Gaia Satellite Research Articles

Gaia18aen: First symbiotic star discovered by Gaia

Context. Besides the astrometric mission of the Gaia satellite, its repeated and high-precision measurements also serve as an all-sky photometric transient survey. The sudden brightenings of the sources are published as Gaia Photometric Science Alerts and are made publicly available, allowing the community to photometrically and spectroscopically follow up on the object. Aims. The goal of this paper is to analyze the nature and derive the basic parameters of Gaia18aen, a transient detected at the beginning of 2018. This object coincides with the position of the emission-line star WRAY 15-136. The brightening was classified as a “nova?” on the basis of a subsequent spectroscopic observation. Methods. We analyzed two spectra of Gaia18aen and collected the available photometry of the object covering the brightenings in 2018 and also the preceding and following periods of quiescence. Based on this observational data, we derived the parameters of Gaia18aen and discussed the nature of the object. Results. Gaia18aen is the first symbiotic star discovered by Gaia satellite. The system is an S-type symbiotic star and consists of an M giant of a slightly super-solar metallicity, where Teff ∼ 3500 K, a radius of ∼230 R⊙, and a high luminosity L ∼ 7400 L⊙. The hot component is a hot white dwarf. We tentatively determined the orbital period of the system ∼487 d. The main outburst of Gaia18aen in 2018 was accompanied by a decrease in the temperature of the hot component. The first phase of the outburst was characterized by the high luminosity L ∼ 27 000 L⊙, which remained constant for about three weeks after the optical maximum, later followed by the gradual decline of luminosity and increase of temperature. Several re-brightenings have been detected on the timescales of hundreds of days.

On-sky silicon photomultiplier detector performance measurements for millisecond to sub-microsecond optical source variability studies

In our Ultra-Fast Astronomy (UFA) program, we aim to improve measurements of variability of astronomical targets on millisecond and shorter time scales. In this work, we present initial on-sky measurements of the performance of silicon photomultiplier detectors (SiPMs) for UFA. We mounted two different SiPMs at the focal plane of the 0.7-meter aperture Nazarbayev University Transient Telescope at the Assy-Turgen Astrophysical Observatory (NUTTelA-TAO), with no filter in front of the detector. The $3mm\times3mm$ SiPM single-channel detectors have a field of view of $2.2716'\times2.2716'$. During the nights of 2019 October 28-29, we measured sky background, bright stars, and an artificial source with a 100Hz flashing frequency. We compared detected SiPM counts with Gaia satellite G-band flux values to show that our SiPMs have a linear response. With our two SiPMs (models S14520-3050VS and S14160-3050HS), we measured a dark current of $\sim$130 and $\sim$85 kilo counts per second (kcps), and a sky background of $\sim$201 and $\sim$203 kcps, respectively. We measured an intrinsic crosstalk of 10.34$\%$ and 10.52$\%$ and derived a 5$\sigma$ sensitivity of 13.9 and 14.0 Gaia G-band magnitude for 200ms exposures, for the two detectors respectively. For a 10 $\mu$s window, and allowing a false alarm rate of once per 100 nights, we derived a sensitivity of 22 detected photons, or 6 Gaia G-band magnitudes. For nanosecond timescales, our detection is limited by crosstalk to 12 detected photons, which corresponds to a fluence of $\sim$155 photons per square meter.

Searching for ZZ Ceti White Dwarfs in the Gaia Survey

Abstract The Gaia satellite recently released parallax measurements for ∼260,000 high-confidence white dwarf candidates, allowing for precise measurements of their physical parameters. By combining these parallaxes with Pan-STARRS and u-band photometry, we measured the effective temperature and stellar mass for all white dwarfs in the Northern Hemisphere within 100 pc of the Sun, and identified a sample of ZZ Ceti white dwarf candidates within the so-called instability strip. We acquired high-speed photometric observations for 90 candidates using the PESTO camera attached to the 1.6 m telescope at the Mont-Mégantic Observatory. We report the discovery of 38 new ZZ Ceti stars, including two very rare ultramassive pulsators. We also identified five possibly variable stars within the strip, in addition to 47 objects that do not appear to show any photometric variability. However, several of those could be variable with an amplitude below our detection threshold, or could be located outside the instability strip due to errors in their photometric parameters. In the light of our results, we explore the trends of the dominant period and amplitude in the M– plane, and briefly discuss the question of the purity of the ZZ Ceti instability strip (i.e., a region devoid of non-variable stars).

