ABSTRACT
The detection of X-ray emission constitutes a reliable and efficient tool for the selection of active galactic nuclei (AGNs), although it may be biased against the most heavily absorbed ...AGNs. Simple mid-infrared (IR) broad-band selection criteria identify a large number of luminous and absorbed AGNs, yet again host contamination could lead to non-uniform and incomplete samples. Spectral energy distribution (SED) decomposition is able to decouple the emission from the AGN versus that from star-forming regions, revealing weaker AGN components. We aim to identify the obscured AGN population in the VIMOS Public Extragalactic Redshift Survey in the Canada–France–Hawaii Telescope Legacy Survey W1 field through SED modelling. We construct SEDs for 6860 sources and identify 160 AGNs at a high confidence level using a Bayesian approach. Using optical spectroscopy, we confirm the nature of ∼85 per cent of the AGNs. Our AGN sample is highly complete (∼92 per cent) compared to mid-IR colour-selected AGNs, including a significant number of galaxy-dominated systems with lower luminosities. In addition to the lack of X-ray emission (80 per cent), the SED fitting results suggest that the majority of the sources are obscured. We use a number of diagnostic criteria in the optical, IR, and X-ray regimes to verify these results. Interestingly, only 35 per cent of the most luminous mid-IR-selected AGNs have X-ray counterparts suggesting strong absorption. Our work emphasizes the importance of using SED decomposition techniques to select a population of type II AGNs, which may remain undetected by either X-ray or IR colour surveys.
Photometric measurements are prone to systematic errors presenting a challenge to low-amplitude variability detection. In search for a general-purpose variability detection technique able to recover ...a broad range of variability types including currently unknown ones, we test 18 statistical characteristics quantifying scatter and/or correlation between brightness measurements. We compare their performance in identifying variable objects in seven time series data sets obtained with telescopes ranging in size from a telephoto lens to 1 m-class and probing variability on time-scales from minutes to decades. The test data sets together include light curves of 127 539 objects, among them 1251 variable stars of various types and represent a range of observing conditions often found in ground-based variability surveys. The real data are complemented by simulations. We propose a combination of two indices that together recover a broad range of variability types from photometric data characterized by a wide variety of sampling patterns, photometric accuracies and percentages of outlier measurements. The first index is the interquartile range (IQR) of magnitude measurements, sensitive to variability irrespective of a time-scale and resistant to outliers. It can be complemented by the ratio of the light-curve variance to the mean square successive difference, 1/..., which is efficient in detecting variability on time-scales longer than the typical time interval between observations. Variable objects have larger 1/... and/or IQR values than non-variable objects of similar brightness. Another approach to variability detection is to combine many variability indices using principal component analysis. We present 124 previously unknown variable stars found in the test data. (ProQuest: ... denotes formulae/symbols omitted.)
The high mass-loss rates of red supergiants (RSGs) drastically affect their evolution and final fate, but their mass-loss mechanism remains poorly understood. Various empirical prescriptions scaled ...with luminosity have been derived in the literature, yielding results with a dispersion of two to three orders of magnitude. We determine an accurate mass-loss rate relation with luminosity and other parameters using a large, clean sample of RSGs. In this way, we shed light into the underlying physical mechanism and explain the discrepancy between previous works. We assembled a sample of 2,219 RSG candidates in the Large Magellanic Cloud, with ultraviolet to mid-infrared photometry in up to 49 filters. We determined the luminosity of each RSG by integrating the spectral energy distribution and the mass-loss rate using the radiative transfer code DUSTY Our derived RSG mass-loss rates range from approximately $10^ odot $ yr$^ $ to $10^ odot $ yr$^ $, mainly depending on the luminosity. The average mass-loss rate is $9.3 odot $ yr$^ $ for $ (L/L_ odot) >4$, corresponding to a dust-production rate of $ odot $ yr$^ $. We established a mass-loss rate relation as a function of luminosity and effective temperature. Furthermore, we found a turning point in the relation of mass-loss rate versus luminosity at approximately $ (L/L_ odot) = 4.4$, indicating enhanced rates beyond this limit. We show that this enhancement correlates with photometric variability. We compared our results with prescriptions from the literature, finding an agreement with works assuming steady-state winds. Additionally, we examined the effect of different assumptions on our models and found that radiatively driven winds result in mass-loss rates higher by two to three orders of magnitude, which is unrealistically high for RSGs. For grain sizes $<0.1 \ the predicted mass-loss rates are higher by a factor of 25-30 than larger grain sizes. Finally, we found that 21<!PCT!> of our sample constitute current binary candidates. This has a minor effect on our mass-loss relation.
