The primary sample of the Gaia Data Release 1 is the Tycho-Gaia Astrometric Solution (TGAS): 2 million Tycho-2 sources with improved parallaxes and proper motions relative to the initial catalog. ...This increased astrometric precision presents an opportunity to find new binary stars and moving groups. We search for high-confidence comoving pairs of stars in TGAS by identifying pairs of stars consistent with having the same 3D velocity using a marginalized likelihood ratio test to discriminate candidate comoving pairs from the field population. Although we perform some visualizations using (bias-corrected) inverse parallax as a point estimate of distance, the likelihood ratio is computed with a probabilistic model that includes the covariances of parallax and proper motions and marginalizes the (unknown) true distances and 3D velocities of the stars. We find 13,085 comoving star pairs among 10,606 unique stars with separations as large as 10 pc (our search limit). Some of these pairs form larger groups through mutual comoving neighbors: many of these pair networks correspond to known open clusters and OB associations, but we also report the discovery of several new comoving groups. Most surprisingly, we find a large number of very wide ( pc) separation comoving star pairs, the number of which increases with increasing separation and cannot be explained purely by false-positive contamination. Our key result is a catalog of high-confidence comoving pairs of stars in TGAS. We discuss the utility of this catalog for making dynamical inferences about the Galaxy, testing stellar atmosphere models, and validating chemical abundance measurements.
We report and discuss the discovery of a significant difference in the chemical abundances of a comoving pair of bright solar-type stars, HD 240430 and HD 240429. The two stars have an estimated 3D ...separation of 0.6 pc ( 0.01 pc projected) at a distance of r 100 pc with nearly identical 3D velocities, as inferred from Gaia TGAS parallaxes and proper motions, and high-precision radial velocity measurements. Stellar parameters determined from high-resolution spectra obtained with the High Resolution Echelle Spectrometer (HIRES) at the Keck Observatory indicate that the two stars are ∼4 Gyr old. The more metal-rich of the two, HD 240430, shows an enhancement of refractory ( K) elements by 0.2 dex and a marginal enhancement of (moderately) volatile elements ( K; , , , , and ). This is the largest metallicity difference found in a wide binary pair to date. Additionally, HD 240430 shows an anomalously high surface lithium abundance ( ), higher than its cooler companion by 0.5 dex. The proximity in phase-space and ages between the two stars suggests that they formed together with the same composition, which is at odds with the observed differences in metallicity and abundance patterns. We therefore suggest that the star HD 240430, "Kronos," accreted of rocky material after birth, selectively enhancing the refractory elements as well as lithium in its surface and convective envelope.
We investigate two-dimensional image decomposition of nearby, morphologically selected early-type galaxies (ETGs). We are motivated by recent observational evidence of significant size growth of ...quiescent galaxies and theoretical development advocating a two-phase formation scenario for ETGs. We find that a significant fraction of nearby ETGs show changes in isophotal shape that require multi-component models. The characteristic sizes of the inner and outer component are ∼3 and ∼15 kpc. The inner component lies on the mass-size relation of ETGs at z ∼ 0.25-0.75, while the outer component tends to be more elliptical and hints at a stochastic buildup process. We find real physical differences between single- and double-component ETGs, with double-component galaxies being younger and more metal-rich. The fraction of double-component ETGs increases with increasing and decreases in denser environments. We hypothesize that double-component systems were able to accrete gas and small galaxies until later times, boosting their central densities, building up their outer parts, and lowering their typical central ages. In contrast, the oldest galaxies, perhaps due to residing in richer environments, have no remaining hints of their last accretion episode.
ABSTRACT
The precision of the Gaia data offers a unique opportunity to study the internal velocity field of star clusters. We develop and validate a forward-modelling method for the internal motions ...of stars in a cluster. The model allows an anisotropic velocity dispersion matrix and linear velocity gradient describing rotation and shear, combines radial velocities available for a subset of stars, and accounts for contamination from background sources via a mixture model. We apply the method to Gaia DR2 data of the Hyades cluster and its tidal tails, dividing and comparing the kinematics of stars within and beyond 10 pc, which is roughly the tidal radius of the cluster. While the velocity dispersion for the cluster is nearly isotropic, the velocity ellipsoid for the tails is clearly elongated with the major axis pointing towards the Galactic centre. We find positive and negative expansions at ≈2σ significance in Galactic azimuthal and vertical directions for the cluster but no rotation. The tidal tails are stretching in a direction tilted from the Galactic centre while equally contracting as the cluster in Galactic vertical direction. The tails have a shear (A) of 16.90 ± 0.92 $\mathrm{m}\, \mathrm{s}^{-1}\, \mathrm{pc}^{-1}$ and a vorticity (B) of −6.48 ± 1.15 $\mathrm{m}\, \mathrm{s}^{-1}\, \mathrm{pc}^{-1}$, values distinct from the local Oort constants. By solving the Jeans equations for flattened models of the Hyades, we show that the observed velocity dispersions are a factor of ≈2 greater than required for virial equilibrium due to tidal heating and disruption. From simple models of the mass loss, we estimate that the Hyades is close to final dissolution with only a further ≲30 Myr left.
