistent across three downscaled EHT sizes tested. These findings suggest that miniaturized EHTs could represent a cost-effective microphysiological system for disease modelling and examining drug responses particularly in secondary screens for drug discovery.
•Co-culture based on three cardiac cell types derived from hiPSCs form functional Engineered Heart Tissues (EHTs).•Miniaturized EHTs with only 16,000 cells maintain functionality.•Three-fold miniaturized EHTs maintain the same contraction kinetics as their larger counterparts.•Downscaling of EHTs does not decrease force delivered per cardiomyocyte.
Gaia Data Release 2 Clementini, G.; Ripepi, V.; Molinaro, R. ...
Astronomy and astrophysics (Berlin),
02/2019, Letnik:
622
Journal Article
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Context.
The
Gaia
second Data Release (DR2) presents a first mapping of full-sky RR Lyrae stars and Cepheids observed by the spacecraft during the initial 22 months of science operations.
Aims.
The ...Specific Objects Study (SOS) pipeline, developed to validate and fully characterise Cepheids and RR Lyrae stars (SOS Cep&RRL) observed by
Gaia
, has been presented in the documentation and papers accompanying the
Gaia
first Data Release. Here we describe how the SOS pipeline was modified to allow for processing the
Gaia
multi-band (
G
,
G
BP
, and
G
RP
) time-series photometry of all-sky candidate variables and produce specific results for confirmed RR Lyrae stars and Cepheids that are published in the DR2 catalogue.
Methods.
The SOS Cep&RRL processing uses tools such as the period–amplitude and the period–luminosity relations in the
G
band. For the analysis of the
Gaia
DR2 candidates we also used tools based on the
G
BP
and
G
RP
photometry, such as the period–Wesenheit relation in (
G
,
G
RP
).
Results.
Multi-band time-series photometry and characterisation by the SOS Cep&RRL pipeline are published in
Gaia
DR2 for 150 359 such variables (9575 classified as Cepheids and 140 784 as RR Lyrae stars) distributed throughout the sky. The sample includes variables in 87 globular clusters and 14 dwarf galaxies (the Magellanic Clouds, 5 classical and 7 ultra-faint dwarfs). To the best of our knowledge, as of 25 April 2018, the variability of 50 570 of these sources (350 Cepheids and 50 220 RR Lyrae stars) has not been reported before in the literature, therefore they are likely new discoveries by
Gaia
. An estimate of the interstellar absorption is published for 54 272 fundamental-mode RR Lyrae stars from a relation based on the
G
-band amplitude and the pulsation period. Metallicities derived from the Fourier parameters of the light curves are also released for 64 932 RR Lyrae stars and 3738 fundamental-mode classical Cepheids with periods shorter than 6.3 days.
ABSTRACT
We present new luminosity–metallicity (LZ; $M_{V}\!-\!\rm Fe/H$ and $M_{G}\!-\!\rm Fe/H$) relations and, for the first time, empirical, Gaia three-band (G, GBP, GRP) ...period-Wesenheit-metallicity (PWZ) relations of RR Lyrae stars (RRLs) derived using a hierarchical Bayesian approach and new accurate parallaxes published for these variables in the Gaia Early Data Release 3 (EDR3). In a previous study, we obtained Bayesian hierarchically derived LZ relations from a sample of about four hundred Milky Way field RRLs with G-band light curves and trigonometric parallaxes published in the Gaia Data Release 2 (DR2), using V mean magnitudes, metallicities, absorptions, and pulsation periods available in the literature. We now extend that study in two directions. First, we update our previous results using trigonometric parallaxes from Gaia EDR3 and incorporate the Bayesian analysis of a first empirical PWZ relation derived using those field RRLs with G, GBP and GRP time-series photometry available in Gaia DR2. Secondly, we use Bayesian inference to derive LZ relations and empirical PW Gaia three-band relations from 385 RRLs belonging to 15 Milky Way globular clusters (GC) with literature-compiled spectroscopic metallicities ranging from −0.36 to −2.39 dex and prior distances extending from 2.2 to 41.2 kpc. From the samples of RRLs analysed in this paper we infer a mean Gaia EDR3 zero-point offset of −0.028 mas with median values ranging from −0.033 (LZ and PWZ models for field stars) to −0.024 mas (LZ model in the V band for GC RRLs).
Abstract
Estimating the atmospheric parameters of M-type stars has been a difficult task due to the lack of simple diagnostics in the stellar spectra. We aim at uncovering good sets of predictive ...features of stellar atmospheric parameters (Teff, log (g), M/H) in spectra of M-type stars. We define two types of potential features (equivalent widths and integrated flux ratios) able to explain the atmospheric physical parameters. We search the space of feature sets using a genetic algorithm that evaluates solutions by their prediction performance in the framework of the BT-Settl library of stellar spectra. Thereafter, we construct eight regression models using different machine-learning techniques and compare their performances with those obtained using the classical χ2 approach and independent component analysis (ICA) coefficients. Finally, we validate the various alternatives using two sets of real spectra from the NASA Infrared Telescope Facility (IRTF) and Dwarf Archives collections. We find that the cross-validation errors are poor measures of the performance of regression models in the context of physical parameter prediction in M-type stars. For R ∼ 2000 spectra with signal-to-noise ratios typical of the IRTF and Dwarf Archives, feature selection with genetic algorithms or alternative techniques produces only marginal advantages with respect to representation spaces that are unconstrained in wavelength (full spectrum or ICA). We make available the atmospheric parameters for the two collections of observed spectra as online material.
Ultracool spectroscopic outliers in Gaia DR3 Cooper, W J; Smart, R L; Jones, H R A ...
Monthly notices of the Royal Astronomical Society,
01/2024, Letnik:
527, Številka:
1
Journal Article
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ABSTRACT
Gaia DR3 provided a first release of RP spectra and astrophysical parameters for ultracool dwarfs (UCDs). We used these Gaia RP spectra and astrophysical parameters to select the most ...outlying UCDs. These objects have spectral types of M7 or later and might be young brown dwarfs or low-metallicity objects. This work aimed to find UCDs that have Gaia RP spectra significantly different to the typical population. However, the intrinsic faintness of these UCDs in Gaia means that their spectra were typically rather low signal-to-noise ratio in Gaia DR3. This study is intended as a proof of concept for future iterations of the Gaia data releases. Based on well-studied subdwarfs and young objects, we created a spectral type-specific color ratio, defined using Gaia RP spectra; this ratio is then used to determine which objects are outliers. We then used the objects kinematics and photometry external to Gaia to cut down the list of outliers into a list of ‘prime candidates’. We produce a list of 58 Gaia RP spectra outliers, seven of which we deem as prime candidates. Of these, six are likely subdwarfs and one is a known young stellar object. Four of six subdwarf candidates were known as subdwarfs already. The two other subdwarf candidates, namely 2MASS J03405673 + 2633447 (sdM8.5) and 2MASS J01204397 + 6623543 (sdM9), are new classifications.
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