OA@INAF - Istituto Nazionale di Astrofisica
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FitteR for Accretion ProPErties of T Tauri stars (FRAPPE): A new approach to use class III spectra to derive stellar and accretion properties
Full text linkContext. Studies of the stellar and accretion properties of classical T Tauri stars (CTTS) require photospheric spectral templates to be compared with. The use of low-activity, slowly rotating field dwarfs or model spectra can be advantageous for the determination of stellar parameters, but it can lead to an overestimate of the mass accretion rate, since both classes of templates do not include the emission of the active chromosphere present in young stars. Observed spectra of non-accreting young stars are best suited to this purpose. Using such templates comes with the downside of a limited number of available templates and observational uncertainties on the properties of the templates. Aims. For this work, we aimed to expand the currently available grid of wide-wavelength coverage observed spectra of non-accreting stars with additional new spectra and an interpolation method that allowed us to obtain a continuous grid of low resolution spectra ranging from spectral type G8 to M9.5, while also mitigating observational uncertainties. This interpolated grid was then implemented in the self-consistent method to derive stellar and accretion properties of CTTS. With the new templates, we aimed to estimate a lower limit on the accretion luminosities that can be obtained through a study of the UV excess emission using observed templates. Methods. We analyzed the molecular photospheric features present in the VLT/X-shooter spectra of the targets to perform a spectral classification, including estimates of their extinction. We applied a non-parametric fitting method to the full grid of observed templates to obtain an interpolated grid of templates. Both the individual templates and interpolated grid are provided to the community. We implemented this grid to improve the method to self-consistently derive stellar and accretion properties of accreting stars. We used the uncertainties on our interpolated grid to estimate a lower limit on the accretion luminosity that we can measure with this method. Results. Our new method, which uses a continuous grid of templates, provides results that are consistent with using individual templates but it significantly improves the reliability of the results in the case of degeneracies associated with the peculiarities of individual observed templates. We find that the measurable accretion luminosities range from ~2.7 dex lower than the stellar luminosity in M5.5 stars to ~1.3 dex lower for G8 stars. For young stars with masses of ~1 M⊙ and ages of 3- 6 Myr this limit translates into an observational limit of the mass accretion rate on the order of 10-10 M⊙/yr. This limit is higher than the lower limit on the measurable mass accretion rate when using the various emission lines present in the spectra of young stars to estimate the accretion rate. An analysis of these emission lines allows us to probe lower accretion rates, pending a revised calibration of the relationships between line and accretion luminosities at low accretion rates. Conclusions. The implementation of an interpolated grid of observed templates allows us to better disentangle degenerate solutions, leading to a more reliable estimate of accretion rates in young accreting stars
E-XQR-30: The evolution of Mg II, C II, and O I across 2 < z < 6
Full text linkIntervening metal absorbers in quasar spectra at z > 6 can be used as probes to study the chemical enrichment of the Universe during the Epoch of Reionization. This work presents the comoving line densities (dn/dX) of low-ionization absorbers, namely, Mg II (2796 Å), C II (1334 Å), and O I (1302 Å) across 2 0.03 Å. For the first time, we present the comoving line densities of 131 weak (W 5, in agreement with previous works. In the context of a decline in the metal enrichment of the Universe at z > 5, the overall evolution in the incidence rates of absorption systems can be explained by a weak - possibly soft fluctuating - ultraviolet background. Our results, thereby, provide evidence for a late reionization continuing to occur in metal-enriched and therefore, biased regions in the Universe
Negative cosmological constant in the dark energy sector: tests from JWST photometric and spectroscopic observations of high-redshift galaxies