Constraints on dark matter-nucleon effective couplings in the presence of kinematically distinct halo substructures using the DEAP-3600 detector

DEAP-3600 is a single-phase liquid argon detector aiming to directly detect Weakly Interacting Massive Particles (WIMPs), located at SNOLAB (Sudbury, Canada). After analyzing data taken during the first year of operation, a null result was used to place an upper bound on the WIMP-nucleon spin-independent, isoscalar cross section. This study reinterprets this result within a Non-Relativistic Effective Field Theory framework, and further examines how various possible substructures in the local dark matter halo may affect these constraints. Such substructures are hinted at by kinematic structures in the local stellar distribution observed by the Gaia satellite and other recent astronomical surveys. These include the Gaia Sausage (or Enceladus), as well as a number of distinct streams identified in recent studies. Limits are presented for the coupling strength of the effective contact interaction operators $\mathcal{O}_1$, $\mathcal{O}_3$, $\mathcal{O}_5$, $\mathcal{O}_8$, and $\mathcal{O}_{11}$, considering isoscalar, isovector, and xenonphobic scenarios, as well as the specific operators corresponding to millicharge, magnetic dipole, electric dipole, and anapole interactions. The effects of halo substructures on each of these operators are explored as well, showing that the $\mathcal{O}_5$ and $\mathcal{O}_8$ operators are particularly sensitive to the velocity distribution, even at dark matter masses above 100 GeV/$c^2$.

The nature of the symbiotic candidate 2MASS J07363415+6538548 in the field of NGC 2403

ABSTRACT New Online Database of Symbiotic Variables includes several poorly characterized objects and candidate symbiotic stars, not only in the Milky Way but also in other galaxies. The goal of the research presented in this paper was to reveal the nature of 2MASS J07363415+6538548, the object discovered as an X-ray source in the field of NGC 2403, and tentatively classified as a symbiotic candidate or a cataclysmic variable. By analysis of available photometric data from ground-based surveys, together with a high-precision photometry from TESS, remarkable astrometric measurements of the Gaia satellite and observations of other surveys spanning from X-rays to infrared, we have found that the object neither not a symbiotic star nor a cataclysmic variable, but rather an active K-type dwarf. The star is located in the distance of 415 pc, has an effective temperature of 4275 K, luminosity of 0.14 L⊙, mass of 0.7 M⊙, and radius of 0.7 R⊙. It has a rotational period ∼ 3 d and is a strong X-ray source with the X-ray luminosity of ∼ $10^{30}\rm \, erg\, s^{-1}$. Gyrochronology and isochrone fitting confirmed that the star is young.

Testing the Accuracy of the Tangent Point Method for Determining the Milky Way’s Inner Rotation Curve

Abstract The tangent point method (TPM) is commonly used to calculate the rotation curve of the inner portion of the Milky Way, relying on the assumption that along one line of sight, the point tangent to the Galactic center is the point where the observed radial velocity is an extremum. This work compares rotation curves derived from the TPM, using neutral hydrogen gas data, and direct measurements of stellar positions and velocities from the Gaia Satellite. The Gaia data indicate that the rotation curve based upon stellar kinematics is much shallower than that previously reported based on the TPM, especially in the inner regions of the Galaxy. Additionally, based on stellar kinematics, the locations of the extremum radial velocities and tangent points often differ. While these results are consistent with previously published simulations, future data from the Gaia satellite will allow for further confidence in determining an accurate rotation curve.

