J. Martin-Pintado

Dense gas and star formation: characteristics of cloud cores associated with water masers

We have observed 150 regions of massive star formation, selected originally by the presence of an H2O maser, in the J = 5 → 4, 3 → 2, and 2 → 1 transitions of CS, and 49 regions in the same transitions of C34S. Over 90% of the 150 regions were detected in the J = 2 → 1 and 3 → 2 transitions of CS, and 75% were detected in the J = 5 → 4 transition. We have combined the data with the J = 7 → 6 data from our original (1992) survey to determine the density by analyzing the excitation of the rotational levels. Using large velocity gradient models, we have determined densities and column densities for 71 of these regions. The gas densities are very high (log n = 5.9), but much less than the critical density of the J = 7 → 6 line. Small maps of 25 of the sources in the J = 5 → 4 line yield a mean diameter of 1.0 pc. Several estimates of the mass of dense gas were made for the sources for which we had sufficient information. The mean virial mass is 3800 M☉. The mean ratio of bolometric luminosity to virial mass (L/M) is 190, about 50 times higher than estimates made using CO emission, suggesting that star formation is much more efficient in the dense gas probed in this study. The depletion time for the dense gas is ~1.3 × 107 yr, comparable to the timescale for gas dispersal around open clusters and OB associations. We find no statistically significant line width-size or density-size relationships in our data. Instead, both line width and density are greater for a given size than would be predicted by the usual relationships. We find that the line width increases with density, the opposite of what would be predicted by the usual arguments. We estimate that the luminosity of our Galaxy (excluding the inner 400 pc) in the CS J = 5 → 4 transition is 15-23 L☉, considerably less than the luminosity in this line within the central 100 pc of NGC 253 and M82. In addition, the ratio of far-infrared luminosity to CS luminosity is higher in M82 than in any cloud in our sample.

Warm H

We present ISO observations of several H2 pure-rotational lines (from S(0) to S(5)) towards a sample of 16 molecular clouds distributed along the central ~ 500 pc of the Galaxy. We also present C18O and 13CO J=1->0 and J=2->1 observations of these sources made with the IRAM-30m telescope. With the CO data we derive H2 densities of 10e(3.5-4.0) cm-3 and H2 column densities of a few 10e22 cm-2. We have corrected the H2 data for ~ 30 magnitudes of visual extinction using a self-consistent method. In every source, we find that the H2 emission exhibits a large temperature gradient. The S(0) and S(1) lines trace temperatures (T) of ~150 K while the S(4) and S(5) lines indicate temperatures of ~ 600K. The warm H2 column density is typically ~1-2 x 10e22 cm-2, and is predominantly gas with T=150 K. This is the first direct estimate of the total column density of the warm molecular gas in the Galactic center region. These warm H2 column densities represent a fraction of ~ 30 % of the gas traced by the CO isotopes emission. The cooling by H2 in the warm component is comparable to that by CO. Comparing our H2 and CO data with available ammonia NH3 observations from literature one obtains relatively high NH3 abundances of a few 10e(-7) in both the warm and the cold gas. A single shock or Photo-Dissociation Region (PDR) cannot explain all the observed H2 lines. Alternatives for the heating mechanisms are discussed.

