F. Strafella

Infrared emission from dust structures surrounding Herbig Ae/Be stars

IR observations in J, H, K, L, M, and 8-13 μm bands of Herbig Ae/Be stars located in the southern hemisphere are presented. These results enlarge an already existing data base, increasing the significance of the correlations which can be obtained from observational parameters. Silicate features detected both in emission and in absorption indicate the presence of dust around these objects. A first analysis based on two-color diagrams, polarization, and luminosity suggests that the spherical geometry for the dust distribution is a more common feature, with respect to the flattened structure

Herschel observations of B1-bS and B1-bN: two first hydrostatic core candidates in the Perseus star-forming cloud

We report far-infrared Herschel observations obtained between 70 μm and 500 μm of two star-forming dusty condensations, [HKM99] B1-bS and [HKM99] B1-bN, in the B1 region of the Perseus star-forming cloud. In the western part of the Perseus cloud, B1-bS is the only source detected in all six PACS and SPIRE photometric bands, but it is not visible in the Spitzer map at 24 μm. B1-bN is clearly detected between 100 μm and 250 μm. We have fitted the spectral energy distributions of these sources to derive their physical properties, and find that a simple greybody model fails to reproduce the observed spectral energy distributions. At least a two-component model is required, consisting of a central source surrounded by a dusty envelope. The properties derived from the fit, however, suggest that the central source is not a Class 0 object. We then conclude that while B1-bS and B1-bN appear to be more evolved than a pre-stellar core, the best-fit models suggest that their central objects are younger than a Class 0 source. Hence, they may be good candidates to be examples of the first hydrostatic core phase. The projected distance between B1-bS and B1-bN is a few Jeans lengths. If their physical separation is close to this value, this pair would allow studying the mutual interactions between two forming stars at a very early stage of their evolution.

A Herschel study of YSO evolutionary stages and formation timelines in two fields of the Hi-GAL survey

We present a first study of the star-forming compact dust condensations revealed by Herschel in the two 2° × 2° Galactic Plane fields centered at [l, b] = [30°, 0°] and [l, b] =[59°, 0°] , respectively, and observed during the science demonstration phase for the Herschel Infrared GALactic plane survey (Hi-GAL) key-project. Compact source catalogs extracted for the two fields in the five Hi-GAL bands (70, 160, 250, 350 and 500 μm) were merged based on simple criteria of positional association and spectral energy distribution (SED) consistency into a final catalog which contains only coherent SEDs with counterparts in at least three adjacent Herschel bands. These final source lists contain 528 entries for the l = 30° field, and 444 entries for the = 59° field. The SED coverage has been augmented with ancillary data at 24 μm and 1.1 mm. SED modeling for the subset of 318 and 101 sources (in the two fields, respectively) for which the distance is known was carried out using both a structured star/disk/envelope radiative transfer model and a simple isothermal grey-body. Global parameters like mass, luminosity, temperature and dust properties have been estimated. The L_(bol)/M_(env) ratio spans four orders of magnitudes from values compatible with the pre-protostellar phase to embedded massive zero-age main sequence stars. Sources in the l = 59° field have on average lower L/M, possibly outlining an overall earlier evolutionary stage with respect to the sources in the l = 30° field. Many of these cores are actively forming high-mass stars, although the estimated core surface densities appear to be an order of magnitude below the 1 g cm^(-2) critical threshold for high-mass star formation.

