J. R. Rizzo

On the chemistry and distribution of HOC+ in M 82: More evidence for extensive PDRs

Context. The molecular gas composition in the inner 1 kpc disk of the starburst galaxy M 82 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. Two explanations have been proposed for such high abundances: the influence of intense UV fields from massive stars, or a significant role of X-Rays. Aims. Our aim is to investigate the origin of the high abundances of reactive ions in M 82. Methods. We have completed our previous 30 m HOC + J = 1 → 0 observations with the higher excitation HCO + and HOC + J = 4 → 3a nd 3→ 2 rotational lines. In addition, we have obtained with the IRAM Plateau de Bure Interferometer (PdBI) a 4 �� resolution map of the HOC + emission in M 82, the first ever obtained in a Galactic or extragalactic source. Results. 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 (n ≥ 10 4 cm −3 ). Therefore, the HOC + emission is arising in the dense PDRs embedded in the M 82 nuclear disk, rather than in the intercloud phase and/or wind. Conclusions. We have improved our previous chemical model of M 82 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) irradiated by the intense interstellar UV field (G0 = 10 4 in units of the Habing field) prevailing in the nucleus of M 82. Most molecular observations (HCO + , HOC + ,C O + , 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 a small number of large molecular clouds suggests that M 82 is an old starburst, where star formation has almost exhausted the molecular gas reservoir.

Gas morphology and energetics at the surface of PDRs: New insights with Herschel observations of NGC 7023

Context. We investigate the physics and chemistry of the gas and dust in dense photon-dominated regions (PDRs), along with their dependence on the illuminating UV field. Aims. Using Herschel/HIFI observations, we study the gas energetics in NGC 7023 in relation to the morphology of this nebula. NGC 7023 is the prototype of a PDR illuminated by a B2V star and is one of the key targets of Herschel. Methods. Our approach consists in determining the energetics of the region by combining the information carried by the mid-IR spectrum (extinction by classical grains, emission from very small dust particles) with that of the main gas coolant lines. In this letter, we discuss more specifically the intensity and line profile of the 158 μm (1901 GHz) [C ii] line measured by HIFI and provide information on the emitting gas. Results. We show that both the [C ii] emission and the mid-IR emission from polycyclic aromatic hydrocarbons (PAHs) arise from the regions located in the transition zone between atomic and molecular gas. Using the Meudon PDR code and a simple transfer model, we find good agreement between the calculated and observed [C ii] intensities. Conclusions. HIFI observations of NGC 7023 provide the opportunity to constrain the energetics at the surface of PDRs. Future work will include analysis of the main coolant line [O i] and use of a new PDR model that includes PAH-related species.

Chemical study of intermediate-mass (IM) Class 0 protostars - CO depletion and N2H+ deuteration

Aims. We are carrying out a physical and chemical study of the protostellar envelopes in a representative sample of IM Class 0 protostars. In our first paper we determined the physical structure (density-temperature radial profiles) of the protostellar envelopes. Here, we study the CO depletion and N2H + deuteration. Methods. We observed the millimeter lines of C 18 O, C 17 O, N2H + and N2D + towards the protostars using the IRAM 30m telescope. Based on these observations, we derived the C 18 O, N2H + and N2D + radial abundance profiles across their envelopes using a radiative

Spectral line survey of the ultracompact HII region Monoceros R2

Context. Ultracompact (UC) Hii regions constitute one of the earliest phases in the formation of a massive star and are characterized by extreme physical conditions (G0 > 105 Habing field and n > 106 cm-3). The UC Hii Mon R2 is the closest example and an excellent target to study the chemistry in these complex regions. Aims: Our goal is to investigate the chemistry of the molecular gas around UC Hii Mon R2 and the variations caused by the different local physical conditions. Methods: We carried out 3 mm and 1 mm spectral surveys using the IRAM 30-m telescope towards three positions that represent different physical environments in Mon R2: (i) the ionization front (IF) at (0″, 0″), and two peaks in the molecular cloud; (ii) molecular Peak 1 (hereafter MP1) at the offset (+15″, -15″); and (iii) molecular Peak 2 (hereafter MP2) at the farther offset (0″, 40″). In addition, we carried out extensive modeling to explain the chemical differences between the three observed regions. Results: We detected more than 30 different species (including isotopologues and deuterated compounds). In particular, we detected SO+ and C4H confirming that ultraviolet (UV) radiation plays an important role in the molecular chemistry of this region. In agreement with this interpretation, we detected the typical photo-dissociation region (PDR) molecules CN, HCN, HCO, C2H, and c-C3H2. There are chemical differences between the observed positions. While the IF and the MP1 have a chemistry similar to that found in high UV field and dense PDRs such as the Orion Bar, the MP2 is similar to lower UV/density PDRs such as the Horsehead nebula. Our chemical modeling supports this interpretation. In addition to the PDR-like species, we detected complex molecules such as CH3CN, H2CO, HC3N, CH3OH, and CH3C2H that are not usually found in PDRs. The sulfur compounds CS, HCS+, C2S, H2CS, SO, and SO2 and the deuterated species DCN and C2D were also identified. The origin of these complex species requires further study. The observed deuteration fractionations, [DCN]/[HCN] ~ 0.03 and [C2D]/[C2H] ~ 0.05, are among the highest in warm regions. Conclusions: Our results show that the high UV/dense PDRs have a different chemistry from the low UV case. Some abundance ratios such as [CO+]/[HCO+] or [HCO]/[HCO+] are good diagnostics for differentiating between them. In Mon R2, we have the two classes of PDRs, a high UV PDR towards the IF and the adjacent molecular bar, and a low-UV PDR, which extends towards the north-west following the border of the cloud. Appendices A and B are available in electronic form at http://www.aanda.org