_\mathsf{2}

In order to study the existence and evolution of circumstellar disks around intermediate-mass stars (M 3M), we have obtained single-dish and interferometric continuum images at 2.6 mm and 1.3 mm of the intermediate-mass protostar NGC 7129 FIRS 2 and of the Herbig Be stars LkH 234 and HD 200775. These objects are representative of the dierent stages of the pre-main sequence evolution with ages ranging from a few 10 3 to 8 10 6 years. Single-dish and interferometric observations of the outflows associated with these sources are also presented. In NGC 7129 FIRS 2, two millimeter sources are required to t the interferometric 1.3 mm continuum emission. Only the most intense of these millimeter objects, FIRS 2 { MM1, seems to be associated with the CO outflow. The second and weaker source, FIRS 2 { MM2, does not present any sign of stellar activity. The single-dish map of the CO outflow presents an unusual morphology with the blue and red lobes separated by an angle of 82 .T he COJ =1 ! 0 interferometric image shows that this unusual morphology is the result of the superposition of two outflows, one of them associated with FIRS 2 { MM1 (the blue lobe in the single-dish map) and the other (the red lobe) with a new infrared source (FIRS 2 { IR) which is not detected in the millimeter continuum images. The interferometric 1.3 mm continuum image of NGC 7129 FIRS 1 reveals that LkH 234 is a member of a cluster of embedded objects. Two millimeter clumps are detected in this far-infrared source. The strongest is spatially coincident with the mid-infrared companion of LkH 234, IRS 6. A new millimeter clump, FIRS 1 { MM1, is detected at an oset ( 3:23 00 ,3 .0 00 )f rom LkH 234. We have not detected any compact source towards LkH 234 with a limit for the mass of a circumstellar disk, MD < 0.1 M. The comparison of the interferometric CO J =1 ! 0 and continuum images reveals that IRS 6 very likely drives the energetic molecular outflow detected towards NGC 7129 FIRS 1 and the (SII) jet. The extremely young object FIRS 1 { MM1 (it has not been detected in the near-and mid-infrared) turns out to be the driving source of the H2 jet. There is no evidence for the existence of a bipolar outflow associated with LkH 234. We have not detected 1.3 mm continuum emission towards HD 200775. Our observations imply a 3- upper limit of < 0.002 M for the mass of a circumstellar disk. This is the lowest upper limit obtained so far in a Herbig Be star. Thus our observations provide new important information on three protostars (IRS 6, FIRS 1 { MM1 and FIRS 2 { MM1), one infrared star (FIRS 2 { IR) and two Herbig Be stars. The luminosities of the protostars are consistent with being intermediate-mass objects (M 3:5{4:5 M). They are surrounded by thick envelopes with masses ranging between2 3:5 M and drive energetic outflows. Circumstellar disks and bipolar outflows are not detected toward the Herbig Be stars. We have obtained an upper limit for the disk/stellar mass ratio, MD=M ,o f< 0.02 in LkH 234 and of < 0.0002 in HD 200775. Our limit in HD 200775 implies that in evolved Herbig Be stars the MD=M ratio is more than two orders of magnitude lower than in T Tauri and Herbig Ae stars. We propose that in massive stars (M 5 M )b oth the dispersal of the outer disk and the energetic mass-loss, occur early in the stellar evolution before the star becomes visible. Some mechanisms for the dispersal of the outer disk are discussed.

in the Galactic center region

In this Letter we present the first images of the emission of SiO and H13CO+ in the nucleus of the starburst galaxy M82. Contrary to other molecular species that mainly trace the distribution of the star-forming molecular gas within the disk, the SiO emission extends noticeably out of the galaxy plane. The bulk of the SiO emission is restricted to two major features. The first feature, referred to as the SiO supershell, is an open shell of 150 pc diameter, located 120 pc west from the galaxy center. The SiO supershell represents the inner front of a molecular shell expanding at ~40 km s-1, produced by mass ejection around a supercluster of young stars containing supernova remnant SNR 41.95+57.5. The second feature is a vertical filament, referred to as the SiO chimney, emanating from the disk at 200 pc east from the galaxy center. The SiO chimney reaches a 500 pc vertical height, and it is associated with the most prominent chimney identified in radio continuum maps. The kinematics, morphology, and fractional abundances of the SiO gas features in M82 can be explained in the framework of shocked chemistry driven by local episodes of gas ejection from the starburst disk. The SiO emission stands out as a privileged tracer of the disk-halo interface in M82. We speculate that the chimney and the supershell, each injecting ~107 M☉ of molecular gas, are two different evolutionary stages in the outflow phenomenon building up the gaseous halo.In this Letter we present the first images of the emission of SiO and H13CO+ in the nucleus of the starburst galaxy M82. Contrary to other molecular species that mainly trace the distribution of the star-forming molecular gas within the disk, the SiO emission extends noticeably out of the galaxy plane. The bulk of the SiO emission is restricted to two major features. The first feature, referred to as the SiO supershell, is an open shell of 150 pc diameter, located 120 pc west from the galaxy center. The SiO supershell represents the inner front of a molecular shell expanding at 40 km/s, produced by mass ejection around a supercluster of young stars containing supernova remnant SNR 41.95+57.5. The second feature is a vertical filament, referred to as the SiO chimney, emanating from the disk at 200 pc east from the galaxy center. The SiO chimney reaches a 500 pc vertical height, and it is associated with the most prominent chimney identified in radio continuum maps. The kinematics, morphology, and fractional abundances of the SiO gas features in M82 can be explained in the framework of shocked chemistry driven by local episodes of gas ejection from the starburst disk.

