A. Rodriguez-Franco

Observational study of reactive ions and radicals in PDRs

We have carried out a survey of reactive ions (CO + , HOC + , HC 1 8 O + , SO + ) and cyclopropenylidene (C 3 H 2 ) in three prototypical photodissociation regions (PDRs), the reflection nebula NGC 7023, the Orion Bar and the planetary nebula (PN) NGC 7027. The reactive ion CO + has been detected in all the targets with fractional abundances ranging from ∼ 10 - 1 1 to ∼a few 10 - 9 . Its spatial distribution in NGC 7023 and the Orion Bar show that CO + arises in the innermost part (A, < 2 mag) of the PDR. In NGC 7027, the CO + lines shows an expansion velocity higher than that of the CO lines. This high expansion velocity is consistent with the CO + emission arising in the high velocity layer of neutral gas which is being accelerated by the ionized gas. Photochemistry determines the chemical composition of this layer. The reactive ions HOC + and SO + have been detected in NGC 7023 and the Orion Bar. In both sources, the fractional abundance of HOC + is enhanced by a factor of ∼ 10 towards the PDRs, with typical abundances, X H O C + = 0.7-3 x 10 - 1 1 . This enhancement produces a decrease of the [HCO + ]/[HOC + ] abundance ratio towards the PDR. In fact, we have derived [HCO + ]/[HOC + ] ∼ 50-120 in NGC 7023, which is the lowest ratio measured thus far. HOC + and SO + have not been detected in NGC 7027. Interestingly, this is the source with the highest CO + abundance, X C O + = 5 × 10 - 9 . This lack of detection is interpreted as due to the peculiar chemistry of C-rich PNs, in which the abundance of oxygenated molecules, in particular H 2 O, is low. We have detected cyclopropenylidene (C 3 H 2 ) in the three target PDRs. Similarly to the reactive ions, the abundance of C 3 H 2 in NGC 7023 and the Orion Bar is a factor of 10-100 higher in the PDRs than in the foreground molecular cloud with peak values ranging from 10 - 1 0 to 10 - 9 . In NGC 7027, we measured the maximum C 3 H 2 abundance with a value of ∼10 - 8 . Similarly to the case of CO + , the high expansion velocities of the C 3 H 2 lines in NGC 7027 suggests that its emission arises in the neutral gas which is being accelerated by the ionized gas. Photodestruction of Polycyclic Aromatic Hydrocarbons (PAHs) is proposed to explain the enhanced C 3 H 2 abundance in these PDRs.

Disks and outflows around intermediate-mass stars and protostars

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.

The history of mass dispersal around Herbig Ae/Be stars

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.

First Evidence of Dusty Disks around Herbig Be Stars

We have carried out a high-sensitivity search for circumstellar disks around Herbig Be stars in the continuum at 1.4 and 2.7 mm using the IRAM interferometer at the Plateau de Bure. In this Letter, we report data on three well-studied B0 stars: MWC 1080, MWC 137, and R Mon. The two latter have also been observed in the continuum at 0.7 and 1.3 cm using the NRAO Very Large Array. We report the detection of circumstellar disks around MWC 1080 and R Mon with masses of Md ~ 0.003 and 0.01 M☉, respectively, while for MWC 137 we estimate a disk mass upper limit of 0.007 M☉. Our results show that the ratio Md/M* is at least an order of magnitude lower in Herbig Be stars than in Herbig Ae and T Tauri stars.

Grain Evolution across the Shocks in the L1448-mm Outflow

The recent detection of shock precursors toward the very young L1448-mm outflow offers us the possibility to study the grain chemistry during the first stages of the shock evolution, constraining the molecules ejected from grains and the species formed in gas phase. Observations of key molecules in the grain chemistry such as SiO, CH3OH, SO, CS, H2S, OCS, and SO2 toward this outflow are presented. The line profiles and the derived abundances show three distinct velocity regimes that trace the shock evolution: the preshock, the shock precursor, and the postshock gas. The SiO, CH3OH, SO, and CS abundances are enhanced with respect to the quiescent gas by 1 order of magnitude in the shock-precursor component, and by 3 orders of magnitude in the postshock gas. The derived SiO and CH3OH abundances are consistent with the recent ejection of these molecules from grains. Since H2S is only enhanced in the shock-precursor component and OCS and SO2 are undetected, SO and CS are the most abundant sulfur-bearing species in the grain mantles of L1448-mm. The ejection of mainly SO and CS rather than H2S or OCS from grains suggests that the sulfur chemistry will depend on the chemical history of the grain mantles in outflows and hot cores.

The gas content of peculiar galaxies: Counterrotators and polar rings

This paper studies the global ISM content in a sample of 104 accreting galaxies, including counterro- tators and polar rings, which spans the entire Hubble sequence. The molecular, atomic and hot gas content of accretors is compared to a newly compiled sample of normal galaxies. We present results of a small survey of the J =1 0 line of 12 CO with the 15 m SEST telescope on a sample of 11 accretors (10 counterrotators and 1 polar ring). The SEST sample is enlarged with published data from 48 galaxies, for which observational evidence of counterrotation in the gas and/or the stars has been found. Furthermore, the available data on a sample of 46 polar ring galaxies has been compiled. In order to explore the existence of an evolutionary path linking the two families of accretors, the gas content of counterrotators and polar rings is compared. It was found that the normalized content of cold gas (Mgas=LB) in polar rings is1 order of magnitude higher than the reference value derived for normal galaxies. The inferred gas masses are sucient to stabilize polar rings through self-gravity. In contrast, it was found that the cold gas content of counterrotators is close to normal for all galaxy types. Although counterrotators and polar rings probably share a common origin, the gas masses estimated here conrm that light gas rings accreted by future counterrotators may have evolved faster than the self-gravitating structures of polar rings. In this scenario, the transformation of atomic into molecular gas could be enhanced near the transition region between the prograde and the retrograde disks, especially in late-type accretors characterized by a high content of primordial gas. This is tentatively conrmed in this work: the measured H2/HI ratio seems larger in counterrotators than in normal or polar ring galaxies for types later than S0s.