Photocenter Shift Effect in Double Stars of Gaia DR2 Database

In order to check the performance of the Gaia satellite when detecting double stars, we attempted to discover a photocenter shift affecting Gaia's second data release (Gaia DR2). We analyzed data of bona-fide double stars from Double and Multiple System Annex catalog and matched them with the Gaia DR2 catalog. We tried to derive a relationship between measurements of visual magnitude and signs of binarity based on parallaxes and proper motions. A comprehensive study of parallaxes indicates the presence of the photocenter shift effect. Due to the proper motion two-dimensional nature, we conducted only a simply study which implies that a real photocenter shift may affect double stars appearing in the Gaia DR2 catalog as single sources.

How unusual is the Milky Way’s assembly history?

ABSTRACT In the lambda cold dark matter (ΛCDM) model of structure formation galactic haloes build-up by accretion of mass and mergers of smaller haloes. The most recent massive merger event experienced by the Milky Way (MW) halo was the accretion of the Large Magellanic Cloud (LMC; which has a stellar mass of ∼109M⊙). Recent analyses of galactic stellar data from the Gaia satellite have uncovered an earlier massive accretion event, the Gaia-Enceladus Sausage (GES), which merged with the MW around 10 Gyr ago. Here, we use the EAGLE cosmological hydrodynamics simulation to study properties of simulated MW-mass haloes constrained to have accretion histories similar to that of the MW, specifically the recent accretion of an ‘LMC’ galaxy and a ‘GES’ merger, with a quiescent period between the GES merger and the infall of the LMC (the ‘LMC and GES’ category). We find that ∼16 per cent of MW-mass haloes have an LMC; ∼5 per cent have a GES event and no further merger with an equally massive object since z = 1; and only 0.65 per cent belong to the LMC and GES category. The progenitors of the MWs in this last category are much less massive than average at early times but eventually catch up with the mean. The LMC and GES category of galaxies naturally end up in the ‘blue cloud’ in the colour–magnitude diagram at z = 0, tend to have a disc morphology and have a larger than average number of satellite galaxies.

The Gaia-ESO survey: the non-universality of the age–chemical-clocks–metallicity relations in the Galactic disc

Context. In the era of large spectroscopic surveys, massive databases of high-quality spectra coupled with the products of the Gaia satellite provide tools to outline a new picture of our Galaxy. In this framework, an important piece of information is provided by our ability to infer stellar ages, and consequently to sketch a Galactic timeline. Aims. We aim to provide empirical relations between stellar ages and abundance ratios for a sample of stars with very similar stellar parameters to those of the Sun, namely the so-called solar-like stars. We investigate the dependence on metallicity, and we apply our relations to independent samples, that is, the Gaia-ESO samples of open clusters and of field stars. Methods. We analyse high-resolution and high-signal-to-noise-ratio HARPS spectra of a sample of solar-like stars to obtain precise determinations of their atmospheric parameters and abundances for 25 elements and/or ions belonging to the main nucleosynthesis channels through differential spectral analysis, and of their ages through isochrone fitting. Results. We investigate the relations between stellar ages and several abundance ratios. For the abundance ratios with a steeper dependence on age, we perform multivariate linear regressions, in which we include the dependence on metallicity, [Fe/H]. We apply our best relations to a sample of open clusters located from the inner to the outer regions of the Galactic disc. Using our relations, we are able to recover the literature ages only for clusters located at RGC > 7 kpc. The values that we obtain for the ages of the inner-disc clusters are much greater than the literature ones. In these clusters, the content of neutron capture elements, such as Y and Zr, is indeed lower than expected from chemical evolution models, and consequently their [Y/Mg] and [Y/Al] are lower than in clusters of the same age located in the solar neighbourhood. With our chemical evolution model and a set of empirical yields, we suggest that a strong dependence on the star formation history and metallicity-dependent stellar yields of s-process elements can substantially modify the slope of the [s/α]–[Fe/H]–age relation in different regions of the Galaxy. Conclusions. Our results point towards a non-universal relation [s/α]–[Fe/H]–age, indicating the existence of relations with different slopes and intercepts at different Galactocentric distances or for different star formation histories. Therefore, relations between ages and abundance ratios obtained from samples of stars located in a limited region of the Galaxy cannot be translated into general relations valid for the whole disc. A better understanding of the s-process at high metallicity is necessary to fully understand the origin of these variations.

