arXiv subjects

Emanuele Bertone

Publications and source records attributed to Emanuele Bertone.

At least 19 recordsLinked to original sources

arXiv2025

GALEX UV survey of magnetic chemically peculiar stars in the Kepler prime field

Magnetic chemically peculiar (mCP) stars are strongly magnetic upper main-sequence stars that exhibit light rotational variability due to an uneven surface distribution of certain peculiar elements, which may appear in phase at certain wavelengths and in antiphase to the flux at other wavelengths. We present a study of the properties of photometric variability of a sample of confirmed mCP stars (mostly Ap/CP2 stars), mCP star candidates, and several non-CP stars in the near ultraviolet and visible wavelength regions based on observational data from the GALEX and Kepler prime missions. Antiphase variations between the near ultraviolet and optical light curves are observed in the majority of mCP stars. We investigate the presence of a correlation of the variability amplitudes in both wavelength regions with effective temperature, surface gravity, and metallicity and calculate model atmospheres, spectral energy distributions and synthetic light curves to connect our findings to theoretical models. While the theoretical calculations show that, at fixed abundances, a clear correlation between the light curve amplitude ratios and effective temperature is expected, our sample does not show any correlation with the investigated properties. This may be due to the highly individualistic abundance patterns of our sample stars, which are the main contributors to the line blanketing in different wavelength bands.

astro-ph.SR↗

arXiv2020

Synthetic spectroscopic indices for identifying multiple stellar populations in globular clusters

We present an investigation of synthetic spectroscopic indices that can plausibly help in identifying the presence of multiple stellar populations in globular clusters. The study is based on a new grid of stellar model atmospheres and high-resolution (R=500,000) synthetic spectra, that consider chemical partitions that have been singled out in Galactic globular clusters. The database is composed of 3472 model atmospheres and theoretical spectra calculated with the collection of Fortran codes DFSYNTHE, ATLAS9 and SYNTHE, developed by Robert L. Kurucz. They cover a range of effective temperature from 4300 to 7000 K, surface gravity from 2.0 to 5.0 dex and four different chemical compositions. A set of 19 spectroscopic indices were calculated from a degraded version (R=2500) of the theoretical spectra dataset. The set includes five indices previously used in the context of globular clusters analyses and 14 indices that we have newly defined by maximizing the capability of differentiating the chemical compositions. We explored the effects of atmospheric parameters on the index values and identified the optimal spectral diagnostics that allow to trace the signatures of objects of different stellar populations, located in the main sequence, the red giant branch and the horizontal branch. We found a suitable set of indices, that mostly involve molecular bands (in particular NH, but also CH and CN), that are very promising for spectroscopically identifying multiple stellar populations in globular clusters.

astro-ph.SR↗

arXiv2017

The GALEX View of "Boyajian's Star" (KIC 8462852)

The enigmatic star KIC 8462852, informally known as "Boyajian's Star", has exhibited unexplained variability from both short timescale (days) dimming events, and years-long fading in the Kepler mission. No single physical mechanism has successfully explained these observations to date. Here we investigate the ultraviolet variability of KIC 8462852 on a range of timescales using data from the GALEX mission that occurred contemporaneously with the Kepler mission. The wide wavelength baseline between the Kepler and GALEX data provides a unique constraint on the nature of the variability. Using 1600 seconds of photon-counting data from four GALEX visits spread over 70 days in 2011, we find no coherent NUV variability in the system on 10-100 second or months timescales. Comparing the integrated flux from these 2011 visits to the 2012 NUV flux published in the GALEX-CAUSE Kepler survey, we find a 3% decrease in brightness for KIC 8462852. We find this level of variability is significant, but not necessarily unusual for stars of similar spectral type in the GALEX data. This decrease coincides with the secular optical fading reported by Montet & Simon (2016). We find the multi-wavelength variability is somewhat inconsistent with typical interstellar dust absorption, but instead favors a R$_V$ = 5.0 $\pm$ 0.9 reddening law potentially from circumstellar dust.

astro-ph.SR↗

arXiv2017

Multiple component decomposition from millimeter single-channel data

We present an implementation of a blind source separation algorithm to remove foregrounds off millimeter surveys made by single-channel instruments. In order to make possible such a decomposition over single-wavelength data: we generate levels of artificial redundancy, then perform a blind decomposition, calibrate the resulting maps, and lastly measure physical information. We simulate the reduction pipeline using mock data: atmospheric fluctuations, extended astrophysical foregrounds, and point-like sources, but we apply the same methodology to the AzTEC/ASTE survey of the Great Observatories Origins Deep Survey-South (GOODS-S). In both applications, our technique robustly decomposes redundant maps into their underlying components, reducing flux bias, improving signal-to-noise, and minimizing information loss. In particular, the GOODS-S survey is decomposed into four independent physical components, one of them is the already known map of point sources, two are atmospheric and systematic foregrounds, and the fourth component is an extended emission that can be interpreted as the confusion background of faint sources.