Mass segregation in the hyades cluster Evans, N Wyn; Oh, Semyeong
Monthly Notices of the Royal Astronomical Society,
04/2022, Volume:
512, Issue:
3
Journal Article
Peer reviewed
Open access
ABSTRACT
Using the Gaia colour–magnitude diagram, we assign masses to a catalogue of 979 confirmed members of the Hyades cluster and tails. By fitting the cumulative mass profile, stars within the ...tidal radius have a Plummer-like profile with half-mass radius rh of 5.75 pc. The tails are extended with rh = 69.35 pc and fall off more slowly than Plummer with density proportional to distance−1.36. The cluster stars are separated into two groups at BP–RP = 2 or 0.56M⊙ to give a high mass (${\bar{M}} = 0.95 M_\odot$) and a low mass (${\bar{M}} = 0.32 M_\odot$) population. We show that: (i) the high mass population has a half-mass radius rh of 4.88 pc, whilst the low mass population has rh = 8.10 pc; (ii) despite the differences in spatial extent, the kinematics and binarity properties of the high and low mass populations are similar. They have isotropic velocity ellipsoids with mean 1d velocity dispersions σ of 0.427 and 0.415 km s−1, respectively. The dynamical state of the Hyades is far from energy equipartition ($\sigma \propto {\bar{M}}^{-1/2}$). We identify a new mass segregation instability for clusters with escape speed V. Populations with $V/\sigma \lesssim 2\sqrt{2}$ can never attain thermal equilibrium and equipartition. This regime encompasses many Galactic open and globular clusters. For the Hyades, there must be an outward energy flux of at least $9.5 \times 10^{-4} M_\odot \, {\rm km^2\, s^{-2} Myr^{-1}}$ to maintain its current configuration. The present mass-loss of 0.26M⊙Myr−1 due to tidal stripping by itself implies a substantial energy flow beyond the required magnitude.
ABSTRACT
For stars with unresolved companions, motions of the centre of light and that of mass decouple, causing a single-source astrometric model to perform poorly. We show that such stars can be ...easily detected with the reduced χ2 statistic, or renormalized unit weight error (RUWE), provided as part of Gaia DR2. We convert RUWE into the amplitude of the image centroid wobble, which, if scaled by the source distance, is proportional to the physical separation between companions (for periods up to several years). We test this idea on a sample of known spectroscopic binaries and demonstrate that the amplitude of the centroid perturbation scales with the binary period and the mass ratio as expected. We apply this technique to the Gaia DR2 data and show how the binary fraction evolves across the Hertzsprung–Russell diagram. The observed incidence of unresolved companions is high for massive young stars and drops steadily with stellar mass, reaching its lowest levels for white dwarfs. We highlight the elevated binary fraction for the nearby blue stragglers and blue horizontal branch stars. We also illustrate how unresolved hierarchical triples inflate the relative velocity signal in wide binaries. Finally, we point out a hint of evidence for the existence of additional companions to the hosts of extrasolar hot Jupiters.
ABSTRACT
We show how astrometric and spectroscopic errors introduced by an unresolved binary system can be combined to give estimates of the binary period and mass ratio. This can be performed ...analytically if we assume we see one or more full orbits over our observational baseline, or numerically for all other cases. We apply this method to Gaia DR3 data, combining the most recent astrometric and spectroscopic data. We compare inferred periods and mass ratios calculated using our method with orbital parameters measured for non-single stars in Gaia DR3 and find good agreement. Finally, we use this method to search the subset of the Gaia DR3 RVS data set with rv_method_used = 1 for compact object candidates. We select sources with significant astrometric and spectroscopic errors (RUWEast > 1.25 and RUWEspec > 2), large inferred mass ratios, and large inferred companion masses (q > 1 and m2 > 3M⊙) giving a catalogue of 4641 candidate hierarchical triples and Main Sequence + Compact Object pairs. We apply more stringent cuts, and impose low levels of photometric variability to remove likely triples (RUWEphot < 2), producing a gold sample of 45 candidates.