We present results of high-cadence monitoring of the optical light curve of the nearby, Type Ia SN 2014J in M 82, using the 2.3 m Aristarchos telescope. B and V-band photometry on days 15–18 after ...tmax(B) was obtained with a cadence of 2 min per band, revealing evidence for rapid variability at the 0.02–0.05 mag level on timescales of 15–60 min on all four nights. The decline slope was measured as steeper in the B-band than in the V-band, and to steadily decrease in both bands from 0.15 mag day-1 (night 1) to 0.04 mag day-1 (night 4) in V, and from 0.19 mag day-1 (night 1) to 0.06 mag day-1 (night 4) in B, corresponding to the onset of the secondary maximum. We propose that rapid variability could be due to one or a combination of the following scenarios: the clumpiness of the ejecta, their interaction with circumstellar material, the asymmetry of the explosion, or the mechanism causing the secondary maximum in the near-infrared light curve. We encourage the community to undertake high-cadence monitoring of future, nearby and bright supernovae to investigate the intraday behaviour of their light curves.
We present a catalog of 1750 massive stars in the Large Magellanic Cloud (LMC), with accurate spectral types compiled from the literature, and a photometric catalog for a subset of 1268 of these ...stars, with the goal of exploring their infrared properties. The photometric catalog consists of stars with infrared counterparts in the Spitzer SAGE survey database, for which we present uniform photometry from 0.3 to 24 Delta *mm in the UBVIJHKs +IRAC+MIPS24 bands. The resulting infrared color-magnitude diagrams illustrate that the supergiant Be, red supergiant, and luminous blue variable (LBV) stars are among the brightest infrared point sources in the LMC, due to their intrinsic brightness, and at longer wavelengths, due to dust. We detect infrared excesses due to free-free emission among ~900 OB stars, which correlate with luminosity class. We confirm the presence of dust around 10 supergiant Be stars, finding the shape of their spectral energy distributions (SEDs) to be very similar, in contrast to the variety of SED shapes among the spectrally variable LBVs. The similar luminosities of Be supergiants (log L/L >= 4) and the rare, dusty progenitors of the new class of optical transients (e.g., SN 2008S and NGC 300 OT), plus the fact that dust is present in both types of objects, suggests a common origin for them. We find the infrared colors for Wolf-Rayet stars to be independent of spectral type and their SEDs to be flatter than what models predict. The results of this study provide the first comprehensive roadmap for interpreting luminous, massive, resolved stellar populations in nearby galaxies at infrared wavelengths.
We present results of the Near-Earth objects Lunar Impacts and Optical TrAnsients (NELIOTA) campaign for lunar impact flashes observed with the 1.2 m Kryoneri telescope. From August 2019 to August ...2023, we report 113 validated and 70 suspected flashes. For the validated flashes, we calculate the physical parameters (masses, radii) of the corresponding projectiles, the temperatures developed during the impacts, and the expected crater sizes. For the multiframe flashes, we present light curves and thermal evolution plots. Using the whole sample of NELIOTA that encompasses 192 validated flashes in total from 2017, the statistics of the physical parameters of the meteoroids, the peak temperatures of the impacts, and the expected crater sizes has been updated. Using this large sample, empirical relations correlating the luminous energies per photometric band were derived and used to roughly estimate the parameters of 92 suspected flashes of the NELIOTA archive. For a typical value of the luminous efficiency, we found that the majority (>75%) of the impacting meteoroids have masses between 1 and 200 g, radii between 0.5 and 3 cm and produced craters up to 3.5 m. 85% of the peak temperatures of the impacts range between 2000 and 4500 K. Statistics regarding the magnitude decline and the cooling rates of the multiframe flashes are also presented. The recalculation of the appearance frequency of meteoroids (lying within the aforementioned ranges of physical parameters) on the Moon yields that the total lunar surface is bombarded with 7.4 sporadic meteoroids per hour and up to 12.6 meteoroids per hour when the Earth-Moon system passes through a strong meteoroid stream. By extrapolating these rates on Earth, the respective rates for various distances from its surface are calculated and used to estimate the probability of an impact of a meteoroid with a hypothetical infrastructure on the Moon, or with a satellite orbiting Earth for various impact surfaces and duration times of the missions.
We present the technical specifications and first results of the ESA-funded, lunar monitoring project “NELIOTA” (NEO Lunar Impacts and Optical TrAnsients) at the National Observatory of Athens, which ...aims to determine the size-frequency distribution of small near-Earth objects (NEOs) via detection of impact flashes on the surface of the Moon. For the purposes of this project a twin camera instrument was specially designed and installed at the 1.2 m Kryoneri telescope utilizing the fast-frame capabilities of scientific Complementary Metal-Oxide Semiconductor detectors (sCMOS). The system provides a wide field-of-view (17.0′ × 14.4′) and simultaneous observations in two photometric bands (R and I), reaching limiting magnitudes of 18.7 mag in 10 s in both bands at a 2.5 signal-to-noise ratio (S/N) level. This makes it a unique instrument that can be used for the detection of NEO impacts on the Moon, as well as for any astronomy projects that demand high-cadence multicolor observations. The wide field-of-view ensures that a large portion of the Moon is observed, while the simultaneous, high-cadence, monitoring in two photometric bands makes possible the determination of the temperatures of the impacts on the Moon’s surface and the validation of the impact flashes from a single site. Considering the varying background level on the Moon’s surface we demonstrate that the NELIOTA system can detect NEO impact flashes at a 2.5 S/N level of ∼12.4 mag in the I-band and R-band for observations made at low lunar phases (∼0.1). We report 31 NEO impact flashes detected during the first year of the NELIOTA campaign. The faintest flash was at 11.24 mag in the R-band (about two magnitudes fainter than ever observed before) at lunar phase 0.32. Our observations suggest a detection rate of 1.96 × 10−7 events km−2 h−1.