On the Circumstellar Matter Distribution around Herbig Ae/Be Stars

The spectral energy distributions (SEDs) observed in Herbig Ae/Be stars are critically analyzed in connection with a spherically symmetric model for matter distribution around the central objects. The possibility that the dusty component is described by an MRN-DL mixture (see the work of Mathis, Rumpl, and Nordsiek; Draine and Lee) and is constituted by particles with mean sizes greater than those typical of the interstellar medium is considered. We find that while spherical models with interstellar-like dust are able to describe the observed SEDs in the visible, infrared, and radio region, they generally fail to fit the submillimeter fluxes. A better agreement is obtained if the absorption coefficient of the dust at far-infrared wavelengths follows a dependence that is shallower than that of the standard MRN-DL mixture. In this case, general agreement among the observed and the model-derived spectral types, distance, and visual extinction is found. Inconsistencies in submillimeter fluxes obtained by different authors are discussed in connection with the different telescopes used and the confusion of the sources. An indication is given for the density distribution around Herbig Ae/Be stars, which, if modeled with a power law, goes as the inverse of the radial distance in most cases.

Microlensing search towards M31

We present the first results of the analysis of data collected during the 1998-99 observational campaign at the 1.3 meter McGraw-Hill Telescope, towards the Andromeda galaxy (M 31), aimed to detect gravitational microlensing effects as a probe for the presence of dark matter in our Galaxy and in the M 31 halo. The analysis is performed using the pixel lensing technique, which consists of the study of flux variations of unresolved sources and has been proposed and implemented by the AGAPE collaboration. We carry out a shape analysis by demanding that the detected flux variations be achromatic and compatible with a Paczynski light curve. We apply the Durbin-Watson hypothesis test to the residuals. Furthermore, we consider the background of variables sources. Finally five candidate microlensing events emerge from our selection. Comparing with the predictions of a Monte Carlo simulation, assuming a standard spherical model for the M 31 and Galactic haloes, and typical values for the MACHO mass, we find that our events are only marginally consistent with the distribution of observable parameters predicted by the simulation.

The Blast Survey of the Vela Molecular Cloud: Physical Properties of the Dense Cores in Vela-D

The Balloon-borne Large-Aperture Submillimeter Telescope (BLAST) carried out a 250, 350, and 500 μm survey of the galactic plane encompassing the Vela Molecular Ridge, with the primary goal of identifying the coldest dense cores possibly associated with the earliest stages of star formation. Here, we present the results from observations of the Vela-D region, covering about 4 deg^2, in which we find 141 BLAST cores. We exploit existing data taken with the Spitzer MIPS, IRAC, and SEST-SIMBA instruments to constrain their (single-temperature) spectral energy distributions, assuming a dust emissivity index β = 2.0. This combination of data allows us to determine the temperature, luminosity, and mass of each BLAST core, and also enables us to separate starless from protostellar sources. We also analyze the effects that the uncertainties on the derived physical parameters of the individual sources have on the overall physical properties of starless and protostellar cores, and we find that there appear to be a smooth transition from the pre- to the protostellar phase. In particular, for protostellar cores we find a correlation between the MIPS24 flux, associated with the central protostar, and the temperature of the dust envelope. We also find that the core mass function of the Vela-D cores has a slope consistent with other similar (sub)millimeter surveys.

Direct estimate of cirrus noise in Herschel Hi-GAL images

In Herschel images of the Galactic plane and many star forming regions, a major factor limiting our ability to extract faint compact sources is cirrus confusion noise, operationally defined as the “statistical error to be expected in photometric measurements due to confusion in a background of fluctuating surface brightness”. The histogram of the flux densities of extracted sources shows a distinctive faint-end cutoff below which the catalog suffers from incompleteness and the flux densities become unreliable. This empirical cutoff should be closely related to the estimated cirrus noise and we show that this is the case. We compute the cirrus noise directly, both on Herschel images from which the bright sources have been removed and on simulated images of cirrus with statistically similar fluctuations. We connect these direct estimates with those from power spectrum analysis, which has been used extensively to predict the cirrus noise and provides insight into how it depends on various statistical properties and photometric operational parameters. We report multi-wavelength power spectra of diffuse Galactic dust emission from Hi-GAL observations at 70 to 500 μm within Galactic plane fields at l = 30° and l = 59° .We find that the exponent of the power spectrum is about −3. At 250 μm, the amplitude of the power spectrum increases roughly as the square of the median brightness of the map and so the expected cirrus noise scales linearly with the median brightness. For a given region, the wavelength dependence of the amplitude can be described by the square of the spectral energy distribution (SED) of the dust emission. Generally, the confusion noise will be a worse problem at longer wavelengths, because of the combination of lower angular resolution and the rising power spectrum of cirrus toward lower spatial frequencies, but the photometric signal to noise will also depend on the relative SED of the source compared to the cirrus.