Interpreting and modelling astronomical catalogues requires an understanding of the catalogues’ completeness or selection function: what properties determine an object’s probability of being ...including in the catalogue? Here we set out to empirically quantify the completeness of the overall catalogue of
Gaia
’s third data release (DR3). This task is not straightforward because
Gaia
is the all-sky optical survey with the highest angular resolution to date and no consistent ground truth exists to allow direct comparisons. However, well-characterised deeper imaging enables an empirical assessment of
Gaia
’s
G
-band completeness across parts of the sky. On this basis, we devised a simple analytical completeness model of
Gaia
as a function of the observed
G
magnitude and position over the sky, which accounts for both the effects of crowding and the complex
Gaia
scanning law. Our model only depends on a single quantity: the median magnitude
M
10
in a patch of the sky of catalogued sources with
astrometric_matched_transits
≤10. We note that
M
10
reflects elementary completeness decisions in the
Gaia
pipeline and is computable from the
Gaia
DR3 catalogue itself and therefore applicable across the whole sky. We calibrated our model using the Dark Energy Camera Plane Survey (DECaPS) and tested its predictions against
Hubble
Space Telescope observations of globular clusters. We found that our model predicts
Gaia
’s completeness values to a few per cent (RMS) across the sky. We make the model available as a part of the
gaiaunlimited
Python package built and maintained by the GaiaUnlimited project.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
Context.
Understanding the intricacies behind the presence and absence of sources in an astronomical catalogue is crucial for the accurate interpretation of astronomical data. In particular, for the ...multi-dimensional
Gaia
data, filters and cuts on different parameters or measurements introduce a selection function that may unintentionally alter scientific conclusions in subtle ways.
Aims.
We aim to develop a methodology to estimate the selection function for different subsamples of stars in the
Gaia
catalogue.
Methods.
Comparing the number of stars in a given subsample to that in the overall
Gaia
catalogue provides an estimate of the subsample membership probability as a function of sky position, magnitude, and colour. The method used to make this estimate must differentiate the stochastic absence of subsample stars from selection effects. When multiplied with the overall
Gaia
catalogue selection function, this provides the total selection function of the subsample.
Results.
We present our new method for estimating the selection function by applying it to the sources in
Gaia
DR3 with heliocentric radial velocity measurements. We also compute the selection function for the stars in the Gaia-Sausage/Enceladus sample, confirming that the apparent asymmetry of its debris across the sky is merely caused by selection effects.
Conclusions.
The method we have developed estimates the selection function of the stars present in a subsample of
Gaia
data, given that the subsample is completely contained in the
Gaia
parent catalogue (for which the selection function is known). This tool is made available in a GaiaUnlimited Python package.
Full text
Available for:
FMFMET, NUK, UL, UM, UPUK
We report spectroscopic measurements of stars in the recently discovered young stellar association Price-Whelan 1 (PW 1), which was found in the vicinity of the Leading Arm (LA) of the Magellanic ...Stream (MS). We obtained Magellan+MIKE high-resolution spectra of the 28 brightest stars in PW 1 and used The Cannon to determine their stellar parameters. We find that the mean metallicity of PW 1 is Fe/H = −1.23 with a small scatter of 0.06 dex and the mean RV is = 276.7 with a dispersion of . Our results are consistent in , , and Fe/H with the young and metal-poor characteristics (116 Myr and Fe/H = −1.1) determined for PW 1 from our discovery paper. We find a strong correlation between the spatial pattern of the PW 1 stars and the LA II gas with an offset of −10 15 in and +1 55 in . The similarity in metallicity, velocity, and spatial patterns indicates that PW 1 likely originated in LA II. We find that the spatial and kinematic separation between LA II and PW 1 can be explained by ram pressure from Milky Way (MW) gas. Using orbit integrations that account for the LMC and MW halo and outer disk gas, we constrain the halo gas density at the orbital pericenter of PW 1 to be and the disk gas density at the midplane at to be . We, therefore, conclude that PW 1 formed from the LA II of the MS, making it a powerful constraint on the MW-Magellanic interaction.
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