arXiv subjects

U. Munari

Publications and source records attributed to U. Munari.

At least 199 records · Page 11Linked to original sources

arXiv2004

Asiago eclipsing binaries program. II. V570 Per

The orbit and physical parameters of the double-lined eclipsing binary V570 Per, discovered by the Hipparcos satellite, are derived with formal errors better than 1% using high resolution Echelle spectroscopy and B, V photoelectric photometry. Atmospheric analysis is performed on spectra at quadrature using synthetic spectroscopy that indicates a [Fe/H]=+0.02+/-0.05 metallicity. V570 Per turns out to be a detached system, with shallow eclipses of Delta B = Delta V =0.13 mag amplitude at a distance of 123+/-3 pc, in fine agreement with Hipparcos 117 (lower lim. 104, upper lim. 132) pc distance. V570 Per is composed by unperturbed components of F2 and F5 spectral types and masses of 1.457+/-0.004 and 1.351+/-0.004 Msun respectively, which do not show surface activity. Both components are still within the Main Sequence band and are dynamically relaxed to co-rotation with the orbital motion. The system is particularly interesting because both components have their masses in the range where transition occurs between convective and radiative cores, and where differences between families of stellar evolutionary tracks show appreciable differences. The position of the components of V570 Per on the temperature-luminosity plane is compared with Padova, Geneva, Granada and Teramo-04 theoretical stellar models. The comparison provides different best fit metallicities and ages, with [Fe/H] ranging from +0.07 to +0.18, and ages from 0.6 to 1.0 Gyr.

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arXiv2004

Taking the pulse of the shortest orbital period binary system RX J0806.3+1527

RX J0806.3+1527 is thought to be a 321s orbital period (the shortest known) double white dwarf binary system. According to the double degenerate binary (DDB) scenario this source is expected to be one of the strongest gravitational wave (GW) emitter candidates. In the last years RX J0806.3+1527 has been studied in great details, through multiwavelength observational campaigns and from the point of view of data analysis result interpretations. We present here the timing results obtained thanks to a 3.5-year long optical monitoring campaign carried out by the Very Large Telescope (VLT) and the Telescopio Nazionale Galileo (TNG) which allowed us to detect and study the orbital period derivative (spin-up at a rate of about 10^-3 s/yr) of the 321s modulation, to detect the linear polarisation (at a level of about 2%), and to study the broad band energy spectrum. The VLT/TNG observational strategy we used allowed us to rely upon a P-Pdot coherent solution which we finally extended backward to the 1994 ROSAT observation of RX J0806.3+1527.

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arXiv2004

Spectroscopic survey of the Galaxy with Gaia I. Design and performance of the Radial Velocity Spectrometer

The definition and optimisation studies for the Gaia satellite spectrograph, the Radial Velocity Spectrometer (RVS), converged in late 2002 with the adoption of the instrument baseline. This paper reviews the characteristics of the selected configuration and presents its expected performance. The RVS is a 2.0 by 1.6 degree integral field spectrograph, dispersing the light of all sources entering its field of view with a resolving power R=11 500 over the wavelength range [848, 874] nm. The RVS will continuously and repeatedly scan the sky during the 5 years of the Gaia mission. On average, each source will be observed 102 times over this period. The RVS will collect the spectra of about 100-150 million stars up to magnitude V~17-18. At the end of the mission, the RVS will provide radial velocities with precisions of ~2 km/s at V=15 and \~15-20 km/s at V=17, for a solar metallicity G5 dwarf. The RVS will also provide rotational velocities, with precisions (at the end of the mission) for late type stars of sigma_vsini ~5 km/s at V~15 as well as atmospheric parameters up to V~14-15. The individual abundances of elements such as Silicon and Magnesium, vital for the understanding of Galactic evolution, will be obtained up to V~12-13. Finally, the presence of the 862.0 nm Diffuse Interstellar Band (DIB) in the RVS wavelength range will make it possible to derive the three dimensional structure of the interstellar reddening.

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arXiv2004

The distance to the Pleiades from orbital solution of the double-lined eclipsing binary HD 23642

Combining precise B,V photoelectric photometry and radial velocities, we have been able to derive a firm orbital solution and accurate physical parameters for the newly discovered eclipsing binary HD 23642 in the Pleiades open cluster. The resulting distance to the binary, and therefore to the cluster, is 132 +/- 2 pc. This closely confirms the distance modulus obtained by classical main sequence fitting methods (m-M = 5.60 or 132 pc), moving cluster techniques and the astrometric orbit of Atlas. This is the first time the distance to a member of the Pleiades is derived by orbital solution of a double-lined eclipsing binary, and it is intended to contribute to the ongoing discussion about the discordant Hipparcos distance to the cluster.

