arXiv subjects

Fabio Favata

Publications and source records attributed to Fabio Favata.

27 records · Page 2Linked to original sources

arXiv2006

The Monitor project: Searching for occultations in young open clusters

The Monitor project is a photometric monitoring survey of nine young (1-200Myr) clusters in the solar neighbourhood to search for eclipses by very low mass stars and brown dwarfs and for planetary transits in the light curves of cluster members. It began in the autumn of 2004 and uses several 2 to 4m telescopes worldwide. We aim to calibrate the relation between age, mass, radius and where possible luminosity, from the K-dwarf to the planet regime, in an age range where constraints on evolutionary models are currently very scarce. Any detection of an exoplanet in one of our youngest targets (<=10Myr) would also provide important constraints on planet formation and migration timescales and their relation to proto-planetary disc lifetimes. Finally, we will use the light curves of cluster members to study rotation and flaring in low-mass pre-main sequence stars. The present paper details the motivation, science goals and observing strategy of the survey. We present a method to estimate the sensitivity and number of detections expected in each cluster, using a simple semi-analytic approach which takes into account the characteristics of the cluster and photometric observations, using (tunable) best-guess assumptions for the incidence and parameter distribution of putative companions, and we incorporate the limits imposed by radial velocity follow-up from medium and large telescopes. We use these calculations to show that the survey as a whole can be expected to detect over 100 young low and very low mass eclipsing binaries, and approx. 3 transiting planets with radial velocity signatures detectable with currently available facilities.

astro-ph↗

arXiv2005

The Origin of T Tauri X-ray Emission: New Insights from the Chandra Orion Ultradeep Project

We use the data of the Chandra Orion Ultradeep Project (COUP) to study the nearly 600 X-ray sources that can be reliably identified with optically well characterized T Tauri stars (TTS) in the Orion Nebula Cluster. We detect X-ray emission from more than 97% of the optically visible late-type (spectral types F to M) cluster stars. This proofs that there is no ``X-ray quiet'' population of late-type stars with suppressed magnetic activity. All TTS with known rotation periods lie in the saturated or super-saturated regime of the relation between activity and Rossby numbers seen for main-sequence (MS) stars, but the TTS show a much larger scatter in X-ray activity than seen for the MS stars. Strong near-linear relations between X-ray luminosities, bolometric luminosities and mass are present. We also find that the fractional X-ray luminosity rises slowly with mass over the 0.1 - 2 M_sun range. The plasma temperatures determined from the X-ray spectra of the TTS are much hotter than in MS stars, but seem to follow a general solar-stellar correlation between plasma temperature and activity level. The large scatter about the relations between X-ray activity and stellar parameters seems to be related to the influence of accretion on the X-ray emission. While the X-ray activity of the non-accreting TTS is consistent with that of rapidly rotating MS stars, the accreting stars are less X-ray active (by a factor of ~2-3 on average) and produce much less well defined correlations than the non-accretors. We discuss possible reasons for the suppression of X-ray emission by accretion and the implications of our findings on long-standing questions related to the origin of the X-ray emission from young stars.

astro-ph↗

arXiv2003

Stellar coronal astronomy - a review

Coronal astronomy is by now a fairly mature discipline, with a quarter century having gone by since the detection of the first stellar X-ray coronal source (Capella), and having benefitted from a series of major orbiting observing facilities. Several observational characteristics of coronal X-ray and EUV emission have been solidly established through extensive observations, and are by now common, almost text-book, knowledge. At the same time the implications of coronal astronomy for broader astrophysical questions (e.g. Galactic structure, stellar formation, stellar structure, etc.) have become appreciated. The interpretation of stellar coronal properties is however still often open to debate, and will need qualitatively new observational data to book further progress. In the present review we try to recapitulate our view on the status of the field at the beginning of a new era, in which the high sensitivity and the high spectral resolution provided by Chandra and XMM-Newton will address new questions which were not accessible before.

astro-ph↗

arXiv2001

Direct determination of quasar redshifts

We present observations of 11 quasars, selected in the range z = 2.2-4.1, obtained with ESA's Superconducting Tunnel Junction (STJ) camera on the WHT. Using a single template quasar spectrum, we show that we can determine the redshifts of these objects to about 1%. A follow-up spectroscopic observation of one QSO for which our best-fit redshift (z = 2.976) differs significantly from the tentative literature value (z ~ 2.30) confirms that the latter was incorrect.

astro-ph↗

arXiv2001

Discovery of X-ray emission from the proto-stellar jet L1551 IRS5 (HH 154)