Full text linkEarly observations with the James Webb Space Telescope (JWST) have revealed the existence of an unexpectedly large abundance of extremely massive galaxies at redshifts z ≳ 5: these are in tension with the predictions not only of the standard ΛCDM cosmology, but also with those of a wide class of dynamical dark energy (DE) models, and are generally in better agreement with models characterized by a phantom behaviour. Here we consider a model, inspired by string theory and the ubiquity of anti-de Sitter vacua therein, featuring an evolving DE component with positive energy density on top of a negative cosmological constant, argued in an earlier exploratory analysis to potentially be able to explain the JWST observations. We perform a robust comparison of this model against JWST data, considering both photometric observations from the CEERS program, and spectroscopic observations from the FRESCO survey. We show that the model is able to accommodate the JWST observations, with a consistency probability of up to 98%, even in the presence of an evolving component with a quintessence-like behaviour (easier to accommodate theoretically compared to phantom DE), while remaining consistent with standard low-redshift probes. Our results showcase the potential of measurements of high-redshift galaxy abundances in tests of fundamental physics, and their complementarity with standard cosmological probes
Age Determination of LAMOST Red Giant Branch Stars Based on the Gradient Boosting Decision Tree Method
Full text linkIn this study, we estimate the stellar ages of LAMOST DR8 red giant branch (RGB) stars based on the gradient boosting decision tree (GBDT) algorithm. We used 2643 RGB stars extracted from the APOKASC-2 asteroseismological catalog as the training data set. After selecting the parameters ([α/Fe], [C/Fe], T eff, [N/Fe], [C/H], log g) highly correlated with age using GBDT, we apply the same GBDT method to the new catalog of more than 590,000 stars classified as RGB stars. The test data set shows that the median relative error is around 11.6% for the method. We also compare the predicted ages of RGB stars with other studies (e.g., based on APOGEE) and find some systematic differences. The final uncertainty is about 15%-30% compared to the ages of open clusters. Then, we present the spatial distribution of the RGB sample with an age determination, which could recreate the expected result, and discuss systematic biases. All these diagnostics show that one can apply the GBDT method to other stellar samples to estimate atmospheric parameters and age
Chronology of our Galaxy from Gaia colour-magnitude diagram fitting (ChronoGal): I. the formation and evolution of the thin disc from the Gaia Catalogue of Nearby Stars
Full text linkContext. The study of the Milky Way is living a golden era thanks to the enormous high-quality datasets delivered by Gaia, and space asteroseismic and ground-based spectroscopic surveys. However, the current major challenge to reconstructing the chronology of the Milky Way is the difficulty to derive precise stellar ages for large samples of stars. The colour-magnitude diagram (CMD) fitting technique offers an alternative to individual age determinations to derive the star formation history (SFH) of complex stellar populations. Aims. Our aim is to obtain a detailed dynamically evolved SFH (deSFH) of the solar neighbourhood, and the age and metallicity distributions that result from it. We define deSFH as the amount of mass transformed into stars, as a function of time and metallicity, in order to account for the population of stars contained in a particular volume. Methods. We present a new package to derive SFHs from CMD fitting tailored to work with Gaia data, called CMDft.Gaia, and we use it to analyse the CMD of the Gaia Catalogue of Nearby Stars (GCNS), which contains a complete census of the (mostly thin disc) stars currently within 100 pc of the Sun. Results. We present an unprecedentedly detailed view of the evolution of the Milky Way disc at the solar radius. The bulk of star formation started 11-10.5 Gyr ago at metallicity around solar, and continued with a slightly decreasing metallicity trend until 6 Gyr ago. Between 6 and 4 Gyr ago, a notable break in the age-metallicity distribution is observed, with three stellar populations with distinct metallicities (sub-solar, solar, and super-solar), possibly indicating some dramatic event in the life of our Galaxy. Star formation then resumed 4 Gyr ago with a somewhat bursty behaviour, metallicity near solar and average star formation rate higher than in the period before 6 Gyr ago. The derived metallicity distribution closely matches precise spectroscopic data, which also show stellar populations deviating from solar metallicity. Interestingly, our results reveal the presence of intermediate-age populations exhibiting both a metallicity typical of the thick disc, approximately [M/H]-0.5, and super-solar metallicity. Conclusions. The many tests performed indicate that, with high-precision photometric and distance data such as that provided by Gaia, CMDft.Gaia is able to achieve a precision of ≲ 10% and an accuracy better than 6% in the dating of stellar populations, even at old ages. A comparison with independent spectroscopic metallicity information shows that metallicity distributions are also determined with high precision, without imposing any a priori metallicity information in the fitting process. This opens the door to obtaining detailed and robust information on the evolution of the stellar populations of the Milky Way over cosmic time. As an example, we provide in this paper an unprecedentedly detailed view of the age and metallicity distributions of the stars within 100 pc of the Sun
He abundance in NGC 1850 A and B: Are we observing the early stage of the formation of multiple populations in a stellar cluster?