Disks and outflows around intermediate-mass stars and protostars

We present a systematic study of the material surrounding intermediate-mass stars. Our sample includes 34 Herbig Ae/Be (HAEBE) stars of dierent ages and luminosities. This is a quite complete representation of the whole class of HAEBE stars and consequently, our conclusions should have a solid statistical meaning. In addition, we have observed 2 intermediate-mass protostars and included published data on 15 protostellar objects in order to determine the evolution of the circumstellar material in the early stages of stellar evolution. All the HAEBE stars have been classied according with the three Types already dened in Fuente et al. (1998): Type I stars are immersed in a dense clump and have associated bipolar outflows, their ages are0.1 Myr; Type II stars are still immersed in the molecular cloud though not in a dense clump, their ages are between af ew 0.1 to af ew Myr; Type III stars have completely dispersed the surrounding material and are located in a cavity of the molecular cloud, their ages are >1 Myr. Our observations are used to reconstruct the evolution of the circumstellar material around intermediate-mass stars and investigate the mass dispersal mechanisms at the dierent stages of the stellar evolution. Our results can be summarized as follows: intermediate-mass stars disperse90% of the mass of the parent clump during the protostellar phase. During this phase, the energetic outflows sweep out the gas and dust forming a biconical cavity while the equatorial material is infalling to feed the circumstellar disk and eventually the protostar. In this way, the density structure of the parent clump remains well described by a density law n/ r with 2 0:08 pc) cavities in the molecular cloud, producing a dramatic change in the morphology of the region. This dierence is easily understood if photodissociation plays an important role in the mass dispersal around these objects.

SiO Chimneys and Supershells in M82

We have detected high-velocity ammonia emission associated with four bipolar molecular outflows excited by young stars (L1157, L1448, IRAS 3282, and NGC 2071). The high-velocity emission is more prominent in the (3, 3) line, although it is also detected in the (1, 1), (2, 2), and (4, 4) lines. We deduce that the accelerated gas is heated to more than 50-100 K, i.e., a factor of 3-7 over the ambient temperatures (which are in the range 12-26 K). Such temperatures, when taken into account in the analysis of previous SiO observations, confirm that the shocked material is rather dense [n(H 2 )∼10 5 cm −3 ]

The history of mass dispersal around Herbig Ae/Be stars

We have observed seven ultracompact H II regions in hydrogen recombination lines in the millimeter band. Toward four of these regions there is a high-velocity (FWHM 60-80 km s-1) component in the line profiles. The high-velocity gas accounts for 35%-70% of the emission measure within the beam. We compare these objects with an additional seven similar sources we have found in the literature. The broad recombination line objects (BRLOs) make up about 30% of all sources in complexes containing ultracompact H II regions. Comparison of spectral line and continuum data implies that the BRLOs coincide with sources with rising spectral indices, ≥0.4 up to 100 GHz. Both the number of BRLOs and their frequency of occurrence within H II region complexes, when coupled with their small size and large internal motions, mean that the apparent contradiction between the dynamical and the population lifetimes for BRLOs is even more severe than for ultracompact H II regions as a whole. We evaluate a number of possible models for the origin of the broad recombination line emission. The lifetime, morphology, and rising spectral index of the sources argue for photoevaporated disks as the cause for BRLOs. Existing models for such regions, however, do not account for the large amounts of ionized gas observed at supersonic velocities.

