Astrophysics

Young associations in star forming regions are stellar systems that allow us to understand the mechanisms that characterise the stars in their early life and what happens around them. In particular, the analysis of the disks and of the exoplanets around young stars allows us to know the key processes that prevail in their evolution and understand the properties of the exoplanets orbiting older stars. The TESS mission is giving us the opportunity to extract and analyse the light curves of association members with high accuracy, but the crowding that affects these regions makes difficult the light curve extraction. In the PATHOS project, cutting-edge tools are used to extract high-precision light curves and identify variable stars and transiting exoplanets in open clusters and associations. In this work, I analysed the light curves of stars in five young ( ≲10 Myr) associations, searching for variables and candidate exoplanets. By using the rotational periods of the association members, I constrained the ages of the five stellar systems ( ∼2 - 10 Myr). I searched for dippers and I investigated the properties of the dust that forms the circumstellar disks. Finally, I searched for transiting signals, finding 6 strong candidate exoplanets. No candidates with radius R P ≲0.9 R J have been detected, in agreement with the expectations. The frequency of giant planets resulted to be ∼ 2-3 %, higher than that expected for field stars ( ≲1 %); the low statistic makes this conclusion not strong, and new investigations on young objects are mandatory to confirm this result.

The original 110 year long 27-day Recurrence Index (original R27) was published more than forty years ago. That index, based on the autocorrelation of consecutive 27-day sets of the geomagnetic aa-index, is a measure of the cycle-to-cycle stability of high speed solar wind structure. During an effort to extend the index, it was discovered that the index could be significantly strengthened by pre-smoothing the geomagnetic aa-index listing used as input. A revised index (revised R27) is presented which clearly shows periods of long term stable solar wind structure toward the end of every sunspot cycle over the last 150 years. The extension of R27 over an interval including the greater part of the space age enables the updating of various studies of long-term solar wind variability based on R27, as well as comparison of R27 with more recently-developed solar-terrestrial parameters.

Whether any OB stars form in isolation is a question central to theories of massive star formation. To address this, we search for tiny, sparse clusters around 210 field OB stars from the Runaways and Isolated O-Type Star Spectroscopic Survey of the SMC (RIOTS4), using friends-of-friends (FOF) and nearest neighbors (NN) algorithms. We also stack the target fields to evaluate the presence of an aggregate density enhancement. Using several statistical tests, we compare these observations with three random-field datasets, and we also compare the known runaways to non-runaways. We find that the local environments of non-runaways show higher aggregate central densities than for runaways, implying the presence of some "tips-of-iceberg" (TIB) clusters. We find that the frequency of these tiny clusters is low, ∼4−5% of our sample. This fraction is much lower than some previous estimates, but is consistent with field OB stars being almost entirely runaway and walkaway stars. The lack of TIB clusters implies that such objects either evaporate on short timescales, or do not form, implying a higher cluster lower-mass limit and consistent with a relationship between maximum stellar mass ( m max ) and the mass of the cluster ( M cl ). On the other hand, we also cannot rule out that some OB stars may form in highly isolated conditions. Our results set strong constraints on the formation of massive stars in relative isolation.

We propose a theory for the MMRD relation of novae, using free-free emission model light curves built on the optically thick wind theory. We calculated ( t 3 , M V,max ) for various sets of ( M ˙ acc , M WD ) , where M V,max is the peak absolute V magnitude, t 3 is the 3-mag decay time from the peak, and M ˙ acc is the mass accretion rate on to the white dwarf (WD) of mass M WD . The model light curves are uniquely characterized by x≡ M env / M sc , where M env is the hydrogen-rich envelope mass and M sc is the scaling mass at which the wind has a certain wind mass-loss rate. For a given ignition mass M ig , we can specify the first point x 0 = M ig / M sc on the model light curve, and calculate the corresponding peak brightness and t 3 time from this first point. Our ( t 3 , M V,max ) points cover well the distribution of existing novae. The lower the mass accretion rate, the brighter the peak. The maximum brightness is limited to M V,max ≳−10.4 by the lowest mass-accretion rate of M ˙ acc ≳1× 10 −11 M ⊙ yr −1 . A significant part of the observational MMRD trend corresponds to the M ˙ acc ∼5× 10 −9 M ⊙ yr −1 line with different WD masses. A scatter from the trend line indicates a variation in their mass-accretion rates. Thus, the global trend of an MMRD relation does exist, but its scatter is too large for it to be a precision distance indicator of individual novae. We tabulate ( t 3 , M V,max ) for many sets of ( M ˙ acc , M WD ) .