astro-ph.IM↗

arXiv2017

Infrared excesses in stars with and without planets using revised ${\it WISE}$ photometry

We present an analysis on the potential prevalence of mid infrared excesses in stars with and without planetary companions. Based on an extended database of stars detected with the ${\it WISE}$ satellite, we studied two stellar samples: one with 236 planet hosts and another with 986 objects for which planets have been searched but not found. We determined the presence of an excess over the photosphere by comparing the observed flux ratio at 22 $μ$m and 12 $μ$m ($f_{22}/f_{12}$) with the corresponding synthetic value, derived from results of classical model photospheres. We found a detection rate of 0.85$\%$ at 22 $μ$m (2 excesses) in the sample of stars with planets and 0.1$\%$ (1 detection) for the stars without planets. The difference of the detection rate between the two samples is not statistically significant, a result that is independent of the different approaches found in the literature to define an excess in the wavelength range covered by ${\it WISE}$ observations. As an additional result, we found that the ${\it WISE}$ fluxes required a normalisation procedure to make them compatible with synthetic data, probably pointing out a revision of the ${\it WISE}$ data calibration.

astro-ph.SR↗

arXiv2017

Mg, Al, Si, Ca, Ti, Fe, and Ni abundance for a sample of solar analogues

We report on the determination of chemical abundances of 38 solar analogues, including 11 objects previously identified as super metal-rich stars. We have measured the equivalent widths for 34 lines of 7 different chemical elements (Mg, Al, Si, Ca, Ti, Fe, and Ni) in high-resolution ($\mathcal{R} \sim 80\,000$) spectroscopic images, obtained at the Observatorio Astrofísico Guillermo Haro (Sonora, Mexico), with the Cananea High-resolution Spectrograph. We derived chemical abundances using ATLAS12 model atmospheres and the Fortran code MOOG. We confirmed the super metallicity status of 6 solar analogues. Within our sample, BD+60 600 is the most metal-rich star ([Fe/H]=+0.35 dex), while for HD 166991 we obtained the lowest iron abundance ([Fe/H]=$-0.53$ dex). We also computed the so-called [Ref] index for 25 of our solar analogues, and we found, that BD+60 600 ([Ref]=+0.42) and BD+28 3198 ([Ref]=+0.34) are good targets for exoplanet search.

astro-ph.SR↗

arXiv2015

Deep GALEX UV Survey of the Kepler Field I: Point Source Catalog

We report observations of a deep near-ultraviolet (NUV) survey of the Kepler field made in 2012 with the Galaxy Evolution Explorer (GALEX) Complete All-Sky UV Survey Extension (CAUSE). The GALEX-CAUSE Kepler survey (GCK) covers 104 square degrees of the Kepler field and reaches limiting magnitude NUV=22.6 at 3σ. Analysis of the GCK survey has yielded a catalog of 669,928 NUV sources, of which 475,164 are cross-matched with stars in the Kepler Input Catalog (KIC). Approximately 327 of 451 confirmed exoplanet host stars and 2614 of 4696 candidate exoplanet host stars identified by Kepler have NUV photometry in the GCK survey. The GCK catalog should enable the identification and characterization of UV-excess stars in the Kepler field (young solar-type and low-mass stars, chromospherically active binaries, white dwarfs, horizontal branch stars, etc.), and elucidation of various astrophysics problems related to the stars and planetary systems in the Kepler field.

astro-ph.SR↗

arXiv2014

The Chromospheric Solar Millimeter-wave Cavity; a Common Property in the Semi-empirical Models

The semi-empirical models of the solar chromosphere are useful in the study of the solar radio emission at millimeter - infrared wavelengths. However, current models do not reproduce the observations of the quiet sun. In this work we present a theoretical study of the radiative transfer equation for four semi- empirical models at these wavelengths. We found that the Chromospheric Solar Milimeter-wave Cavity (CSMC), a region where the atmosphere becomes locally optically thin at millimeter wavelengths, is present in the semi-empirical models under study. We conclude that the CSMC is a general property of the solar chromosphere where the semi-empirical models shows temperature minimum.