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arXiv2004

SB2 and eclipsing binaries with GAIA and RAVE

The expected performance of GAIA satellite on eclipsing binaries is reviewed on the basis of (a) combined Hipparcos and ground-based observations mimicking GAIA data harvest, and (b) accurate simulations using the latest instrument model. It is found that for a large majority of the 16000 SB2 eclipsing binaries that GAIA will discover at magnitudes V<13, the orbital solutions and physical parameters will be derived with formal accuracies better than 2%. For the same stars the GAIA parallax errors will be about 5 micro-arcsec, i.e. an error of 0.5% at 1 kpc, which will allow to iteratively refine of the parameters and physics used in orbital modeling. The detectability of SB2 binaries by the already up and running spectral survey RAVE is discussed. It is found that all F-to-M SB2 binaries showing a velocity separation >=35 km/sec and a luminosity ratio >=0.5 will be recognized as such.

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arXiv2004

Asiago eclipsing binaries program. I. V432 Aur

The orbit and physical parameters of the previously unsolved eclipsing binary V432 Aur, discovered by Hipparcos, have been derived with errors better than 1% from extensive Echelle spectroscopy and B, V photometry. Synthetic spectral analysis of both components has been performed, yielding T_eff and log g in close agreement with the orbital solution, a metallicity [Z/Z_sun]=-0.60 and rotational synchronization for both components. Direct comparison on the theoretical L, T_eff plane with the Padova evolutionary tracks and isochrones for the masses of the two components (1.22 and 1.08 M_sun) provides a perfect match and a 3.75 Gyr age. The more massive and cooler component is approaching the base of the giant branch and displays a probable pulsation activity with an amplitude of Delta V = 0.075 mag and Delta rad.vel. = 1.5 km/sec. With a T_eff = 6080 K it falls to the red of the nearby instability strip populated by delta Sct and gamma Dor types of pulsating variables. Orbital modeling reveals a large and bright surface spot on it. The pulsations activity and the large spot(s) suggest the presence of macro-turbulent motions in its atmosphere. They reflect in a line broadening that at cursory inspection could be taken as indication of a rotation faster than synchronization, something obviously odd for an old, expanding star.

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arXiv2003

Polarimetric evolution of V838 Mon

We present the results of our polarimetric and spectropolarimetric monitoring of V838 Monocerotis, performed at Asiago and Crimean observatories during and after the multiple outbursts that occurred in January-March 2002. The polarization of the object is mainly due to interstellar polarization (P=2.48%). Intrinsic polarization up to 0.7% at 5000 A is present during the second maximum of the object (February 2002). This intrinsic component increases toward shorter wavelengths but our limited spectral coverage (5000-7500 A) does not allow conclusive inferences about its origin. A strong depolarization across the Halpha profile is observed. The interstellar polarization gives a lower limit to the reddening of E(B-V)>0.28, with E(B-V)=0.5 being the most probable value. A normal ratio of total to selective absorption (R_V=3.22+-0.17) was derived from the wavelength of maximum interstellar polarization. This suggests a low (if any) contribution by circumstellar material with peculiar dust to gas ratio. A polarimetric map of a portion of the light echo shows a complex polarization distribution reaching P_max=45%.

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arXiv2003

Kinematics and binaries in young stellar aggregates. II. NGC 6913 = M29

Between 1996 and 2003 we have obtained 226 high resolution spectra of 16 stars in the field of the young open cluster NGC 6913, to the aim of constraining its main properties and study its internal kinematics. Twelve of the program stars turned out to be members, one of them probably unbound. Nine are binaries (one eclipsing and another double lined) and for seven of them the observations allowed to derive the orbital elements. All but two of the nine discovered binaries are cluster members. In spite of the young age (a few Myr), the cluster already shows signs that could be interpreted as evidence of dynamical relaxation and mass segregation. However, they may be also the result of an unconventional formation scenario. The dynamical (virial) mass as estimated from the radial velocity dispersion is larger than the cluster luminous mass, which may be explained by a combination of the optically thick interstellar cloud that occults part of the cluster, the unbound state or undetected very wide binary orbit of some of the members that inflate the velocity dispersion and a high inclination for the axis of a possible cluster angular momentum. All discovered binaries are hard enough to survive average close encounters within the cluster and do not yet show sign of relaxation of the orbital elements to values typical of field binaries.