We present a detailed three-dimensional (3D) view of a prominence eruption, coronal loop expansion, and coronal mass ejections (CMEs) associated with an M4.4 flare that occurred on 2011 March 8 in the active region NOAA 11165. Full-disk H α images of the flare and filament ejection were successfully obtained by the Flare Monitoring Telescope (FMT) following its relocation to Ica University, Peru. Multiwavelength observation around the H α line enabled us to derive the 3D velocity field of the H α prominence eruption. Features in extreme ultraviolet were also obtained by the Atmospheric Imager Assembly onboard the {\it Solar Dynamic Observatory} and the Extreme Ultraviolet Imager on board the {\it Solar Terrestrial Relations Observatory - Ahead} satellite. We found that, following collision of the erupted filament with the coronal magnetic field, some coronal loops began to expand, leading to the growth of a clear CME. We also discuss the succeeding activities of CME driven by multiple interactions between the expanding loops and the surrounding coronal magnetic field.

We report the discovery of the closest known black hole candidate as a binary companion to V723 Mon. V723 Mon is a nearby ( d??60pc ), bright ( V??.3 ~mag), evolved ( T eff,giant ??440 ~K, and L giant ??73 L ??) red giant in a high mass function, f(M)=1.72±0.01 M ??, nearly circular binary ( P=59.9 d, e?? ). V723 Mon is a known variable star, previously classified as an eclipsing binary, but its All-Sky Automated Survey (ASAS), Kilodegree Extremely Little Telescope (KELT), and Transiting Exoplanet Survey Satellite (TESS) light curves are those of a nearly edge-on ellipsoidal variable. Detailed models of the light curves constrained by the period, radial velocities and stellar temperature give an inclination of 87.0 ??+ 1.7 ????1.4 ??, a mass ratio of q??.33±0.02 , a companion mass of M comp =3.04±0.06 M ??, a stellar radius of R giant =24.9±0.7 R ??, and a giant mass of M giant =1.00±0.07 M ??. We identify a likely non-stellar, diffuse veiling component with contributions in the B and V -band of ??3% and ??4% , respectively. The SED and the absence of continuum eclipses imply that the companion mass must be dominated by a compact object. We do observe eclipses of the Balmer lines when the dark companion passes behind the giant, but their velocity spreads are low compared to observed accretion disks. The X-ray luminosity of the system is L X ??.6? 10 29 ergs s ?? , corresponding to L/ L edd ??10 ?? . The simplest explanation for the massive companion is a single compact object, most likely a black hole in the "mass gap".

We present results from a circular polarisation survey for radio stars in the Rapid ASKAP Continuum Survey (RACS). RACS is a survey of the entire sky south of δ=+ 41 ??being conducted with the Australian Square Kilometre Array Pathfinder telescope (ASKAP) over a 288 MHz wide band centred on 887.5 MHz. The data we analyse includes Stokes I and V polarisation products to an RMS sensitivity of 250 μ Jy PSF ?? . We searched RACS for sources with fractional circular polarisation above 6 per cent, and after excluding imaging artefacts, polarisation leakage, and known pulsars we identified radio emission coincident with 33 known stars. These range from M-dwarfs through to magnetic, chemically peculiar A- and B-type stars. Some of these are well known radio stars such as YZ CMi and CU Vir, but 23 have no previous radio detections. We report the flux density and derived brightness temperature of these detections and discuss the nature of the radio emission. We also discuss the implications of our results for the population statistics of radio stars in the context of future ASKAP and Square Kilometre Array surveys.