astro-ph.SR↗

arXiv2014

The Limb Brightening and its Relationship with the Millimeter-wave Cavity

Through a detailed theoretical analysis of the local emission at millimeter,sub-millimeter and infrared wavelength regimes (from \~ 10 GHz up to \~ 10 THz), we found that, associated with the temperature minimum, there is an optically thin cavity surrounded by two regions of high local emissivity. We call this structure the Chromospheric Solar Millimeter Cavity (CSMC). In order to search for traces of this cavity in the available radio observations on the solar limb, we have developed a robust method to associate the radiation at different heights with the observed brightness temperatures. We foresee that this approach will allow us to determine the relationship between the CSMC and the solar limb brightening.

astro-ph.SR↗

arXiv2014

The Relation between the Radial Temperature Profile in the Chromosphere and the Solar Spectrum at Centimeter, Millimeter, Sub-millimeter, and Infrared Wavelengths

Solar observations from millimeter to ultraviolet wavelengths show that there is a temperature minimum between photosphere and chromosphere. Analysis based on semi-empirical models locate this point at about 500 km over the photosphere. The consistency of these models has been tested by means of millimeter to infrared observations. In the present work, we show that variations of the theoretical radial temperature profile near the temperature minimum impacts the brightness temperature at centimeter, submillimeter, and infrared wavelengths, but the millimeter wavelength emission remains unchanged. We found a region between 500 and 1000 km over the photosphere that remains hidden to observations at the frequencies under study in this work.

astro-ph.SR↗

arXiv2013

Mg II h+k Flux - Rotational Period Correlation for G-type stars

We present an analysis of the correlation between the mid-UV Mg II h and k emission lines and measured rotational periods of G-type stars. Based on IUE and HST high resolution spectra of a sample of 36 stars, we derive an exponential function that best represents the correlation. We found that the variation of the Mg II h + k fluxes is about a factor of 2.5 larger than that of Ca II H+K, indicating that the UV features are more sensitive to the decline of rotational period. The comparison of UV-predicted rotational periods with those derived from empirical Prot - Ca II H+K flux calibrations are consistent, with some scatter at large periods, where the emission are less intense. We present newly derived rotational periods for 15 G-type stars.

astro-ph.SR↗

arXiv2013

Searching for IR excesses in Sun-like stars observed by WISE

We present the results of a search of infrared excess candidates in a comprehensive (29\,000 stars) magnitude limited sample of dwarf stars, spanning the spectral range F2-K0, and brighter than V$=$15 mag. We searched the sample within the {\em WISE} all sky survey database for objects within 1 arcsecond of the coordinates provided by SIMBAD database and found over 9\,000 sources detected in all {\em WISE} bands. This latter sample excludes objects that are flagged as extended sources and those images which are affected by various optical artifacts. For each detected object, we compared the observed W4/W2 (22$μ$m/4.6$μ$m) flux ratio with the expected photospheric value and identified 197 excess candidates at 3$σ$. For the vast majority of candidates, the results of this analysis represent the first reported evidence of an IR excess. Through the comparison with a simple black-body emission model, we derive estimates of the dust temperature, as well as of the dust fractional luminosities. For more than 80% of the sample this temperature is higher than 120 K, suggesting the presence of warm circumstellar dust. Complementary observations at longer wavelengths (far-IR and sub-mm) are required for better characterising and explaining the origin of this emission.

astro-ph.SR↗

arXiv2013

Targeting Young Stars with Kepler: Planet Formation, Migration Mechanisms and the Early History of Planetary Systems

This white paper discusses a repurposed mission for the Kepler spacecraft that focusses on solving outstanding problems in planet formation and evolution by targeting the study of the hot Jupiter population of young stars. This mission can solve the question of the mode of migration of hot Jupiters, address the problem of whether Jupiters form by hot-start (gravitational instability) or cold-start (core accretion) mechanisms, and provide a wealth of data on the early stages of planetary system evolution during the active phases of stars which impact planetary habitability. In one year of observations of three weeks dwell time per field, Kepler would increase by more than an order of magnitude the number of known hot Jupiters, which can be followed up with fast cadence observations to to search for transit timing variations and to perform asteroseismological characterization of the host stars. This mission scenario continues to operate Kepler in the photometric monitoring mode for which it was designed, and is generally flexible with regards to field selection enabling prioritization of fuel usage and attitude control constraints.

astro-ph.EP↗

arXiv2012

Stellar lifetime and ultraviolet properties of the old metal-rich Galactic open cluster NGC6791: a pathway to understand the UV upturn of elliptical galaxies