M31 PIXEL LENSING EVENT OAB-N2: A STUDY OF THE LENS PROPER MOTION

We present an updated analysis of the M31 pixel lensing candidate event OAB-N2 previously reported by Calchi Novati et al. Here we take advantage of new data both astrometrical and photometrical. For astrometry: using archival 4 m KPNO and Hubble Space Telescope/WFPC2 data we perform a detailed analysis of the event source whose result, although not fully conclusive on the source magnitude determination, is confirmed by the following light curve photometry analysis. For photometry: first, unpublished WeCAPP data allow us to confirm OAB-N2, previously reported only as a viable candidate, as a well-constrained pixel lensing event. Second, this photometry enables a detailed analysis in the event parameter space including the effects due to a finite source size. The combined results of these analyses allow us to put a strong lower limit on the lens proper motion. This outcome favors the MACHO lensing hypothesis over self-lensing for this individual event and points the way toward distinguishing between the MACHO and self-lensing hypotheses from larger data sets.

CANDIDATE MICROLENSING EVENTS FROM M31 OBSERVATIONS WITH THE LOIANO TELESCOPE

Microlensing observations toward M31 are a powerful tool for the study of the dark matter population in the form of MACHOs both in the Galaxy and the M31 halos, a still unresolved issue, as well as for the analysis of the characteristics of the M31 luminous populations. In this work, we present the second-year results of our pixel lensing campaign carried out toward M31 using the 152 cm Cassini telescope in Loiano. We have established an automatic pipeline for the detection and the characterization of microlensing variations. We have carried out a complete simulation of the experiment and evaluated the expected signal, including an analysis of the efficiency of our pipeline. As a result, we select 1-2 candidate microlensing events (according to different selection criteria). This output is in agreement with the expected rate of M31 self-lensing events. However, the statistics are still too low to draw definitive conclusions on MACHO lensing.

Modeling the Continuum Emission from the Circumstellar Environment of Herbig Ae/Be Stars

This paper discusses a model for the continuum emission of the Herbig Ae/Be stars in the light of an updated set of observational data spanning 5 orders of magnitude in wavelength and including the low-resolution spectra obtained with the Short Wavelength Spectrometer and Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO). The model is used to reproduce the continuum emission of the 36 Herbig Ae/Be stars included in the list by The and coworkers and observed by ISO. The circumstellar matter responsible for the observed spectral energy distributions has been investigated by comparing the set of the observations with the model spectra computed for different possible distributions of circumstellar matter. Cases have been considered with the circumstellar regions partially evacuated along the polar axis by the action of the stellar wind, a phenomenon that is quite common in these pre-main-sequence objects. The inclusion of the polar cavities indirectly allows geometries in which a small-scale disklike structure around the central star is present. The possible coexistence of two different density profiles, in the inner and the outer region of the envelope, respectively, has been also considered. The comparison of the computed models with the observed spectral energy distributions selects the parameter values in such a way that the larger dust grains are preferentially associated with the later spectral types. We find that 17 objects are reasonably fitted, eight of which with a purely spherical model and the remaining nine with the inclusion of the polar cavities. For 10 further objects the fit is worse, and for the remaining nine, almost all associated to IR companions, our model is clearly inappropriate. A linear relationship is suggested between the logarithm of the initial density n0 and the exponent p of the power law n(r) = n0(R*/r)p adopted for the circumstellar density distribution.