Assessment of stochastic and deterministic models of 6304 quasar lightcurves from SDSS Stripe 82

The optical light curves of many quasars show variations of tenths of a magnitude or more on time scales of months to years. This variation often cannot be described well by a simple deterministic model. We perform a Bayesian comparison of over 20 deterministic and stochastic models on 6304 QSO light curves in SDSS Stripe 82. We include the damped random walk (or Ornstein-Uhlenbeck [OU] process), a particular type of stochastic model which recent studies have focused on. Further models we consider are single and double sinusoids, multiple OU processes, higher order continuous autoregressive processes, and composite models. We find that only 29 out of 6304 QSO lightcurves are described significantly better by a deterministic model than a stochastic one. The OU process is an adequate description of the vast majority of cases (6023). Indeed, the OU process is the best single model for 3462 light curves, with the composite OU process/sinusoid model being the best in 1706 cases. The latter model is the dominant one for brighter/bluer QSOs. Furthermore, a non-negligible fraction of QSO lightcurves show evidence that not only the mean is stochastic but the variance is stochastic, too. Our results confirm earlier work that QSO light curves can be described with a stochastic model, but place this on a firmer footing, and further show that the OU process is preferred over several other stochastic and deterministic models. Of course, there may well exist yet better (deterministic or stochastic) models which have not been considered here.Comment: accepted by AA, 12 pages, 11 figures, 4 table

Constraints on the cosmological coupling of black holes from Gaia

Recent work has suggested that black holes (BHs) could be cosmologically coupled to the accelerated expansion of the universe, potentially becoming a candidate for dark energy. This would imply BH mass growth following the cosmological expansion, with the masses of individual BHs growing as $M_{\rm BH}\propto (1+z)^3$. In this letter, we discuss the binary systems Gaia BH1 and Gaia BH2, which contain $\sim 9\,M_{\odot}$ BHs orbited by $\sim 1\,M_{\odot}$ stars in widely-separated orbits. The ages of both systems can be constrained by the properties of the luminous stars. If BH masses are indeed growing as $(1+z)^3$, the masses of both BHs at formation would have been significantly smaller than today. We find a 77% probability that the mass of the BH in Gaia BH2 would have been below $2.2M_\odot$ at formation. This is below the classical Tolman-Oppenheimer-Volkov limit, though it is not yet clear if BHs subject to cosmological coupling should obey this limit. For Gaia BH1, the same probability is 70%. This analysis is consistent with results from two BHs in the globular cluster NGC3201, but unlike the NGC3201 BHs, the Gaia BHs have well-constrained inclinations and thus firm upper mass limits. The discovery of more BHs in binary systems with Gaia astrometry in the coming years will allow us to test the cosmological coupling hypothesis decisively.Comment: submitted 8th March 2023; accepted 2nd May 2023; 4 pages, 3 figure

A system-technical method for the safeguarding of simulations during the design process

Inhalt dieser Arbeit ist die Definition einer Methodik zur Entwicklung von Unterstützungs-systemen zur Absicherung konstruktionsbegleitender Simulationen in CAD-Systemen. Der Fokus der Vorgehensweise liegt auf der systemtechnischen Umsetzung im Sinne der modell-basierten Systementwicklung (MBSE). Die Methode teilt sich auf in die Erstellung einer Wis-sensbasis, die Systementwicklung mittels der System Modeling Language (SysML) und die Implementierung ins CAD-System. Mit diesem Leitfaden soll gerade im Bereich der Formalisierung und der Implementierung von KBE-Systemen der Entwickler solcher Systeme bei der Kommunikation im Team und der Implementierung unterstützt werden. Zur Qualifizierung werden praktische Beispiele gezeigt, wie unterschiedliche KBE-Techniken aus einem neutralen Systemmodell abgeleitet werden können. Die Validierung erfolgt an einem aktuellen Forschungsprojekt aus dem Bereich der spritzgussgerechten Konstruktion.Content of this thesis is the definition of a method for the development of support systems to safeguard simulations during the design process in CAD systems. The focus of the proce-dure is on the technical implementation in terms of model-based system development (MBSE). The method is divided into creating a knowledge base, system development using the System Modeling Language (SysML) and implementation into the CAD system. This guide is intended to support the developers of such systems in team communication and implementation, especially in the area of formalization and implementation of KBE systems. For qualification, practical examples of how different KBE techniques can be derived from a neutral system model are shown. The validation is carried out at a current research project in the field of injection-moulded part design

Robust Data-driven Metallicities for 175 Million Stars from Gaia XP Spectra

We derive and publish data-driven estimates of stellar metallicities [M/H] for 175 million stars with low-resolution XP spectra published in Gaia DR3. The [M/H] values, along with Teff and logg, are derived using the XGBoost algorithm, trained on stellar parameters from APOGEE, augmented by a set of very metal-poor stars. XGBoost draws on a number of data features: the full set of XP spectral coefficients, narrowband fluxes derived from XP spectra, and broadband magnitudes. In particular, we include CatWISE magnitudes, as they reduce the degeneracy of Teff and dust reddening. We also include the parallax as a data feature, which helps constrain logg and [M/H]. The resulting mean stellar parameter precision is 0.1 dex in [M/H], 50 K in Teff, and 0.08 dex in logg. This all-sky [M/H] sample is substantially larger than published samples of comparable fidelity across -3<[M/H]<+0.5. Additionally, we provide a catalog of over 17 million bright (G<16) red giants whose [M/H] are vetted to be precise and pure. We present all-sky maps of the Milky Way in different [M/H] regimes that illustrate the purity of the dataset, and demonstrate the power of this unprecedented sample to reveal the Milky Way's structure from its heart to its disk.Comment: 19 pages, 18 figures, accepted by ApJ

