Roberto Viotti

DETECTION OF GAMMA-RAY EMISSION FROM THE ETA-CARINAE REGION

We present the results of extensive observations by the gamma-ray AGILE satellite of the Galactic region hosting the Carina nebula and the remarkable colliding wind binary Eta Carinae (η Car) during the period 2007 July-2009 January. We detect a gamma-ray source (1AGL J1043–5931) consistent with the position of η Car. If 1AGL J1043–5931 is associated with the Car system, our data provide the long sought first detection above 100 MeV of a colliding wind binary. The average gamma-ray flux above 100 MeV and integrated over the preperiastron period 2007 July-2008 October is F γ = (37 ± 5) × 10–8 ph cm–2 s–1 corresponding to an average gamma-ray luminosity of L γ = 3.4 × 1034 erg s–1 for a distance of 2.3 kpc. We also report a two-day gamma-ray flaring episode of 1AGL J1043–5931 on 2008 October 11-13 possibly related to a transient acceleration and radiation episode of the strongly variable shock in the system.

The ultraviolet spectrum of Eta Carinae

An atlas of the high-resolution UV spectrum of Eta Car from 1200 to 1974 A and from 2200 to 3230 A is presented, based on IUE observations made between 1978 and 1980. The fluxes and equivalent widths of the emission and absorption features, and the stellar continuum in line-free regions are presented. The profiles displayed by the most intense emission suggest line formation in an asymmetric envelope. Many of the observed features may be explained if Eta Car is an intermediate, possible binary, F-type hypergiant in a short living stage, which holds a massive wind heated by dissipation of mechanical energy. 61 refs.

The high energy X-ray tail of η Car revealed by BeppoSAX ⋆

We report on the June 2000 long (100 ks) BeppoSAX exposure that unveiled a new very high energy component of the X-ray spectrum of η Car above 10 keV extending to at least 50 keV. We find that the 2-150 keV spectrum is best reproduced by a thermal + non-thermal model. The thermal component dominates the 2-10 keV spectral range with kTh = 5.5 ± 0.3 keV and log NHh = 22.68 ± 0.01. The spectrum displays a prominent iron emission line centred at 6.70 keV. Its equivalent width of 0.94 keV, if produced by the thermal source, gives a slightly sub-solar iron abundance ((Fe/H) = −0.15 ± 0.02). The high energy tail above 10 keV is best fitted by a power law with a photon index of 2.42 ± 0.04. The integrated 13-150 keV luminosity of ∼12 Lis comparable to that of the 2-10 keV thermal component (19 L� ). The present result can be explained, in the η Car binary star scenario, by Comptonisation of low frequency radiation by high energy electrons, probably generated in the colliding wind shock front, or in instabilities in the wind of the S Dor primary star. It is possible that the high energy tail had largely weakened near the minimum of the 5.53 yr cycle. Probably, the thermal component has a longer recovery time like that of the highest excitation optical emission lines. Both features can be associated with the large absorption measured by BeppoSAX at phase 0.05.

The origin of the supersoft X-ray-optical/UV flux anticorrelation in the symbiotic binary AG Draconis

Context. AG Draconis produces a strong supersoft X-ray emission. The X-ray and optical/UV fluxes are in strict anticorrelation throughout the active and quiescent phases. Aims. We identify the source of the X-ray emission and reveal the nature of the observed flux anticorrelation. Methods. We used X-ray and UV observations with XMM-Newton, far-UV spectroscopy from FUSE, low- and high-resolution IUE spectra, and optical/near-IR spectroscopic and/or photometric observations. We modeled the spectral energy distribution and broad wings of the O vi λ1032 ,λ 1038 and He ii λ1640 lines by the electron-scattering during the maximum of the 2003 burst, and the subsequent transition and quiescent phase. Results. The X-ray-near-IR energy distribution at different levels of the star’s brightness confirmed the observed flux anticorrelation quantitatively and showed that the optical bursts are associated to an increase in the nebular component of radiation. The profile-fitting analysis revealed a significant increase in the mean particle density around the hot star from ∼2.6 × 10 10 cm −3 during quiescent phase to ∼1.1 × 10 12 cm −3 during the burst. Conclusions. The supersoft X-ray emission is produced by the white dwarf photosphere. The X-ray and far-UV fluxes make it possible to determine its temperature unambiguously. The supersoft X-ray-optical/UV flux anticorrelation is caused by the variable wind from the hot star. The enhanced hot star wind gives rise to the optical bursts by reprocessing high-energy photons from the Lyman continuum to the optical/UV.

Toward Understanding the B[e] Phenomenon. II. New Galactic FS CMa Stars*

FS CMa stars form a group of objects with the B[e] phenomenon that were previously known as unclassified B[e] stars or B[e] stars with warm dust (B[e]WD) until recently. They exhibit strong emission-line spectra and strong IR excesses, most likely due to recently formed circumstellar dust. These properties have been suggested to be due to ongoing or recent rapid mass exchange in binary systems with hot primaries and various types of secondaries. The first paper of this series reported an analysis of the available information about previously known Galactic objects with the B[e] phenomenon, the initial selection of the FS CMa group objects, and a qualitative explanation of their properties. This paper reports the results of our new search for more FS CMa objects in the IRAS Point Source Catalog. We present new photometric criteria for identifying FS CMa stars as well as the first results of our observations of nine new FS CMa group members. With this addition, the FS CMa group has now 40 members, becoming