Context. Only a small number of high mass stars (>30 M⊙) have fundamental parameters (i.e. masses and radii) measured with high enough accuracy from eclipsing binaries to constrain formation and ...evolutionary models of massive stars. Aims. This work aims to increase this limited sample, by studying the four massive eclipsing binary candidates discovered by Bonanos in the young massive cluster Westerlund 1. Methods. We present new follow-up echelle spectroscopy of these binaries and models of their light and radial velocity curves. Results. We obtain fundamental parameters for the eight component stars, finding masses that span a range of 10−40 M⊙, and contributing accurate fundamental parameters for one additional very massive star, the 33 M⊙ component of W13. WR77o is found to have a ~40 M⊙ companion, which provides a second dynamical constraint on the mass of the progenitor of the magnetar known in the cluster. We also use W13 to estimate the first, direct, eclipsing binary distance to Westerlund 1 and therefore the magnetar and find it to be at 3.7 ± 0.6 kpc. Conclusions. Our results confirm previous evidence for a high mass for the progenitor of the magnetar. In addition, the availability of eclipsing binaries with accurate parameters opens the way for direct, independent, high precision eclipsing binary distance measurements to Westerlund 1.
We present a catalog of 5324 massive stars in the Small Magellanic Cloud (SMC), with accurate spectral types compiled from the literature, and a photometric catalog for a subset of 3654 of these ...stars, with the goal of exploring their infrared properties. The photometric catalog consists of stars with infrared counterparts in the Spitzer SAGE-SMC survey database, for which we present uniform photometry from 0.3to24 Delta *mm in the UBVIJHKs +IRAC+MIPS24 bands. We compare the color-magnitude diagrams and color-color diagrams to those of stars in the Large Magellanic Cloud (LMC), finding that the brightest infrared sources in the SMC are also the red supergiants, supergiant Be (sgBe) stars, luminous blue variables, and Wolf-Rayet stars, with the latter exhibiting less infrared excess, the red supergiants being less dusty and the sgBe stars being on average less luminous. Among the objects detected at 24 Delta *mm in the SMC are a few very luminous hypergiants, four B-type stars with peculiar, flat spectral energy distributions, and all three known luminous blue variables. We detect a distinct Be star sequence, displaced to the red, and suggest a novel method of confirming Be star candidates photometrically. We find a higher fraction of Oe and Be stars among O and early-B stars in our SMC catalog, respectively, when compared to the LMC catalog, and that the SMC Be stars occur at higher luminosities. We estimate mass-loss rates for the red supergiants, confirming the correlation with luminosity even at the metallicity of the SMC. Finally, we confirm the new class of stars displaying composite A & F type spectra, the sgBe nature of 2dFS1804 and find the F0 supergiant 2dFS3528 to be a candidate luminous blue variable with cold dust.
Studies of the massive star population in galaxies beyond the Local Group are the key to understanding the link between their numbers and modes of star formation in different environments. We present ...the analysis of the massive star population of the galaxies NGC 1326A, NGC 1425, and NGC 4548 using archival images obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 in the F555W and F814W filters. Through high-precision point spread function fitting photometry for all sources in the three fields, we identified 7640 candidate blue supergiants, 2314 candidate yellow supergiants, and 4270 candidate red supergiants. We provide an estimate of the ratio of blue to red supergiants for each field as a function of galactocentric radius. Using Modules for Experiments in Stellar Astrophysics (MESA) at solar metallicity, we defined the luminosity function and estimated the star formation history of each galaxy. We carried out a variability search in the V and I filters using three variability indexes: the median absolute deviation, the interquartile range, and the inverse von Neumann ratio. This analysis yielded 243 new variable candidates with absolute magnitudes ranging from MV = −4 to −10 mag. We classified the variable stars based on their absolute magnitude and their position on the color–magnitude diagram using the MESA evolutionary tracks at solar metallicity. Our analysis yielded 8 candidate variable blue supergiants, 12 candidate variable yellow supergiants, 21 candidate variable red supergiants, and 4 candidate periodic variables.
PDF