Non-local thermodynamic equilibrium stellar spectroscopy with 1D and 3D models - II. Chemical properties of the Galactic metal-poor disc and the halo

From exploratory studies and theoretical expectations it is known that simplifying approximations in spectroscopic analysis (LTE, 1D) lead to systematic biases of stellar parameters and abundances. These biases depend strongly on surface gravity, temperature, and, in particular, for LTE vs. non-LTE (NLTE) on metallicity of the stars. Here we analyse the [Mg/Fe] and [Fe/H] plane of a sample of 326 stars, comparing LTE and NLTE results obtained using 1D hydrostatic models and averaged models. We show that compared to the NLTE benchmark, all other three methods display increasing biases towards lower metallicities, resulting in false trends of [Mg/Fe] against [Fe/H], which have profound implications for interpretations by chemical evolution models. In our best NLTE model, the halo and disc stars show a clearer behaviour in the [Mg/Fe] - [Fe/H] plane, from the knee in abundance space down to the lowest metallicities. Our sample has a large fraction of thick disc stars and this population extends down to at least [Fe/H] ~ -1.6 dex, further than previously proven. The thick disc stars display a constant [Mg/Fe] ~ 0.3 dex, with a small intrinsic dispersion in [Mg/Fe] that suggests that a fast SN Ia channel is not relevant for the disc formation. The halo stars reach higher [Mg/Fe] ratios and display a net trend of [Mg/Fe] at low metallicities, paired with a large dispersion in [Mg/Fe]. These indicate the diverse origin of halo stars from accreted low-mass systems to stochastic/inhomogeneous chemical evolution in the Galactic halo.Comment: 15 pages, accepted for publication in Ap

A Gaia DR2 Mock Stellar Catalog

We present a mock catalog of Milky Way stars, matching in volume and depth the content of the Gaia data release 2 (GDR2). We generated our catalog using Galaxia, a tool to sample stars from a Besancon Galactic model, together with a realistic 3D dust extinction map. The catalog mimicks the complete GDR2 data model and contains most of the entries in the Gaia source catalog: 5-parameter astrometry, 3-band photometry, radial velocities, stellar parameters, and associated scaled nominal uncertainty estimates. In addition, we supplemented the catalog with extinctions and photometry for non-Gaia bands. This catalog can be used to prepare GDR2 queries in a realistic runtime environment, and it can serve as a Galactic model against which to compare the actual GDR2 data in the space of observables. The catalog is hosted through the virtual observatory GAVO's Heidelberg data center service and thus can be queried using ADQL as for GDR2 data.Comment: 8 pages, 7 figures, 9 queries, accepted by PASP, catalog is available from http://dc.g-vo.org/tableinfo/gdr2mock.main and topcat queriabl

A Gaia early DR3 mock stellar catalog: Galactic prior and selection function

We present a mock stellar catalog, matching in volume, depth and data model the content of the planned Gaia early data release 3 (Gaia EDR3). We have generated our catalog (GeDR3mock) using galaxia, a tool to sample stars from an underlying Milky Way (MW) model or from N-body data. We used an updated Besan\c{c}on Galactic model together with the latest PARSEC stellar evolutionary tracks, now also including white dwarfs. We added the Magellanic clouds and realistic open clusters with internal rotation. We empirically modelled uncertainties based on Gaia DR2 (GDR2) and scaled them according to the longer baseline in Gaia EDR3. The apparent magnitudes were reddened according to a new selection of 3D extinction maps. To help with the Gaia selection function we provide all-sky magnitude limit maps in G and BP for a few relevant GDR2 subsets together with the routines to produce these maps for user-defined subsets. We supplement the catalog with photometry and extinctions in non-Gaia bands. The catalog is available in the Virtual Observatory and can be queried just like the actual Gaia EDR3 will be. We highlight a few capabilities of the Astronomy Data Query Language (ADQL) with educative catalog queries. We use the data extracted from those queries to compare GeDR3mock to GDR2, which emphasises the importance of adding observational noise to the mock data. Since the underlying truth, e.g. stellar parameters, is know in GeDR3mock, it can be used to construct priors as well as mock data tests for parameter estimation. All code, models and data used to produce GeDR3mock are linked and contained in galaxia_wrap, a python package, representing a fast galactic forward model, able to project MW models and N-body data into realistic Gaia observables.Comment: 22 pages, 20 figures, accepted by PASP, catalog info and download and ADQL interface: http://dc.g-vo.org/tableinfo/gedr3mock.main ; relevant github repositories: https://github.com/jan-rybizki/Galaxia_wrap ; https://github.com/jan-rybizki/gdr2_completenes