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arXiv2003

Evaluating GAIA performances on eclipsing binaries. III. Orbits and stellar parameters for UW LMi, V432 Aur and CN Lyn

The orbits and physical parameters of three detached F and G-type eclipsing binaries have been derived combining Hipparcos H(P) photometry with 8480-8740 Ang ground-based spectroscopy, simulating the photometric+spectroscopic observations that the GAIA mission will obtain. Tycho B(T) and V(T) light curves are too noisy to be modeled for the three targets, and only mean Tycho colors are retained to constrain the temperature. No previous combined photometric+spectroscopic solution exists in literature for any of the three targets. Quite remarkably, CN Lyn turned out to be an equal masses F5 triple system. Distances from the orbital solutions agree within the astrometric error with the Hipparcos parallaxes.

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arXiv2003

Unveiling the Nature of the 321 s Modulation in RX J0806.3+1527: Near-Simultaneous Chandra and VLT Observations

We report on the first near-simultaneous X-ray and optical observations of RX J0806.3+1527. The source is believed to be a 321s orbital period ultra-compact binary system hosting an X-ray emitting white dwarf. Data were obtained with Chandra and the ESO Very Large Telescope (VLT) in November 2001. We found an optical/X-ray phase-shift in the periodic modulation of about 0.5, strongly favoring the existance of two distinct emission regions in the two bands (for the pulsed fluxes). The Chandra data allow us to study, for the first time, the spectral continuum of RX J0806.3+1527 in soft X-rays. This was well fitted by a blackbody spectrum with kT about 65eV and hydrogen column density of N_H about 5 x 10^20 cm^-2. The average (unabsorbed) source 0.1-2.5 keV luminosity during the modulation-on is L_X about 5 x 10^32 erg s^-1 (assuming a distance of 500pc). Such a value is lower than the luminosity expected if stable mass transfer between two white dwarfs were driven by gravitational radiation. Evidence for absorption-like features are present in the phase-averaged spectrum at about 0.53, 0.64, and 1.26 keV, which may be attributed to heavy elements (C and N). We compare and discuss these findings with other binary systems hosting an accreting white dwarf.

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arXiv2003

NGC 6738: not a real open cluster

A photometric, astrometric and spectroscopic investigation of the poorly studied open cluster NGC 6738 has been performed in order to ascertain its real nature. NGC 6738 is definitely not a physical stellar ensemble: photometry does not show a defined mean sequence, proper motions and radial velocities are randomly distributed, spectro-photometric parallaxes range between 10 and 1600 pc, and the apparent luminosity function is identical to that of the surrounding field. NGC 6738 therefore appears to be an apparent concentration of a few bright stars projected on patchy background absorption.

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arXiv2003

Lithium in the Symbiotic Mira V407 Cyg

We report an identification of the lithium resonance doublet LiI 6708A in the spectrum of V407 Cyg, a symbiotic Mira with a pulsation period of about 745 days. The resolution of the spectra used was R~18500 and the measured equivalent width of the line is ~0.34A. It is suggested that the lithium enrichment is due to hot bottom burning in the intermediate mass AGB variable, although other possible origins cannot be totally ruled out. In contrast to lithium-rich AGB stars in the Magellanic clouds, ZrO 5551A, 6474A absorption bands were not found in the spectrum of V407Cyg. These are the bands used to classify the S-type stars at low-resolution. Although we identified weak ZrO 5718A, 6412A these are not visible in the low-resolution spectra, and we therefore classify the Mira in V407 Cyg as an M type. This, together with other published work, suggests lithium enrichment can precede the third dredge up of s-process enriched material in galactic AGB stars.

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arXiv2003

Evaluating GAIA performances on eclipsing binaries. II. Orbits and stellar parameters for V781 Tau, UV Leo and GK Dra

The orbits and physical parameters of three close, double-lined G0 eclipsing binaries have been derived combining H_P, V_T, B_T photometry from the Hipparcos/Tycho mission with 8480-8740 Åground-based spectroscopy. The setup is mimicking the photometric and spectroscopic observations that should be obtained by GAIA. The binaries considered here are all of G0 spectral type, but each with its own complications: V781 Tau is an overcontact system with components of unequal temperature, UV Leo shows occasional surface spots and GK Dra contains a delta-Scuti variable. Such peculiarities will be common among binaries to be discovered by GAIA. We find that the values of masses, radii and temperatures for such stars can be derived with a 1-2% accuracy using the adopted GAIA-like observing mode.