Full text linkWe present the result of a sample of B-stars in the Large Magellanic Cloud
young double stellar cluster NGC 1850 A and NGC 1850 B, observed with the
integral-field spectrograph at the Very Large Telescope, the Multi Unit
Spectroscopic Explorer. We compare the observed equivalent widths (EWs) of four
He lines (4922 , 5015 , 6678
, and 7065 ) with the ones
determined from synthetic spectra computed with different He mass fraction
(Y=0.25, 0.27, 0.30 and 0.35) with the code SYNSPEC, that takes into account
the non-LTE effect. From this comparison, we determined the He mass fraction of
the B stars, finding a not homogeneous distribution. The stars can be divided
in three groups, He-weak (Y 0.24) and the He-normal (0.24 Y
0.26) belonging to the MS of NGC 1850 A, and the He-rich stars
(0.33 Y 0.38) situated in the MS associated to NGC 1850
B. We have analyzed the stellar rotation as possible responsible of the
anomalous features of the He lines in the He-rich stars. We provide a simple
analysis of the differences between the observed EWs and the ones obtained from
the theoretical models with different rotation velocity (V = 0 and 250
Km/s). The resolution of the MUSE spectra do not allow to get a conclusive
result, however our analysis support the He-enhanced hypothesis
Rapid Dimming Followed by a State Transition: A Study of the Highly Variable Nuclear Transient AT 2019avd over 1000+ Days
Full text linkThe tidal disruption of a star around a supermassive black hole (SMBH) offers a unique opportunity to study accretion onto an SMBH on a human timescale. We present results from our 1000+ days monitoring campaign of AT 2019avd, a nuclear transient with tidal-disruption-event-like properties, with NICER, Swift, and Chandra. Our primary finding is that approximately 225 days following the peak of the X-ray emission, there is a rapid drop in luminosity exceeding 2 orders of magnitude. This X-ray dropoff is accompanied by X-ray spectral hardening, followed by a plateau phase of 740 days. During this phase, the spectral index decreases from 6.2 ± 1.1 to 2.3 ± 0.4, while the disk temperature remains constant. Additionally, we detect pronounced X-ray variability, with an average fractional rms amplitude of 47%, manifesting over timescales of a few dozen minutes. We propose that this phenomenon may be attributed to intervening clumpy outflows. The overall properties of AT 2019avd suggest that the accretion disk evolves from a super-Eddington to a sub-Eddington luminosity state, possibly associated with a compact jet. This evolution follows a pattern in the hardness–intensity diagram similar to that observed in stellar-mass BHs, supporting the mass invariance of accretion–ejection processes around BHs
Comparing Recent Pulsar Timing Array Results on the Nanohertz Stochastic Gravitational-wave Background
Full text linkThe Australian, Chinese, European, Indian, and North American pulsar timing array (PTA) collaborations recently reported, at varying levels, evidence for the presence of a nanohertz gravitational-wave background (GWB). Given that each PTA made different choices in modeling their data, we perform a comparison of the GWB and individual pulsar noise parameters across the results reported from the PTAs that constitute the International Pulsar Timing Array (IPTA). We show that despite making different modeling choices, there is no significant difference in the GWB parameters that are measured by the different PTAs, agreeing within 1σ. The pulsar noise parameters are also consistent between different PTAs for the majority of the pulsars included in these analyses. We bridge the differences in modeling choices by adopting a standardized noise model for all pulsars and PTAs, finding that under this model there is a reduction in the tension in the pulsar noise parameters. As part of this reanalysis, we "extended" each PTA's data set by adding extra pulsars that were not timed by that PTA. Under these extensions, we find better constraints on the GWB amplitude and a higher signal-to-noise ratio for the Hellings–Downs correlations. These extensions serve as a prelude to the benefits offered by a full combination of data across all pulsars in the IPTA, i.e., the IPTA's Data Release 3, which will involve not just adding in additional pulsars but also including data from all three PTAs where any given pulsar is timed by more than a single PTA...