We aim to unravel the interior mixing profile of the pulsating eclipsing binary KIC9850387 by comparing its dynamical parameters and the parameters derived through a combination of evolutionary and asteroseismic modelling. We created a grid of stellar evolutionary models using the stellar evolutionary code \textsc{mesa} and performed an isochrone-cloud (isocloud) based evolutionary modelling of the system. We then generated a grid of pulsational models using the stellar pulsation code \textsc{gyre} based on the age constraints from the evolutionary modelling. Finally, we performed asteroseismic modelling of the observed ??1 and ??2 period-spacing patterns, utilising different combinations of observational constraints, merit functions, and asteroseismic observables to obtain strong constraints on the interior properties of the primary star. Through a combination of asteroseismic modelling and dynamical constraints, we found that the system comprises two main-sequence components at an age of 1.2±0.1 Gyr. We also observed that asteroseismic modelling provided stronger constraints on the interior properties than evolutionary modelling. Overall, we found high levels of interior mixing and posited that this is a result of intrinsic non-tidal mixing mechanisms due to a similar observed behaviour in single stars. We investigated the high-frequency regime of KIC9850387 and found evidence of the surface effect as well as rotational splitting in the form of a prograde-retrograde dipole g 1 mode doublet with a missing zonal mode. We find that the dynamical parameters and the parameters extracted from the asteroseismic modelling of period-spacing patterns are only barely compliant, reinforcing the need for homogeneous analyses of samples of pulsating eclipsing binaries that aim to calibrating interior mixing profiles.

This paper presents a database of the spectroscopic- and photometric- spectral energy distributions (spec-SEDs and phot-SEDs) of the progenitors of core-collapse supernovae (CCSNe). Both binary- and single-star progenitors are included in the database. The database covers the initial metallicity ( Z ) range of 0.0001--0.03, mass range of 8--25 \dsm{}, binary mass ratio range of 0--1, and orbital period range of 0.1--10000\,days. The low-resolution spec-SEDs and phot-SEDs of single- and binary-star CCSN progenitors are included in the database. These data can be used for studying the basic parameters, e.g., metallicity, age, initial and final masses of CCSN progenitors. It can also be used for studying the effects of different factors on the determination of parameters of CCSN progenitors. When the database is used for fitting the SEDs of binary-star CCSN progenitors, it is strongly suggested to determine the metallicity and orbital period in advance, while it is not necessary for single-star progenitors.

We construct from Gaia eDR3 an extensive catalog of spatially resolved binary stars within ??1 kpc of the Sun, with projected separations ranging from a few au to 1 pc. We estimate the probability that each pair is a chance alignment empirically, using the Gaia catalog itself to calculate the rate of chance alignments as a function of observables. The catalog contains 1.3 (1.1) million binaries with >90% (>99%) probability of being bound, including 16,000 white dwarf -- main sequence (WD+MS) binaries and 1,400 WD+WD binaries. We make the full catalog publicly available, as well as the queries and code to produce it. We then use this sample to calibrate the published Gaia DR3 parallax uncertainties, making use of the binary components' near-identical parallaxes. We show that these uncertainties are generally reliable for faint stars ( G??8 ), but are underestimated significantly for brighter stars. The underestimates are generally ??0% for isolated sources with well-behaved astrometry, but are larger (up to 80%) for apparently well-behaved sources with a companion within ?? arcsec, and much larger for sources with poor astrometric fits. We provide an empirical fitting function to inflate published ? ? values for isolated sources. The public catalog offers wide ranging follow-up opportunities: from calibrating spectroscopic surveys, to precisely constraining ages of field stars, to the masses and the initial-final mass relation of white dwarfs, to dynamically probing the Galactic tidal field.