Francesco Pala | Università di Sassari (original) (raw)

Papers by Francesco Pala

SPIE Proceedings, 2008

We present the status of the Sardinia Radio Telescope (SRT) project, a new general purpose, fully... more We present the status of the Sardinia Radio Telescope (SRT) project, a new general purpose, fully steerable 64 m diameter parabolic radiotelescope capable to operate with high efficiency in the 0.3-116 GHz frequency range. The instrument is the result of a scientific ...

The Astrophysical Journal, 1994

... AND FRANCESCO PALLA Osservatorio Astrofisico di Arcetri, Lgo Enrico Fermi 5, 1-50125 Firenze,... more ... AND FRANCESCO PALLA Osservatorio Astrofisico di Arcetri, Lgo Enrico Fermi 5, 1-50125 Firenze, Italy; galli®arcetri.astro.it; palla@arcetri.astro.it Osci&amp;amp;amp;it Sm Osservatorio Astronomico di Collurania, 1-64100 Teramo, Italy; FEDERICO FERRiNI Dipartimento di Fisica, Sezione di ...

The Astrophysical Journal, 1997

The Astrophysical Journal, 1998

IRAS 23385+6053 is a Young Stellar Object with luminosity ∼ 1.6 × 10 4 L at a kinematic distance ... more IRAS 23385+6053 is a Young Stellar Object with luminosity ∼ 1.6 × 10 4 L at a kinematic distance of 4.9 kpc. This candidate precursor of an ultracompact Hii region is associated with a millimeter source detected at the JCMT but is undetected at centimeter wavelengths with the VLA. We observed this source with the OVRO millimeter array at 3.4 mm in the continuum, HCO + (1 → 0), H 13 CO + (1 → 0) and SiO(v=0, 2 → 1) line emission, and with CAM aboard ISO at 6.75µm and 15µm. The IRAS source is coincident with a 3.4 mm compact (r core 0.048 pc) and massive (M 370 M) core, which is undetected at 15µm to a 3σ level of 6 mJy; this is compatible with the derived H 2 column density of ∼ 2 × 10 24 cm −2 and the estimated visual extinction A V ∼2000 mag. We find L submm /L bol ∼ 3 × 10 −3 and M env /M 1, typical of Class 0 objects. The source is also associated with a compact outflow characterized by a size ∼ < r core , a dynamical timescale of ∼ < 7 × 10 3 years, and a mass loss rateṀ ∼ > 10 −3 M yr −1 .

The Astrophysical Journal, 2011

Monthly Notices of the Royal Astronomical Society, 2007

Chandra gratings spectra of a sample of 15 massive OB stars were analyzed under the basic assumpt... more Chandra gratings spectra of a sample of 15 massive OB stars were analyzed under the basic assumption that the X-ray emission is produced in an ensemble of shocks formed in the winds driven by these objects. Shocks develop either as a result of radiation-driven instabilities or due to confinement of the wind by relatively strong magnetic field, and since they are radiative, a simple model of their X-ray emission was developed that allows a direct comparison with observations. According to our model, the shock structures (clumps, complete or fractional shells) eventually become 'cold' clouds in the X-ray sky of the star. As a result, it is expected that for large covering factors of the hot clumps, there is a high probability for X-ray absorption by the 'cold' clouds, resulting in blue-shifted spectral lines. Our analysis has revealed that such a correlation indeed exists for the considered sample of OB stars. As to the temperature characteristics of the X-ray emission plasma, the studied OB stars fall in two groups: (i) one with plasma temperature limited to ∼0.1-0.4 keV; (ii) the other wtih X-rays produced in plasmas at considerably higher temperatures. We argue that the two groups correspond to different mechanisms for the origin of X-rays: in radiative-driven instability shocks and in magnetically-confined wind shocks, respectively.

Arxiv preprint astro-ph/9811210, 1998

Rendiconti Lincei, 2011

We apply state-resolved molecular kinetics to study the chemical evolution of the primordial Univ... more We apply state-resolved molecular kinetics to study the chemical evolution of the primordial Universe. In particular, the vibrationally resolved kinetics of the most abundant molecular species, H2 and its cation H2+, is calculated by solving a master equation. The results show that the vibrational level populations of these two species are not in local thermal equilibrium for a wide range

