arXiv subjects

T. Alonso-Albi

Publications and source records attributed to T. Alonso-Albi.

24 records · Page 2Linked to original sources

arXiv2009

Circumstellar disks around Herbig Be stars

We completed a search for circumstellar disks around Herbig Be stars using the NRAO Very Large Array (VLA) and the IRAM Plateau de Bure (PdB) interferometers. We present our new VLA and PdBI data for the three objects MWC 297, Z CMa, and LKHa 215. We constructed the SED from near-IR to centimeter wavelengths by adding our millimeter and centimeter data to the available data at other wavelengths, mainly Spitzer images. The entire SED was fitted using a disk+envelope model. In addition, we compiled all the disk millimeter observations in the literature and completed a statistical analysis of all the data. We show that the disk mass is usually only a small percentage (less than 10%) of the mass of the entire envelope in HBe stars. For the disks, there are large source-to-source variations. Two disks in our sample, R Mon and Z CMa, have similar sizes and masses to those found in T Tauri and Herbig Ae stars. The disks around MWC 1080 and MWC 297 are, however, smaller (rout<100 AU). We did not detect the disks towards MWC 137 and LkHa 215 at millimeter wavelengths, which limits the mass and the size of the possible circumstellar disks. A comparison between our data and previous results for T Tauri and Herbig Ae stars indicates that although massive disks (0.1 Msun) are found in young objects (10^4 yr), the masses of the disks around Herbig Be stars are usually 5-10 times lower than those around lower mass stars. We propose that disk photoevaporation is responsible for this behavior. In Herbig Be stars, the UV radiation disperses the gas in the outer disk on a timescale of a few 10^5 yr. Once the outer part of the disk has vanished, the entire gaseous disk is photoevaporated on a very short timescale (10^5 yr) and only a small, dusty disk consisting of large grains remains.

astro-ph↗

arXiv2008

On the chemistry and distribution of HOC+ in M82: More evidence for extensive PDRs

The molecular gas composition in the inner 1 kpc disk of the starburst galaxy M82 resembles that of Galactic Photon Dominated Regions (PDRs). In particular, large abundances of the reactive ions HOC+ and CO+ have been measured in the nucleus of this galaxy. To investigate the origin of the large abundances of reactive ions in M82, we have completed our previous 30m HOC+ J=1-0 observations with the higher excitation HCO+ and HOC+ J=4-3 and 3-2 rotational lines. In addition, we have obtained with the IRAM Plateau de Bure Interferometer (PdBI) a 4" resolution map of the HOC+ 1-0 emission, the first ever obtained in a Galactic or extragalactic source. Our HOC+ interferometric image shows that the emission of the HOC+ 1-0 line is mainly restricted to the nuclear disk, with the maxima towards the E. and W. molecular peaks. In addition, line excitation calculations imply that the HOC+ emission arises in dense gas. Therefore, the HOC+ emission is arising in the dense PDRs embedded in the M82 nuclear disk, rather than in the intercloud phase and/or wind. We have improved our previous chemical model of M82 by (i) using the new version of the Meudon PDR code, (ii) updating the chemical network, and (iii) considering two different types of clouds (with different thickness). Most molecular observations (HCO+, HOC+, CO+, CN, HCN, H3O+) are well explained assuming that ~ 87 % of the mass of the molecular gas is forming small clouds (Av=5 mag) while only ~ 13 % of the mass is in large molecular clouds (Av=50 mag). Such small number of large molecular clouds suggests that M82 is an old starburst, where star formation has almost exhausted the molecular gas reservoir.

astro-ph↗

arXiv2008

The dusty disk around VV Ser

We have carried out observations at millimeter and centimeter wavelengths towards VV Ser using the Plateau de Bure Interferometer and the Very Large Array. This allows us to compute the SED from near infrared to centimeter wavelengths. The modeling of the full SED has provided insight into the dust properties and a more accurate value of the disk mass. The mass of dust in the disk around VV Ser is found to be about 4 10^(-5) Msun, i.e. 400 times larger than previous estimates. Moreoever, the SED can only be accounted for assuming dust stratification in the vertical direction across the disk. The existence of small grains (0.25--1 micron) in the disk surface is required to explain the emission at near- and mid-infrared wavelengths. The fluxes measured at millimeter wavelengths imply that the dust grains in the midplane have grown up to very large sizes, at least to some centimeters.

astro-ph↗

arXiv2007

Protostellar clusters in intermediate-mass (IM) star forming regions

The transition between the low density groups of T Tauri stars and the high density clusters around massive stars occurs in the intermediate-mass (IM) range (M$_*$$\sim$2--8 M$_\odot$). High spatial resolution studies of IM young stellar objects (YSO) can provide important clues to understand the clustering in massive star forming regions. Aims: Our aim is to search for clustering in IM Class 0 protostars. The high spatial resolution and sensitivity provided by the new A configuration of the Plateau de Bure Interferometer (PdBI) allow us to study the clustering in these nearby objects. Methods: We have imaged three IM Class 0 protostars (Serpens-FIRS 1, IC 1396 N, CB 3) in the continuum at 3.3 and 1.3mm using the PdBI. The sources have been selected with different luminosity to investigate the dependence of the clustering process on the luminosity of the source. Results: Only one millimeter (mm) source is detected towards the low luminosity source Serpens--FIRS 1. Towards CB 3 and IC1396 N, we detect two compact sources separated by $\sim$0.05 pc. The 1.3mm image of IC 1396 N, which provides the highest spatial resolution, reveal that one of these cores is splitted in, at least, three individual sources.

astro-ph↗

arXiv2007

New 13CO observations towards the Keplerian gaseous disk around R Mon

We present new high angular resolution observations of the 13CO 1-0 rotational line towards the HBe star R Mon, obtained with the IRAM Bure Interferometer, along with the previous results in the observed transitions 12CO 1-0 and 2-1. We have used a flat disk model to fit the 12CO 1-0 and 2-1, and 13CO 1-0 emission, in a strip perpendicular to the outflow axis. The model assumes standard abundances (X(12CO) = 8x10^-5, X(13CO) = 9x10^7), radial potential temperature and density laws (T (K) = T0 r^-q, rho (cm^-3) = rho0 r^-p), and Local Thermodynamic Equilibrium. The 13CO and 12CO emission is consistent with a flat disk at an inclination angle of 20 degrees in Keplerian rotation around the star. The gaseous disk is fitted with a mass of 0.014 Msun, an outer radius of 1500 AU, a temperature of 4500 K at the inner radius (1 AU), and values of q=0.62 and p=1.3 for the indexes of the temperature and density laws. This new 13CO observations allow us to conclude that the disk around R Mon is flat. Our result confirms previous works suggesting a predominant flat geometry in disks around early Be stars.

astro-ph↗

arXiv2006

A Keplerian gaseous disk around the B0 star R Mon

We present high-angular resolution observations of the circumstellar disk around the massive Herbig Be star R Mon (M~8 Msun) in the continuum at 2.7mm and 1.3mm and the CO 1->0 and 2->1 rotational lines. Based on the new 1.3mm continuum image we estimate a disk mass (gas+dust) of 0.007 Msun and an outer radius of <150 AU. Our CO images are consistent with the existence of a Keplerian rotating gaseous disk around this star. Up to our knowledge, this is the most clear evidence for the existence of Keplerian disks around massive stars reported thus far. The mass and physical characteristics of this disk are similar to thoseof the more evolved T Tauri stars and indicate a shorter timescale for the evolution and dispersal of circumstellar disks around massive stars which lose most of their mass before the star becomes visible.

gr-qc↗