Masses of the components of SB2 binaries observed with Gaia – V. Accurate SB2 orbits for 10 binaries and masses of the components of 5 binaries

ABSTRACT Double-lined spectroscopic binaries (SB2s) are one of the main sources of stellar masses, as additional observations are only needed to give the inclinations of the orbital planes in order to obtain the individual masses of the components. For this reason, we are observing a selection of SB2s using the SOPHIE spectrograph at the Haute-Provence observatory in order to precisely determine their orbital elements. Our objective is to finally obtain masses with an accuracy of the order of one per cent by combining our radial velocity (RV) measurements and the astrometric measurements that will come from the Gaia satellite. We present here the RVs and the re-determined orbits of 10 SB2s. In order to verify the masses, we will derive from Gaia, we obtained interferometric measurements of the ESO VLTI for one of these SB2s. Adding the interferometric or speckle measurements already published by us or by others for four other stars, we finally obtain the masses of the components of five binary stars, with masses ranging from 0.51 to 2.2 solar masses, including main-sequence dwarfs and some more evolved stars whose location in the HR diagram has been estimated.

Physical parameters of selected Gaia mass asteroids

Context. Thanks to the Gaia mission, it will be possible to determine the masses of approximately hundreds of large main belt asteroids with very good precision. We currently have diameter estimates for all of them that can be used to compute their volume and hence their density. However, some of those diameters are still based on simple thermal models, which can occasionally lead to volume uncertainties as high as 20–30%. Aims. The aim of this paper is to determine the 3D shape models and compute the volumes for 13 main belt asteroids that were selected from those targets for which Gaia will provide the mass with an accuracy of better than 10%. Methods. We used the genetic Shaping Asteroids with Genetic Evolution (SAGE) algorithm to fit disk-integrated, dense photometric lightcurves and obtain detailed asteroid shape models. These models were scaled by fitting them to available stellar occultation and/or thermal infrared observations. Results. We determine the spin and shape models for 13 main belt asteroids using the SAGE algorithm. Occultation fitting enables us to confirm main shape features and the spin state, while thermophysical modeling leads to more precise diameters as well as estimates of thermal inertia values. Conclusions. We calculated the volume of our sample of main-belt asteroids for which the Gaia satellite will provide precise mass determinations. From our volumes, it will then be possible to more accurately compute the bulk density, which is a fundamental physical property needed to understand the formation and evolution processes of small Solar System bodies.

Gaia’s detectability of black hole–main sequence star binaries formed in open clusters

Abstract Black hole–main sequence star (BH–MS) binaries are one of the targets of future data releases of the astrometric satellite Gaia. They are supposed to be formed in two main sites: a galactic field and star clusters. However, previous work has never predicted the number of BH–MS binaries originating in the latter sites. In this paper we estimate the number of BH–MS binaries formed in open clusters and detectable with Gaia based on the results of N-body simulations. By considering interstellar extinction in the Milky Way and observational constraints, we predict that ∼10 BH–MS binaries are observable. We also find that chemical abundance patterns of companion MSs will help us to identify the origin of the binaries as star clusters. Such MSs are not polluted by outflows of the BH progenitors, such as stellar winds and supernova ejecta. Chemical anomalies might be a good test to confirm the origin of binaries with relatively less massive MSs (≲5 M⊙), short orbital periods (∼1.5 yr), and higher eccentricities (e ≳0.1).

Binary deviations from single object astrometry

ABSTRACT Most binaries are undetected. Astrometric reductions of a system using the assumption that the object moves like a single point mass can be biased by unresolved binary stars. The discrepancy between the centre of mass of the system (which moves like a point mass) and the centre of light (which is what we observe) introduces additional motion. We explore the extent to which binary systems affect single object model fit to astrometric data. This tells us how observations are diluted by binaries and which systems cause the largest discrepancies – but it also allows us to make inferences about the binarity of populations based on observed astrometric error. By examining a sample of mock observations, we show that binaries with periods close to 1 yr can mimic parallax and thus bias distance measurements, while long-period binaries can introduce significant apparent proper motion. While these changes can soak up some of the error introduced by the binary, the total deviation from the best-fitting model can be translated into a lower limit on the on-sky separation of the pair. Throughout, we link these predictions to data from the Gaia satellite, while leaving the conclusions generalizable to other surveys.