The present status of four luminous variables in M 33

Context. Understanding the origin of the instabilities of LBVs is important for shedding light on the late evolutionary stages of massive stars and on the chemical evolution of galaxies. Aims. To investigate the physical nature of variable stars in the upper H–R diagram, we performed a spectrophotometric study of the Romano’s star GR 290 and the Hubble-Sandage variables A, B, and C in the close galaxy M 33. Methods. New spectroscopic and photometric data were employed in conjunction with already published data of these stars in order to derive spectral types, energy distribution and bolometric luminosities. Results. The yellow hypergiant Var A is still at minimum, with a ∼G-type spectrum and strong Hα emission (Weq �− 35 A). Var B is in a low luminosity hot state (V = 17.5, B − V = −0.15) with very strong Hα emission (Weq = −310 A). Its absolute bolometric luminosity is 0.6 × 10 6 L� . Var C, at V = 16.4, is fainter than in the mid 1980s, but its spectrum shows the typical features of LBVs at maximum, a spectrum that is very rich in Fe ii emission lines. Its Lbol is about 0.7 × 10 6 L� . The Romano’s star GR 290 has a rich hot emission-line spectrum and is very bright with Lbol = 3 × 10 6 L� . During 2004 the star brightened by ∼half magnitude in each of the BVR filters. Conclusions. Our observations confirm that Var A probably is an intermediate type hypergiant star surrounded by an expanding envelope with a collisionally excited hydrogen emission, largely obscured by dusty disk and nebula. In recent years, Var B has undergone a blueward transition in the H–R diagram, probably at constant bolometric luminosity, while Var C is in a post-maximum phase with an η Car-type spectrum. GR 290 is notable for its spectrum and luminosity, and it is likely to develop ample spectral variations in the near future, similar to those observed in AG Car.

Toward Understanding the B[e] Phenomenon. III. Properties of the Optical Counterpart of IRAS?00470+6429

FS?CMa type stars are a group of Galactic objects with the B[e] phenomenon. They exhibit strong emission-line spectra and infrared excesses, which are most likely due to recently formed circumstellar dust. The group content and identification criteria were described in the first two papers of the series. In this paper we report our spectroscopic and photometric observations of the optical counterpart of IRAS?00470+6429 obtained in 2003-2008. The optical spectrum is dominated by emission lines, most of which have P?Cyg type profiles. We detected significant brightness variations, which may include a regular component, and variable spectral line profiles in both shape and position. The presence of a weak Li I 6708?? line in the spectrum suggests that the object is most likely a binary system with a B2-B3 spectral-type primary companion of a luminosity log L/L ? = 3.9 ? 0.3 and a late-type secondary companion. We estimate a distance toward the object to be 2.0 ? 0.3 kpc from the Sun.

Digitization and Scientific Exploitation of the Italian and Vatican Astronomical Plate Archives

There is a widespread interest to digitize the precious information contained in the astronomical plate archives, both for the preservation of their content and for its fast distribution to all interested researchers in order to achieve their better scientific exploitation. This paper presents the first results of our large-scale project to digitize the archive of plates of the Italian Astronomical Observatories and of the Specola Vaticana. Similar systems, composed by commercial flat-bed retro-illuminated scanners plus dedicated personal computers and acquisition and analysis software, have been installed in all participating Institutes. Ad-hoc codes have been developed to acquire the data, to test the suitability of the machines to our scientific needs, and to reduce the digital data in order to extract the astrometric, photometric and spectroscopic content. Two more elements complete the overall project: the provision of high quality BVRI CCD sequences in selected fields with the Campo Imperatore telescopes, and the distribution of the digitized information to all interested researchers via the Web. The methods we have derived in the course of this project have been already applied successfully to plates taken by other Observatories, for instance at Byurakan and at Hamburg.

AG Draconis observed with XMM-Newton

Context. AG Draconis is the brightest symbiotic star in X-rays and one of the prototypes of the supersoft X-ray source class. Aims. Study of the X-ray spectrum of this peculiar binary system, covering both quiescence and activity periods, is necessary to investigate the physics of the high temperature spectral component, and to unveil the origin of the outbursts. Methods. X-ray and UV observations with XMM-Newton during 2003–2005 and coordinated optical spectrophotometric monitoring, together with archive data, are employed to derive the behaviour of the high energy source of the AG Dra system during different orbital and activity phases. Results. During quiescence the X-ray emission is very soft and is close in strength to the previous ROSAT observations, with an

The 2006 hot phase of Romano's star (GR 290) in M 33

Understanding the nature of the instabilities of LBVs is important to understand the late evolutionary stages of very massive stars. We investigate the long term, S Dor-type variability of the luminous blue variable GR290 (Romanos star) in M33, and its 2006 minimum phase. New spectroscopic and photometric data taken in November and December 2006 were employed in conjunction with already published data on GR290 to derive the physical structure of GR290 in different phases and the time scale of the variability. We find that by the end of 2006, GR 290 had reached the deepest visual minimum so far recorded. Its present spectrum resembles closely that of the Of/WN9 stars, and is the hottest so far recorded in this star (and in any LBV as well), while its visual brightness decreased by about 1.4 mag. This first spectroscopic record of GR290 during a minimum phase confirms that, similarly to AG Car and other LBVs, the star is subject to ample S Dor-type variations, being hotter at minimum, suggesting that the variations take place at constant bolometric luminosity.