The Astrophysical Journal Supplement Series, 2009

We present a new analysis of the stellar population of the Orion Nebula Cluster (ONC) based on mu... more We present a new analysis of the stellar population of the Orion Nebula Cluster (ONC) based on multi-band optical photometry and spectroscopy. We study the color-color diagrams in BVI, plus a narrowband filter centered at 6200 Å, finding evidence that intrinsic color scales valid for main-sequence dwarfs are incompatible with the ONC in the M spectral-type range, while a better agreement is found employing intrinsic colors derived from synthetic photometry, constraining the surface gravity value as predicted by a pre-main-sequence isochrone. We refine these model colors even further, empirically, by comparison with a selected sample of ONC stars with no accretion and no extinction. We consider the stars with known spectral types from the literature, and extend this sample with the addition of 65 newly classified stars from slit spectroscopy and 182 M-type from narrowband photometry; in this way, we isolate a sample of about 1000 stars with known spectral type. We introduce a new method to self-consistently derive the stellar reddening and the optical excess due to accretion from the location of each star in the BVI color-color diagram. This enables us to accurately determine the extinction of the ONC members, together with an estimate of their accretion luminosities. We adopt a lower distance for the Orion Nebula than previously assumed, based on recent parallax measurements. With a careful choice of also the spectral-type-temperature transformation, we produce the new Hertzsprung-Russell diagram of the ONC population, more populated than previous works. With respect to previous works, we find higher luminosity for late-type stars and a slightly lower luminosity for early types. We determine the age distribution of the population, peaking from ∼2 to ∼3 Myr depending on the model. We study the distribution of the members in the massage plane and find that taking into account selection effects due to incompleteness, removes an apparent correlation between mass and age. We derive the initial mass function for low-and intermediatemass members of the ONC, which turns out to be model dependent and shows a turnover at M 0.2 M .

Astronomy & Astrophysics, 2007

Context. T Tau stars display different X-ray properties depending on whether they are accreting (... more Context. T Tau stars display different X-ray properties depending on whether they are accreting (classical T Tau stars; CTTS) or not (weak-line T Tau stars; WTTS). X-ray properties may provide insight into the accretion process between disk and stellar surface. Aims. We use data from the XMM-Newton Extended Survey of the Taurus molecular cloud (XEST) to study differences in X-ray properties between CTTS and WTTS. Methods. XEST data are used to perform correlation and regression analysis between X-ray parameters and stellar properties. Results. We confirm the existence of a X-ray luminosity (L X) vs. mass (M) relation, L X ∝ M 1.69 ± 0.11 , but this relation is a consequence of X-ray saturation and a mass vs. bolometric luminosity (L *) relation for the TTS with an average age of 2.4 Myr. X-ray saturation indicates L X = const.L * , although the constant is different for the two subsamples: const. = 10 −3.73 ± 0.05 for CTTS and const. = 10 −3.39 ± 0.06 for WTTS. Given a similar L * distribution of both samples, the X-ray luminosity function also reflects a real X-ray deficiency in CTTS, by a factor of ≈2 compared to WTTS. The average electron temperatures T av are correlated with L X in WTTS but not in CTTS; CTTS sources are on average hotter than WTTS sources. At best marginal dependencies are found between X-ray properties and mass accretion rates or age. Conclusions. The most fundamental properties are the two saturation laws, indicating suppressed L X for CTTS. We speculate that some of the accreting material in CTTS is cooling active regions to temperatures that may not significantly emit in the X-ray band, and if they do, high-resolution spectroscopy may be required to identify lines formed in such plasma, while CCD cameras do not detect these components. The similarity of the L X vs. T av dependencies in WTTS and main-sequence stars as well as their similar X-ray saturation laws suggests similar physical processes for the hot plasma, i.e., heating and radiation of a magnetic corona.

Astronomy & Astrophysics, 2010

Context. Abundances of iron-peak and α-elements are poorly known in Orion, and the available meas... more Context. Abundances of iron-peak and α-elements are poorly known in Orion, and the available measurements yield contradictory results. Aims. We measure accurate and homogeneous elemental abundances of the Orion subgroups ONC and OB1b, and search for abundance differences across the Orion complex. Methods. We present flames/uves spectroscopic observations of 20 members of the ONC and OB1b. We measured radial velocity, veiling, effective temperature using two spectroscopic methods, and determined the chemical abundances of Fe, Na, Al, Si, Ca, Ti, and Ni using the code MOOG. We also performed a new consistent analysis of spectra previously analyzed by our group. Results. We find three new binaries in the ONC, two in OB1b, and three non-members in OB1b (two of them most likely being OB1a/25 Ori members). Veiling only affects one target in the ONC, and the effective temperatures derived using two spectroscopic techniques agree within the errors. The ONC and OB1b are characterized by a small scatter in iron abundance, with mean [Fe/H] values of −0.11 ± 0.08 and −0.05 ± 0.05, respectively. We find a small scatter in all the other elemental abundances. We confirm that P1455 is a metal-rich star in the ONC. Conclusions. We conclude that the Orion metallicity is not above the solar value. The OB1b group might be slightly more metal-rich than the ONC; on the other hand, the two subgroups have similar almost solar abundances of iron-peak and α-elements with a high degree of homogeneity.