The SRG/eROSITA all-sky survey. First X-ray catalogues and data release of the western Galactic hemisphere
Full text linkThe eROSITA telescope array aboard the Spektrum Roentgen Gamma (SRG) satellite began surveying the sky in December 2019, with the aim of producing all-sky X-ray source lists and sky maps of an unprecedented depth. Here we present catalogues of both point-like and extended sources using the data acquired in the first six months of survey operations (eRASS1; completed June 2020) over the half sky whose proprietary data rights lie with the German eROSITA Consortium. We describe the observation process, the data analysis pipelines, and the characteristics of the X-ray sources. With nearly 930 000 entries detected in the most sensitive 0.2-2.3 keV energy range, the eRASS1 main catalogue presented here increases the number of known X-ray sources in the published literature by more than 60%, and provides a comprehensive inventory of all classes of X-ray celestial objects, covering a wide range of physical processes. A smaller catalogue of 5466 sources detected in the less sensitive but harder 2.3-5 keV band is the result of the first true imaging survey of the entire sky above 2 keV. We present methods to identify and flag potential spurious sources in the catalogues, which we applied for this work, and we tested and validated the astrometric accuracy via cross-comparison with other X-ray and multi-wavelength catalogues. We show that the number counts of X-ray sources in eRASSl are consistent with those derived over narrower fields by past X-ray surveys of a similar depth, and we explore the number counts variation as a function of the location in the sky. Adopting a uniform all-sky flux limit (at 50% completeness) of F05-2 keV > 5 × 10−14 erg s−1 cm−2, we estimate that the eROSITA all-sky survey resolves into individual sources about 20% of the cosmic X-ray background in the 1-2 keV range. The catalogues presented here form part of the first data release (DR1) of the SRG/eROSITA all-sky survey. Beyond the X-ray catalogues, DR1 contains all detected and calibrated event files, source products (light curves and spectra), and all-sky maps. Illustrative examples of these are provided. The catalogue is available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/682/A34</A
The ALMaQUEST Survey XI: a strong but non-linear relationship between star formation and dynamical equilibrium pressure
Full text linkWe present the extended ALMA MaNGA QUEnching and STar formation survey (ALMaQUEST), a combination of the original 46 ALMaQUEST galaxies plus new ALMA observations for a further 20 interacting galaxies. Three well-studied scaling relations are fit to the 19 999 star-forming spaxels in the extended sample, namely the resolved Schmidt-Kennicutt relation, the resolved star-forming main-sequence and the resolved molecular gas main sequence. We additionally investigate the relationship between the dynamical equilibrium pressure (PDE) and star formation rate surface density (ΣSFR), which we refer to as the resolved PDE (rPDE) relation. Contrary to previous studies that have focussed on normal star-forming galaxies and found an approximately linear rPDE relation, the presence of more vigourously star-forming galaxies in the extended ALMaQUEST sample reveals a marked turnover in the relation at high pressures. Although the scatter around the linear fit to the rPDE relation is similar to the other three relations, a random forest analysis, which can extract non-linear dependences, finds that PDEis unambiguously more important than either or Σ⋆ for predicting ΣSFR. We compare the observed rPDE relation to the prediction of the pressure-regulated feedback-modulated (PRFM) model of star formation, finding that galaxies residing on the global SFMS do indeed closely follow the rPDE relation predicted by the PRFM theory. However, galaxies above and below the global SFMS show significant deviations from the model. Galaxies with high SFR are instead consistent with models that include other contributions to turbulence in addition to the local star formation feedback