Near Infrared polarimetry of a sample of YSOs

Our goal is to study the physical properties of the circumstellar environment of young stellar objetcs (YSOs). In particular, the determination of the scattering mechanism can help to constrain the optical depth of the disk and/or envelope in the near infrared. We used the IAGPOL imaging polarimeter along with the CamIV infrared camera at the LNA observatory to obtain near infrared polarimetry measurements at the H band of a sample of optically visible YSOs, namely, eleven T Tauri stars and eight Herbig Ae/Be stars. An independent determination of the disk (or jet) orientation was obtained for twelve objects from the literature. The circumstellar optical depth could be then estimated comparing the integrated polarization position angle (PA) with the direction of the major axis of the disk projected in the plane of the sky. In general, optically thin disks have polarization PA perpendicular to the disk plane. In contrast, optically thick disks produce polarization PA parallel to the disks. Among the T Tauri stars, three are consistent with optically thin disks (AS 353A, RY Tau and UY Aur) and five with optically thick disks (V536 Aql, DG Tau, DO Tau, HL Tau and LkHalpha 358). Among the Herbig Ae/Be stars, two stars show evidence of optically thin disk (Hen 3-1191 and VV Ser) and two of optically thick disks (PDS 453 and MWC 297). Our results seem consistent with the fact that optically thick disks at near infrared bands are associated more likely with younger YSOs. Marginal evidence of polarization reversal is found in RY Tau, RY Ori, WW Vul, and UY Aur. On the first three cases this feature can be associated to the UXOR phenomenon. Correlations with the IRAS colours and the spectral index yielded evidence of an evolutionary segregation with the disks tend to be optically thin when they are older.Comment: 15 pages, 15 figures, accepted in Astronomy and Astrophysic

ISO spectroscopy of disks around Herbig Ae/Be stars

We have investigated the infrared spectra of all 46 Herbig Ae/Be stars for which spectroscopic data is available in the ISO data archive. Our quantitative analysis of these spectra focusses on the emission bands linked to polycyclic aromatic hydrocarbons (PAHs), the amorphous 10 micron silicate band and the crystalline silicate band at 11.3 micron. We have detected PAH emission in 57% of the Herbig stars in our sample. Clear examples of differences in the PAH spectra are present within our sample, indicating differences in PAH size, chemistry and/or ionization. Amorphous silicate emission was detected in the spectra of 52% of the sample stars, amorphous silicate absorption in 13%. We have detected crystalline silicate emission in 11 stars (24% of our sample), of which four (9%) also display strong PAH emission. We have classified the sample sources according to the strength of their mid-IR energy distribution. The systems with stronger mid-infared (20-100 um) excesses relative to their near-infrared (1-5 um) excess display significantly more PAH emission than those with weaker mid-infrared excesses. This provides strong observational support for the disk models by Dullemond (2002), in which systems with a flaring disk geometry display a strong mid-infrared excess, whereas those with disks that are strongly shadowed by the puffed-up inner rim of the disk only display modest amounts of mid-infrared emission. The PAH emission is expected to be produced mainly in the part of the disk atmosphere that is directly exposed to radiation from the central star. In this model, self-shadowed disks should display weaker PAH emission than flared disks, consistent with our observations.Comment: 27 pages, 26 figures, A&A accepted (22/06/2004

Disks around CQ Tau and MWC 758: dense PDR or gas dispersal?

The overall properties of disks surrounding intermediate PMS stars (HAe) are not yet well constrained by current observations. The disk inclination, which significantly affect SED modeling, is often unknown. We attempted to resolve the disks around CQ Tau and MWC 758, to provide accurate constraints on the disk parameters, in particular the temperature and surface density distribution. We report arcsecond resolution observations of dust and CO line emissions with the IRAM array. The disk properties are derived using a standard disk model. We use the Meudon PDR code to study the chemistry. The two disks share some common properties. The mean CO abundance is low despite disk temperatures above the CO condensation temperature. Furthermore, the CO surface density and dust opacity have different radial dependence. The CQ Tau disk appears warmer, and perhaps less dense than that of MWC 758. Modeling the chemistry, we find that photodissociation of CO is a viable mechanism to explain the low abundance. The photospheric flux is not sufficient for this: a strong UV excess is required. In CQ Tau, the high temperature is consistent with expectation for a PDR. The PDR model has difficulty explaining the mild temperatures obtained in MWC 758, for which a low gas-to-dust ratio is preferred. A yet unexplored alternative could be that, despite currently high gas temperatures, CO remains trapped in grains, as the models suggest that large grains can be cold enough to prevent thermal desorption of CO. The low inclination of the CQ Tau disk, ~30^\circ, challenges previous interpretations given for the UX Ori - like luminosity variations of this star. We conclude that CO cannot be used as a simple tracer of gas-to-dust ratio, the CO abundance being affected by photodissociation, and grain growth.Comment: Accepted for publication in Astronomy & Astrophysic