We have for the first time detected X-ray emission associated with a proto-stellar jet, on the jet emanating from L1551 IRS5. The IRS5 proto-star is hidden beyond a very large absorbing column density, making the direct observation of the jet's emission possible. The observed X-ray emission is likely associated with the shock ``working surface'', i.e. the interface between the jet and the circumstellar medium. The X-ray luminosity emanating from the jet is moderate, at LX ~ 3 times 10^29 erg/s, a significant fraction of the luminosity normally associated with the coronal emission from young stars. The spectrum of the X-ray emission is compatible with thermal emission from a hot plasma, with T ~ 0.5 MK, fully compatible with the temperature expected (on the basis of the jet's velocity) for the shock front produced by the jet hitting the circumstellar medium.

astro-ph↗

arXiv2001

Analysis of astronomical data from optical superconducting tunnel junctions

Currently operating optical superconducting tunnel junction (STJ) detectors, developed in ESA, can simultaneously measure the wavelength (delta lambda = 50 nm at 500 nm) and arrival time (to within ~5 micros) of individual photons in the range 310-720 nm with an efficiency of ~70%, and with count rates of order 5,000 photons per second per junction. A number of STJ junctions placed in an array format generates four-dimensional data: photon arrival time, energy, and array element (X,Y). Such STJ cameras are ideally suited for, e.g., high time- resolution spectrally-resolved monitoring of variable sources or low-resolution spectroscopy of faint extragalactic objects. The reduction of STJ data involves detector efficiency correction, atmo- spheric extinction correction, sky background subtraction, and, unlike that of data from CCD-based systems, a more complex energy calibration, barycentric arrival time correction, energy range selection, and time binning; these steps are, in many respects, analogous to procedures followed in high-energy astro- physics. This paper discusses these calibration steps in detail using a repre- sentative observation of the cataclysmic variable UZ Fornacis; these data were obtained with ESA's S-Cam2 6x6-pixel device. We furthermore discuss issues re- lated to telescope pointing and guiding, differential atmospheric refraction, and atmosphere-induced image motion and image smearing (`seeing') in the focal plane. We also present a simple and effective recipe for extracting the evolu- tion of atmospheric seeing with time from any science exposure, and discuss a number of caveats in the interpretation of STJ-based time-binned data, such as light curves and hardness ratio plots.

astro-ph↗

arXiv2001

TTauri Stars in the Large Magellanic Cloud: a combined HST and VLT effort

The combination of the unprecedented spatial resolution attainable with WFPC2 on board HST and of the large collecting area of the VLT makes it possible to study in detail the low mass pre-Main Sequence stars in galaxies other than our own. Here we present the results of our studies of two star forming environments in our closest galactic neighbor, the Large Magellanic Cloud: the region around Supernova 1987A and the double cluster NGC 1850.

astro-ph↗

arXiv2000

Planet Detection Capabilities of the Eddington Mission

Eddington is a space mission for extrasolar planet finding and for asteroseismic observations. It has been selected by ESA as an F2/F3 reserve mission with a potential implementation in 2008-13. Here we describe Eddington's capabilities to detect extrasolar planets, with an emphasis on the detection of habitable planets. Simulations covering the instrumental capabilities of Eddington and the stellar distributions in potential target fields lead to predictions of about 10,000 planets of all sizes and temperatures, and a few tens of terrestrial planets that are potentially habitable. Implications of Eddington for future larger scale missions are briefly discussed.

astro-ph↗

arXiv1999

The corona of the dMe flare star AD Leo

We have studied the X-ray emission (both the quiescent component and the flares) of the dM3e star AD Leo, analyzing the Einstein IPC, ROSAT PSPC and ASCA SIS observations. Using a consistent method which explicitly considers sustained heating we have analyzed six flares with sufficient statistics, deriving constraints on the physical parameters of the flaring regions. In all cases the flaring loops are likely compact (L approx 0.3 R*), and confined to a rather narrow range of sizes, incompatible with the large (L >= R*) tenuous loops claimed by previous analyses of flares on AD Leo and other similar stars. The flaring loops appear to have a larger cross section (beta = r/L approx. 0.3) than customarily assumed (e.g. beta 0.1). All flares show evidence of significant heating during the decay phase. Although the derived peak pressures are high (up to P approx. 10^4 dyne/cm^2) with a peak temperature of approx. 50 MK, the magnetic fields required to confine such loops and to produce the observed flare luminosity are relatively modest (B approx. 1 to 2 kG) and fully compatible with the photospheric magnetic fields measured in several flare stars. If the narrow range of loop sizes obtained is extrapolated to the quiescent structures responsible for the active corona, the latter can be naturally scaled up from the solar case through a modest (a factor of 10) increase in pressure in otherwise solar-like active structures with a small surface filling factor (approx 5%). The quiescent component of the corona shows no evidence for abundance peculiarities with respect to the photosphere, and the quiescent coronal luminosity is remarkably constant (with variations of less than a factor of 2) across the almost 20 yr span of the observations discussed here.

astro-ph↗