Search results for "Disks"
showing 10 items of 118 documents
Pre-main sequence stars with disks in the Eagle Nebula observed in scattered light
2010
NGC6611 and its parental cloud, the Eagle Nebula (M16), are well-studied star-forming regions, thanks to their large content of both OB stars and stars with disks and the observed ongoing star formation. We identified 834 disk-bearing stars associated with the cloud, after detecting their excesses in NIR bands from J band to 8.0 micron. In this paper, we study in detail the nature of a subsample of disk-bearing stars that show peculiar characteristics. They appear older than the other members in the V vs. V-I diagram, and/or they have one or more IRAC colors at pure photospheric values, despite showing NIR excesses, when optical and infrared colors are compared. We confirm the membership of…
Chronology of star formation and disk evolution in the Eagle Nebula
2010
Massive SFR are characterized by intense ionizing fluxes, strong stellar winds and supernovae explosions, all of which have important effects on the surrounding media, on the star-formation (SF) process and on the evolution of YSOs and their disks. We present a multiband study of the massive young cluster NGC6611 and M16, to study how OB stars affect the early stellar evolution and the SF. We search for evidence of triggered SF by OB stars in NGC6611 on a large spatial scale (~10 pc) and how the efficiency of disks photoevaporation depends on the central stars mass. We assemble a multiband catalog with photometric data, from B band to 8.0micron, and X-ray data obtained with 2 new and 1 arch…
Inhomogeneous Accretion Flow in X-ray Binary Pulsars
2005
We analyze the power spectrum of SAX J1808.4‐3658, the first accreting millisecond binary pulsar discovered, to look for a broadening in the wings of the harmonic line, in analogy to what had been previously found in some high mass X‐ray binaries. We indeed detect a broadening at the base of the 401 Hz peak, which is consistent with the convolution of the low frequency noise present in the power spectrum with the harmonic line. We interpret this as the result of a coupling between a fraction of the aperiodic and periodic variability, suggesting that at least part of the noise originates in a region close to the neutron star surface at the magnetic poles.
BeppoSAX observation of 4U 1705-44: detection of hard X-ray emission in the soft state
2007
4U 1705-44 is one of the best studied type I X-ray burster and atoll sources. Since it covers a wide range in luminosity (from a few to 50 x 10^{36} erg s^{-1}) and shows clear spectral state transitions, it represents a good laboratory to test the accretion models proposed for atoll sources. We analysed the energy spectrum accumulated with BeppoSAX observations (43.5 ksec) in August 2000 when the source was in a soft spectral state. The continuum of the wide band energy spectrum is well described by the sum of a blackbody (kT_{bb}~0.56 keV) and a Comptonized component (seed-photon temperature kT_W~1 keV, electron temperature kT_e~2.7 keV, and optical depth ~11). A hard tail was detected at…
Correlation between the spatial distribution of the circumstellar disks and the massive stars in the open cluster NGC 6611. Compiled catalog and clus…
2006
Context: the observation of young stars with circumstellar disks suggests that the disks are dissipated, starting from the inner region, by the radiation of the central star and eventually by the formation of rocky planetesimals, over a time scale of several million years. It was also shown that strong UV radiation emitted by nearby massive stars can heat a circumstellar disk up to some thousand degrees, inducing the photoevaporation of the gas. This process strongly reduces the dissipation time scale. Aims: we study whether there exists a correlation between the spatial distribution of stars with circumstellar disks and the position of massive stars with spectral class earlier than B5, in …
The complex enviroment around Cir X-1
2008
We present the results of an archival 54 ks long Chandra observation of the peculiar source Cir X–1 during the phase passage 0.223-0.261, based on the phase zero passage at the periastron, of its orbital period. We focus on the study of detected emission and absorption features using the High Energy Transmission Grating Spectrometer on board of the Chandra satellite. A comparative analysis of X-ray spectra, selected at different flux levels of the source, allows us to distinguish between a very hard state, at a low countrate, and a brighter, softer, highly absorbed spectrum during episodes of flaring activity, when the unabsorbed source luminosity is about three times the value in the hard …
Magnetohydrodynamic Modeling of the Accretion Shocks in Classical T Tauri Stars: The Role of Local Absorption in the X-Ray Emission
2014
We investigate the properties of X-ray emission from accretion shocks in classical T Tauri stars (CTTSs), generated where the infalling material impacts the stellar surface. Both observations and models of the accretion process reveal several aspects that are still unclear: the observed X-ray luminosity in accretion shocks is below the predicted value, and the density versus temperature structure of the shocked plasma, with increasing densities at higher temperature, deduced from the observations, is at odds with that proposed in the current picture of accretion shocks. To address these open issues we investigate whether a correct treatment of the local absorption by the surrounding medium …
A Broad Iron Line in the Chandra High Energy Transmission Grating Spectrum of 4U 1705-44
2005
We present the results of a Chandra 30 ks observation of the low-mass X-ray binary and atoll source 4U 1705-44. Here we concentrate on the study of discrete features in the energy spectrum at energies below ~3 keV, as well as on the iron Kalpha line, using the High Energy Transmission Grating Spectrometer on board the Chandra satellite. Below 3 keV, three narrow emission lines are found at 1.47, 2.0, and 2.6 keV. The 1.47 and 2.6 keV lines are probably identified with Lyalpha emission from Mg XII and S XVI, respectively. The identification of the feature at ~2.0 keV is uncertain because of the presence of an instrumental feature at the same energy. The iron Kalpha line at ~6.5 keV is found …
Fluctuation-Limited Noise in a Superconducting Transition-Edge Sensor
2003
In order to investigate the origin of the until now unaccounted excess noise and to minimize the uncontrollable phenomena at the transition in x-ray microcalorimeters we have developed superconducting transition-edge sensors into an edgeless geometry, the so-called Corbino disk, with superconducting contacts in the center and at the outer perimeter. The measured rms current noise and its spectral density can be modeled as resistance noise resulting from fluctuations near the equilibrium superconductor-normal metal boundary. Peer reviewed
A Preliminary Analysis of a New Chandra Observation (ObsID 6148) of Cir X-1
2008
We present the preliminary spectral analysis of a 25 ks long Chandra observation of the peculiar source Cir X–1 near the periastron passage. We estimate more precise coordinates of the source compatible with the optical and radio counterpart coordinates. We detect emission lines associated to Mg XII, Si XIII, Si XIV, S XV, S XVI Ar XVII, Ar XVIII, Ca XIX, Ca XX, Fe XXV, Fe XXVI showing a redshift of 470 km s−1. The more intense emission features at 6.6 keV show a double‐peaked shape that can be modelled with two or three Gaussian lines.