Detection of Reactive Ions in the Ultracompact H II Regions Monoceros R2 and G29.96–0.02

We report the first detection of the reactive ions CO+ and HOC+ toward ultracompact (UC) H II regions, particularly in Mon R2 and G29.96-0.02. We have observed two positions in Mon R2, namely, the peak of the UC H II region and the surrounding high-density molecular cloud. CO+ and HOC+ were detected at the UC H II region but not at the molecular cloud, as expected if both ions arise in the photodissociation region (PDR) surrounding the UC H II region. The measured column densities are on the order of 1011 cm-2 in both sources, which yields a strikingly low [HCO+]/[HOC+] abundance ratio of 460 in Mon R2. These values are similar to those found in some other well-known PDRs, like NGC 7023 and the Orion bar. We briefly discuss the chemical implications of these results.

Tracing the Shock Precursors in the L1448-mm/IRS 3 Outflows

We present the detection of the SiO ν = 0, J = 2-1, and J = 3-2 lines and of the HCO 101-000, J = 3/2-1/2, F = 2-1 line at ambient velocities toward the molecular outflows in L1448-mm and L1448 IRS 3. This is the first detection of HCO in a dark cloud. We have also measured lines of H13CO+, H13CN, HN13C, CH3OH, and N2H+. While the HCO and the SiO lines have the narrowest profiles with line widths of ~0.5 km s-1, the other lines have widths of ~1 km s-1. Toward L1448-mm, all lines except those of SiO and HCO show two distinct velocity components centered at 4.7 and 5.2 km s-1. HCO is only observed in the 4.7 km s-1 cloud and SiO in the 5.2 km s-1 component. The SiO abundance is ~10-11 in the 5.2 km s-1 clouds, 1 order of magnitude larger than in the 4.7 km s-1 component and in other dark clouds. The HCO abundance is ~10-11, similar to that predicted by the ion-molecule reactions models for the quiescent gas in dark clouds. The large change in the SiO/HCO abundance ratio (>150) from the 4.7 to the 5.2 km s-1 component, and the distribution and kinematics of the SiO emission toward L1448-mm suggest that the ambient SiO is associated with the molecular outflows. We propose that the narrow line widths and the abundances of SiO in the ambient gas are produced by the interaction of the magnetic and/or radiative precursors of the shocks with the clumpy preshocked ambient gas.

A New Intermediate-Mass Protostar in the Cepheus A HW2 Region

We present the discovery of the first molecular hot core associated with an intermediate-mass protostar in the Cep A HW2 region. The hot condensation was detected from single-dish and interferometric observations of several high-excitation rotational lines (from 100 to 880 K above the ground state) of SO2 in the ground vibrational state and of HC3N in the vibrationally excited states v7 = 1 and v7 = 2. The kinetic temperature derived from both molecules is ~160 K. The high angular resolution observations (125 × 099) of the SO2 J = 287, 21 → 296, 24 line (488 K above the ground state) show that the hot gas is concentrated in a compact condensation with a size of ~06 (~430 AU), located 04 (300 AU) east from the radio jet HW2. The total SO2 column density in the hot condensation is ~1018 cm-2, with an H2 column density ranging from ~1023 to 6 × 1024 cm-2. The H2 density and the SO2 fractional abundance must be larger than 107 cm-3 and 2 × 10-7, respectively. The most likely alternatives for the nature of the hot and very dense condensation are discussed. From the large column densities of hot gas, the detection of the HC3N vibrationally excited lines, and the large SO2 abundance, we favor the interpretation of a hot core heated by an intermediate-mass protostar of 103 L☉. This indicates that the Cep A HW2 region contains a cluster of very young stars.

THE EVOLUTION OF MOLECULAR LINE PROFILES INDUCED BY THE PROPAGATION OF C-SHOCK WAVES

We present the first results of the expected variations of the molecular line emission arising from material recently affected by C-shocks (shock precursors). Our parametric model of the structure of C-shocks has been coupled with a radiative transfer code to calculate the molecular excitation and line profiles of shock tracers such as SiO, and of ion and neutral molecules such as H{sup 13}CO{sup +} and HN{sup 13}C, as the shock propagates through the unperturbed medium. Our results show that the SiO emission arising from the early stage of the magnetic precursor typically has very narrow line profiles slightly shifted in velocity with respect to the ambient cloud. This narrow emission is generated in the region where the bulk of the ion fluid has already slipped to larger velocities in the precursor as observed toward the young L1448-mm outflow. This strongly suggests that the detection of narrow SiO emission, and of an ion enhancement in young shocks, is produced by the magnetic precursor of C-shocks. In addition, our model shows that the different velocity components observed toward this outflow can be explained by the coexistence of different shocks at different evolutionary stages, within the same beam of the single-dish observations.