High-Velocity Hot Ammonia in Bipolar Outflows

We present observations of C2H5OH toward molecular clouds in Sgr A and Sgr B2 and associated with thermal and nonthermal features in the Galactic center (GC). C2H5OH emission in Sgr A and Sgr B2 is widespread but not uniform. C2H5OH emission is much weaker, or it is not detected in some molecular clouds in both complexes, in particular those with radial velocities between 70 and 120 km s-1. While most of the clouds associated with the thermal features do not show C2H5OH emission, that associated with the nonthermal radio arc shows emission. The fractional abundance of C2H5OH in most of the clouds with radial velocities between 0 and 70 km s-1 in Sgr A and Sgr B2 is relatively high, of a few times 10-8. The C2H5OH abundance decreases by more than a factor of 10 (10-9) in the clouds associated with the thermal features. The large abundance of C2H5OH in the gas phase indicates that C2H5OH has formed in grains and released to gas phase by shocks in the last ~105 yr. The implications of this finding in the origin of the shocks in the GC is briefly discussed.

Low-Velocity Ionized Winds from Regions around Young O Stars

We present an analysis of selected ne structure lines (Neii 12.8 m, Neiii 15.6 m, Siii 18.7 and 33.5 m, Oiii 52 and 88 m, Nii 122 ma nd Niii 57 m) observed with the Infrared Space Observatory (ISO) toward the Radio Arc in the Galactic center region (GCR). Most of the data were retrieved from the ISO Data Archive. We study the density of the ionized gas and the large scale ionization structure in a region of 30 30 pc 2 by means of the Oiii 52 m/88 m, Siii 18.7 m/33.5 m, Neiii 15.6 m/Neii12.8 ma nd Niii 57 m/Nii 122 m line ratios. The electron densities (ne) derived from the Oiii lines ratio indicate the presence of di use ionized material withne of 10 1:8 2:6 cm 3 .T he Neiii 15.6 m/Neii 12.8 ma nd Niii 57 m/Nii 122 m line ratios vary from source to source from 0.05 to 0.30 and from 0.3 to 2.5, respec- tively. The Niii 57/Nii 122 m ratio show two clear gradients, one pointing toward the Quintuplet cluster and the other pointing toward the Arches cluster. We have used a simple model to explain the ionization structure observed in the Niii/Nii and Neiii/Neii lines ratios. The model shows that the large scale ionization of the whole region can be accounted for by the UV radiation produced by the Quintuplet and the Arches cluster. Any other ionization mechanism should play a minor role. We also investigate the influence of the clusters on the bubble of warm dust (hereafter Radio Arc Bubble, RAB) seen in the Midcourse Space Experiment (MSX) infrared images. We nd that the warm dust is well correlated with the ionized gas indicating that the dust is also heated by the radiation from both clusters. Furthermore, the elliptical rather than circular symmetry of some structures like the Thermal Filaments can also be explained when one considers the combined eects of both the Arches and the Quintuplet clusters. We have also found that the RAB is lled with continuum emission of hard X-rays and with emission from the 6.4 keV line of neutral or low ionized Fe. We briefly discuss the implications of these ndings on the structure and morphology of the GCR interstellar medium and the possible origin of the RAB.

Large-Scale Grain Mantle Disruption in the Galactic Center

The CO J = 2 - 1 and J = 0 emission of the circumstellar envelope of Mira has been mapped. Three velocity components are found at 44.0, 46.8, and 48.1 km/s. The emission is interpreted as arising from a standard circumstellar envelope and a less extended bipolar outflow. The spherical circumstellar envelope has an expansion velocity of 3 km/s, a mass-loss rate of 1 x 10 to the -7th solar mass/yr, and a total molecular mass of 0.0004 solar mass. 29 refs.