Physical structure of the envelopes of intermediate-mass protostars

Context. Intermediate mass protostars provide a bridge between low- and high-mass protostars. Furthermore, they are an important component of the UV interstellar radiation field. Despite their relevance, little is known about their formation process. Aims. We present a systematic study of the physical structure of five intermediate mass, candidate Class 0 protostars. Our two goals are to shed light on the first phase of intermediate mass star formation and to compare these protostars with low- and high-mass sources. Methods. We derived the dust and gas temperature and density profiles of the sample. We analysed all existing continuum data on each source and modelled the resulting SED with the 1D radiative transfer code DUSTY. The gas temperature was then predicted by means of a modified version of the code CHT96. Results. We found that the density profiles of five out of six studied intermediate mass envelopes are consistent with the predictions of the “inside-out” collapse theory. We compared several physical parameters, like the power law index of the density profile, the size, the mass, the average density, the density at 1000 AU and the density at 10 K of the envelopes of low-, intermediate, and high-mass protostars. When considering these various physical parameters, the transition between the three groups appears smooth, suggesting that the formation processes and triggers do not substantially differ.

Herschel/HIFI observations of CO, H2O and NH3 in Monoceros R2

Context. Mon R2, at a distance of 830 pc, is the only ultracompact Hii region (UCHii) where the associated photon-dominated region (PDR) can be resolved with Herschel. Owing to its brightness and proximity, it is one of the best-suited sources for investigating the chemistry and physics of highly UV-irradiated PDRs. Aims. Our goal is to estimate the abundance of H2 Oa nd NH 3 in this region and investigate their origin. Methods. We present new observations ([Cii], 12 CO, 13 CO, C 18 O, o-H2O, p-H2O, o-H 18 O and o-NH3) obtained with the HIFI instrument onboard Herschel and the IRAM-30 m telescope. We investigated the physical conditions in which these lines arise by analyzing their velocity structure and spatial variations. Using a large velocity gradient approach, we modeled the line intensities and derived an average abundance of H2 Oa nd NH 3 across the region. Finally, we modeled the line profiles with a non-local radiative transfer model and compared these results with the abundance predicted by the Meudon PDR code. Results. The variations of the line profiles and intensities indicate complex geometrical and kinematical patterns. In several tracers ([Cii], CO 9 → 8a nd H 2O) the line profiles vary significantly with position and have broader line widths toward the Hii region. The H2O lines present strong self-absorption at the ambient velocity and emission in high-velocity wings toward the Hii region. The emission in the o-H 18 O ground state line reaches its maximum value around the Hii region, has smaller linewidths and peaks at the velocity of the ambient cloud. Its spatial distribution shows that the o-H 18 O emission arises in the PDR surrounding the Hii region. By modeling the o-H 18 O emission and assuming the standard [ 16 O]/[ 18 O] = 500, we derive a mean abundance of o-H2 Oo f∼10 −8 relative to H2. The ortho-H2O abundance, however, is larger (∼1 × 10 −7 ) in the high-velocity wings detected toward the Hii region. Possible explanations for this larger abundance include an expanding hot PDR and/or an outflow. Ammonia seems to be present only in the envelope of the core with an average abundance of ∼2 × 10 −9 relative to H2. Conclusions. The Meudon PDR code, which includes only gas-phase chemical networks, can account for the measured water abundance in the high velocity gas as long as we assume that it originates from a 1 mag hot expanding layer of the PDR, i.e. that the outflow has only a minor contribution to this emission. To explain the water and ammonia abundances in the rest of the cloud, the molecular freeze out and grain surface chemistry would need to be included.