Orbital Evidences for Dark-matter-free Milky Way Dwarf Spheroidal Galaxies

Abstract The nature of Milky Way dwarf spheroidals (MW dSphs) has been questioned, in particular whether they are dominated by dark matter (DM). Here we investigate an alternative scenario, for which tidal shocks are exerted by the MW to DM-free dSphs after a first infall of their gas-rich progenitors, and for which theoretical calculations have been verified by pure N-body simulations. Whether or not the dSphs are on their first infall cannot be resolved on the sole basis of their star formation history. In fact, gas removal may cause complex gravitational instabilities, and near-pericenter passages can give rise to tidal disruptive processes. Advanced precision with the Gaia satellite in determining both their past orbital motions and the MW velocity curve is, however, providing crucial results. First, tidal shocks explain why DM-free dSphs are found preferentially near their pericenter, where they are in a destructive process, while their chance to be long-lived satellites is associated with a very low probability P ∼ 2 × 10−7, which is at odds with the current DM-dominated dSph scenario. Second, most dSph binding energies are consistent with a first infall. Third, the MW tidal shocks that predict the observed dSph velocity dispersions are themselves predicted in amplitude by the most accurate MW velocity curve. Fourth, tidal shocks accurately predict the forces or accelerations exerted at half-light radii of dSphs, including the MW and the Magellanic System gravitational attractions. The above is suggestive of dSphs that are DM-free and tidally shocked near their pericenters, which may provoke a significant quake in our understanding of near-field cosmology.

Velocity substructure from Gaia and direct searches for dark matter

Data from the Gaia satellite show that the solar neighbourhood of the Milky Way's stellar halo is imprinted with substructure from several accretion events. Evidence of these events is found in "the Shards", stars clustering with high significance in both action space and metallicity. Stars in the Shards share a common origin, likely as ancient satellite galaxies of the Milky Way, so will be embedded in dark matter (DM) counterparts. These "Dark Shards" contain two substantial streams (S1 and S2), as well as several retrograde, prograde and lower energy substructures. The retrograde stream S1 has a very high Earth-frame speed of $\sim 550$ km s$^{-1}$ while S2 moves on a prograde, but highly polar orbit and enhances the peak of the speed distribution at around $300$ km s$^{-1}$. The presence of the Dark Shards locally leads to modifications of many to the fundamental properties of experimental DM signals. The~S2 stream in particular gives rise to an array of effects in searches for axions and in the time dependence of nuclear recoils: shifting the peak day, inducing nonsinusoidal distortions, and increasing the importance of the gravitational focusing of DM by the Sun. Dark Shards additionally bring new features for directional signals, while also enhancing the DM flux towards Cygnus.

Local RR Lyrae stars: native and alien

ABSTRACT Measurements of [Fe/H] and radial velocity are presented for 89 RR Lyrae (RRL) candidates within 6 kpc of the Sun. After the removal of two suspected non-RRLs, these stars were added to an existing data base, which yielded 464 RRLs with [Fe/H] on a homogeneous scale. Using data from the Gaia satellite (Data Release 2), we calculated the positions and space velocities for this sample. These data confirm the existence of a thin disc of RRL with [α/Fe] ∼ solar. The majority of the halo RRLs with large total energies have near-zero angular momenta about the Z-axis. Kinematically, these stars closely resemble the Gaia-Sausage/Gaia-Enceladus stars that others have proposed are debris from the merger of a large galaxy with the Milky Way. The metallicity and period distributions of the RRLs and their positions in the period–amplitude diagram suggest that this disrupted galaxy was as massive as the Large Magellanic Cloud and possibly greater.