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arXiv2003

An energetic stellar outburst accompanied by circumstellar light echoes

Some classes of stars, including supernovae and novae, undergo explosive outbursts that eject stellar material into space. In 2002, the previously unknown variable star V838 Monocerotis brightened suddenly by a factor of about 10^4. Unlike a supernova or nova, V838 Mon did not explosively eject its outer layers; rather, it simply expanded to become a cool supergiant with a moderate-velocity stellar wind. Superluminal light echoes were discovered as light from the outburst propagated into surrounding, pre-existing circumstellar dust. Here we report high-resolution imaging and polarimetry of the light echoes, which allow us to set direct geometric distance limits to the object. At a distance of >6 kpc, V838 Mon at its maximum brightness was temporarily the brightest star in the Milky Way. The presence of the circumstellar dust implies that previous eruptions have occurred, and spectra show it to be a binary system. When combined with the high luminosity and unusual outburst behavior, these characteristics indicate that V838 Mon represents a hitherto unknown type of stellar outburst, for which we have no completely satisfactory physical explanation.

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arXiv2002

Bragg gratings in multi-mode fiber optics for wavelength calibration of GAIA and RAVE spectra

We propose a new technique, the use of FBGs (fiber Bragg gratings), for accurate, easy and low cost wavelength calibration of GAIA, RAVE and follow-ups spectra at local Observatories. FBGs mark the spectra with absorption lines, freely defined in number and position during the fibers manufacturing. The process goes in parallel with the science exposure and through the same optical train and path, thus ensuring the maximum return in wavelength calibration accuracy. Plans to manufacture and test FBGs for the CaII/Paschen region are underway at the Astronomical Observatory of Padova.

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arXiv2002

Criteria for spectral classification of cool stars in the near-IR GAIA wavelength region

The far-red portion of the spectrum offers bright prospects for an accurate classification of cool stars, like the giant components of symbiotic stars. The 8480--8740 Ang region, free from telluric absorptions and where the GAIA Cornerstone mission by ESA will record spectra for 3x10^8 stars, is investigated on the base of available observed and synthetic spectral atlases. We have identified and calibrated diagnostic line ratios useful to derive the effective temperature (spectral type) and gravity (luminosity class) for cool stars observed at spectral resolutions 10,000 <= lambda/delta-lambda <= 20,000, bracketing that eventually chosen for GAIA. A few are presented here.

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arXiv2002

V838 Mon and the new class of stars erupting into cool supergiants (SECS)

V838 Mon has undergone one of the most mysterious stellar outbursts on record. The spectrum at maximum closely resembled a cool AGB star, evolving toward cooler temperatures with time, never reaching optically thin conditions or a nebular stage. The latest spectral type recorded is M8-9. The amplitude peaked at DeltaV=9 mag, with the outburst evolution being characterized by a fast rise, three maxima over four months, and a fast decay (possibly driven by dust condensation in the ejecta). BaII, LiI and s-element lines were prominent in the outburst spectra. Strong and wide (500 km/sec) P-Cyg profiles affected low ionization species, while Balmer lines emerged to modest emission only during the central phase of the outburst. A light-echo discovered expanding around the object constrains its distance to 790+/-30 pc, providing M_V=+4.45 in quiescence and M_V=-4.35 at optical maximum (dependent on the still uncertain E(B-V)=0.5 reddening). The visible progenitor resembled a somewhat under-luminous F0 main sequence star, that did not show detectable variability over the last half century. V838 Mon together with M31-RedVar and V4332 Sgr seems to define a new class of astronomical objects: stars that erupt into cool supergiants (SECS). They do not develop optically thin or nebular phases, and deep P-Cyg profiles denounce large mass loss at least in the early outburst phases. Their visible progenitors are located close to the Main Sequence, away from the post-AGB region. After the outburst, the remnants still closely resemble the precursors (same brightness, same spectral type). Many more similar objects could be buried among poorly studied novae that have been classified as Miras on the base of a single spectrum at maximum brightness.

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arXiv2002

Far-red spectroscopy of peculiar stars and the GAIA mission

The coming GAIA Cornerstone mission by ESA will provide micro-arcsec astrometry, 10 bands photometry and far-red spectroscopy for a huge number of stars in the Galaxy (10^9). GAIA spectroscopy will cover the range 8480--8740 Ang, which includes the CaII triplet and the head of the Paschen series. In this paper we address the diagnostic potential of this wavelength range toward detection and characterization of peculiar stars.

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