The evolutionary properties of the old metal-rich Galactic open cluster NGC6791 are assessed, based on deep UB photometry and 2Mass JK data. For 4739 stars in the cluster, bolometric luminosity and effective temperature have been derived from theoretical (U-B) and (J-K) color fitting. The derived H-R diagram has been matched with the UVBLUE grid of synthetic stellar spectra to obtain the integrated SED of the system, together with a full set UV (Fanelli) and optical (Lick) narrow-band indices. The cluster appears to be a fairly good proxy of standard elliptical galaxies, although with significantly bluer infrared colors, a shallower 4000A Balmer break, and a lower Mg2 index. The confirmed presence of a dozen hot stars, along their EHB evolution, leads the cluster SED to consistently match the properties of the most active UV-upturn galaxies, with 1.7+/-0.4% of the total bolometric luminosity emitted shortward of 2500A. The cluster Helium abundance results Y=0.30 +/-0.04, while the Post-MS implied stellar lifetime from star number counts fairly agrees with the theoretical expectations from both the Padova and BASTI stellar tracks. A Post-MS fuel consumption of 0.43 +/- 0.01 M_sun is found for NGC6791 stars, in close agreement with the estimated mass of cluster He-rich white dwarfs. Such a tight figure may lead to suspect that a fraction of the cluster stellar population does actually not reach the minimum mass required to effectively ignite He in the stellar core.

astro-ph.GA↗

arXiv2011

Optical Spectroscopic ATLAS of the MOJAVE/2cm AGN Sample (1)

We present an optical spectroscopic atlas at intermediate resolution (8-15A) for 123 core-dominated radio-loud active galactic nuclei with relativistic jets, drawn from the MOJAVE/2cm sample at 15GHz. It is the first time that spectroscopic and photometric parameters for a large sample of such type of AGN are presented. The atlas includes spectral parameters for the emission lines Hb, [O III] 5007, Mg II 2798 and/or C IV 1549 and corresponding data for the continuum, as well as the luminosities and equivalent widths of the Fe II UV/optical. It also contains the homogeneous photometric information in the B-band for 242 sources of the sample, with a distribution peak at BJ=18.0 and a magnitude interval of 11.1< BJ <23.7.

astro-ph.CO↗

arXiv2011

Open Issues on the Synthesis of Evolved Stellar Populations at Ultraviolet Wavelengths

In this paper we briefly review three topics that have motivated our (and others') investigations in recent years within the context of evolutionary population synthesis techniques. These are: The origin of the FUV up-turn in elliptical galaxies, the age-metallicity degeneracy, and the study of the mid-UV rest-frame spectra of distant red galaxies. We summarize some of our results and present a very preliminary application of a UV grid of theoretical spectra in the analysis of integrated properties of aged stellar populations. At the end, we concisely suggest how these topics can be tackled once the World Space Observatory enters into operation in the midst of this decade.

astro-ph.CO↗

arXiv2011

Statistical properties of the GALEX spectroscopic stellar sample

The GALEX General Data Release 4/5 includes 174 spectroscopic tiles, obtained from slitless grism observations, for a total of more than 60,000 ultraviolet spectra. We have determined statistical properties of the sample of GALEX stars. We have defined a suitable system of spectroscopic indices, which measure the main mid-UV features at the GALEX low spectral resolution and we have employed it to determine the atmospheric parameters of of stars in the range 4500 -1 dex.

astro-ph.SR↗

arXiv2009

Snapshot metallicity estimate of resolved stellar systems through Lick Fe5270 diagnostic

We outline a new method to derive a "snapshot" metallicity estimate of stellar systems (providing one resolves at least the brightest part of the CMD) just on the basis of low-resolution (i.e., 6-8A FWHM) spectroscopy of a small stellar sample. Our method relies on the Fe5270 Lick index measurements and takes advantage of the special behavior of this spectral feature, that reaches its maximum strength among the ubiquitous component of K-type giants. This makes the Fe5270(max} estimate a robust and model-independent tracer of cluster [Fe/H], being particularly insensitive to the age of the stellar population. A comparison of the Fe5270(max) distribution derived from globular and open clusters, as well as from the field giant population in the Galaxy disk, confirms a tight correlation of the index maximum vs. cluster [Fe/H] allover the entire metallicity range for stellar population with [Fe/H] >~ -2.0. Relying on a theoretical calibration of the feature, we trust to effectively infer cluster metallicity within a typical uncertainty of 0.1-0.2dex, depending on RGB luminosity sampling of the observations.

astro-ph.SR↗