Detection of the nearest Jupiter analogue in radial velocity and astrometry data

ABSTRACT The presence of Jupiter is crucial to the architecture of the Solar system and models underline this to be a generic feature of planetary systems. We find the detection of the difference between the position and motion recorded by the contemporary astrometric satellite Gaia and its precursor Hipparcos can be used to discover Jupiter-like planets. We illustrate how observations of the nearby star ϵ Indi A giving astrometric and radial velocity data can be used to independently find the orbit of its suspected companion. The radial velocity and astrometric data provide complementary detections which allow for a much stronger solution than either technique would provide individually. We quantify ϵ Indi A b as the closest Jupiter-like exoplanet with a mass of 3 MJup on a slightly eccentric orbit with an orbital period of 45 yr. While other long-period exoplanets have been discovered, ϵ Indi A b provides a well-constrained mass and along with the well-studied brown dwarf binary in orbit around ϵ Indi A means that the system provides a benchmark case for our understanding of the formation of gas giant planets and brown dwarfs.

Searching for ZZ Ceti white dwarfs in the Gaia survey

AbstractThe Gaia satellite recently released parallax measurements for nearly 400,000 white dwarf stars, allowing for precise measurements of their physical parameters. By combining these parallaxes with Pan-STARRS and CFIS-u photometry, we measured the effective temperatures and surface gravities for all white dwarfs within 100 pc and identified a sample of ZZ Ceti white dwarf candidates within the instability strip. We report the results of a photometric follow-up, currently under way, aimed at identifying new ZZ Ceti stars among this sample using the PESTO camera attached to the 1.6-m telescope at the Mont Mégantic Observatory. Our goal is to verify that ZZ Ceti stars occupy a region in the logg-Teff plane where no nonvariable stars are found, supporting the idea that ZZ Ceti pulsators represent a phase through which all hydrogen-line (DA) white dwarfs must evolve.

Direct detection anomalies in light of GAIA data

Measurements from the Gaia satellite have greatly increased our knowledge of the dark matter velocity distributions in the Solar neighborhood. There is evidence for multiple cold structures nearby, including a high-velocity stream counterrotating relative to the Sun. This stream could significantly alter the spectrum of recoil energies and increase the annual modulation of dark matter in direct detection experiments such as DAMA/Libra. We reanalyze the experimental limits from Xenon1T, CDMSlite, PICO-60 and COSINE-100, and compare them to the results of the DAMA/Libra experiment. While we find that this new component of the dark matter velocity distribution can greatly improve the fit to the DAMA/Libra data, both spin-independent and spin-dependent interpretations of the DAMA/Libra signal with elastic and inelastic scattering continue to be ruled out by the null results of other experiments, in particular Xenon1T.

Mapping the Stellar Halo with the H3 Spectroscopic Survey

Abstract Modern theories of galaxy formation predict that the Galactic stellar halo was hierarchically assembled from the accretion and disruption of smaller systems. This hierarchical assembly is expected to produce a high degree of structure in the combined phase and chemistry space; this structure should provide a relatively direct probe of the accretion history of our Galaxy. Revealing this structure requires precise 3D positions (including distances), 3D velocities, and chemistry for large samples of stars. The Gaia satellite is delivering proper motions and parallaxes for >1 billion stars to G ≈ 20. However, radial velocities and metallicities will only be available to G ≈ 15, which is insufficient to probe the outer stellar halo (≳10 kpc). Moreover, parallaxes will not be precise enough to deliver high-quality distances for stars beyond ∼10 kpc. Identifying accreted systems throughout the stellar halo therefore requires a large ground-based spectroscopic survey to complement Gaia. Here we provide an overview of the H3 Stellar Spectroscopic Survey, which will deliver precise stellar parameters and spectrophotometric distances for ≈200,000 stars to r = 18. Spectra are obtained with the Hectochelle instrument at the MMT, which is configured for the H3 Survey to deliver resolution R ≈ 23,000 spectra covering the wavelength range 5150–5300 Å. The survey is optimized for stellar halo science and therefore focuses on high Galactic latitude fields ( ), sparsely sampling 15,000 sq. degrees. Targets are selected on the basis of Gaia parallaxes, enabling very efficient selection of bona fide halo stars. The survey began in the fall of 2017 and has collected 88,000 spectra to-date. All of the data, including the derived stellar parameters, will eventually be made publicly available via the survey website: h3survey.rc.fas.harvard.edu.