GRB 980425 host: [C II], [O I], and CO lines reveal recent enhancement of star formation due to atomic gas inflow

Context. Accretion of gas from the intergalactic medium is required to fuel star formation in galaxies. We have recently suggested that this process can be studied using host galaxies of gamma-ray bursts (GRBs). Aims: Our aim is to test this possibility by studying in detail the properties of gas in the closest galaxy hosting a GRB (980425). Methods: We obtained the first ever far-infrared (FIR) line observations of a GRB host, namely Herschel/PACS resolved [C II] 158 μm and [O I] 63 μm spectroscopy, and an APEX/SHeFI CO(2-1) line detection and ALMA CO(1-0) observations of the GRB 980425 host. Results: The GRB 980425 host has elevated [C II]/FIR and [O I]/FIR ratios and higher values of star formation rates (SFR) derived from line ([C II], [O I], Hα) than from continuum (UV, IR, radio) indicators. [C II] emission exhibits a normal morphology, peaking at the galaxy centre, whereas [O I] is concentrated close to the GRB position and the nearby Wolf-Rayet region. The high [O I] flux indicates that there is high radiation field and high gas density at these positions, as derived from modelling of photo-dissociation regions. The [C II]/CO luminosity ratio of the GRB 980425 host is close to the highest values found for local star-forming galaxies. Indeed, its CO-derived molecular gas mass is low given its SFR and metallicity, but the [C II]-derived molecular gas mass is close to the expected value. Conclusions: The [O I] and H I concentrations and the high radiation field and density close to the GRB position are consistent with the hypothesis of a very recent (at most a few tens of Myr ago) inflow of atomic gas triggering star formation. In this scenario dust has not had time to build up (explaining high line-to-continuum ratios). Such a recent enhancement of star formation activity would indeed manifest itself in high SFRline/SFRcontinuum ratios because the line indicators are sensitive only to recent (≲10 Myr) activity, whereas the continuum indicators measure the SFR averaged over much longer periods ( 100 Myr). Within a sample of 32 other GRB hosts, 20 exhibit SFRline/SFRcontinuum> 1 with a mean ratio of 1.74 ± 0.32. This is consistent with a very recent enhancement of star formation that is common among GRB hosts, so galaxies that have recently experienced inflow of gas may preferentially host stars exploding as GRBs. Therefore GRBs may be used to select a unique sample of galaxies that is suitable for the investigation of recent gas accretion. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Deuteration around the ultracompact HII region Monoceros R2

Context. The massive star-forming region Monoceros R2 (Mon R2) hosts the closest ultra-compact Hii region, where the photondominated region (PDR) between the ionized and molecular gas can be spatially resolved with current single-dish telescopes. Aims. We aim at studying the chemistry of deuterated molecules toward Mon R2 to determine the deuterium fractions around a highUV irradiated PDR and investigate the chemistry of these species. Methods. We used the IRAM-30 m telescope to carry out an unbiased spectral survey toward two important positions (namely IF and MP2) in Mon R2 at 1, 2, and 3 mm. This spectral survey is the observational basis of our study of the deuteration in this massive starforming region. Our high spectral resolution observations (∼0.25–0.65 km s −1 ) allowed us to resolve the line profiles of the different species detected. Results. We found a rich chemistry of deuterated species at both positions of Mon R2, with detections of C2D, DCN, DNC, DCO + , D2CO, HDCO, NH2D, and N2D + and their corresponding hydrogenated species and rarer isotopologs. The high spectral resolution of our observations allowed us to resolve three velocity components: the component at 10 km s −1 is detected at both positions and seems associated with the layer most exposed to the UV radiation from IRS 1; the component at 12 km s −1 is found toward the IF position and seems related to the foreground molecular gas; finally, a component at 8.5 km s −1 is only detected toward the MP2 position, most likely related to a low-UV irradiated PDR. We derived the column density of the deuterated species (together with their hydrogenated counterparts), and determined the deuterium fractions as Dfrac = [XD]/[XH]. The values of Dfrac are around 0.01 for all the observed species, except for HCO + and N2H + , which have values 10 times lower. The values found in Mon R2 are similar to those measured in the Orion Bar, and are well explained with a pseudo-time-dependent gas-phase model in which deuteration occurs mainly via ion-molecule reactions with H2D + ,C H 2D + and C2HD + . Finally, the [H 13 CN]/[HN 13 C] ratio is very high (∼11) for the 10 km s −1 component, which also agree with our model predictions for an age of ∼0.01 to a few 0.1 Myr. Conclusions. The deuterium chemistry is a good tool for studying the low-mass and high-mass star-forming regions. However, while low-mass star-forming regions seem well characterized with Dfrac(N2H + )o rDfrac(HCO + ), a more complete chemical modeling is required to date massive star-forming regions. This is due to the higher gas temperature together with the rapid evolution of massive protostars.

Herschel observations in the ultracompact HII region Mon R2 - Water in dense photon-dominated regions (PDRs)

Context. Monoceros R2, at a distance of 830 pc, is the only ultracompact H ii region (UC H ii) where the photon-dominated region (PDR) between the ionized gas and the molecular cloud can be resolved with Herschel. Therefore, it is an excellent laboratory to study the chemistry in extreme PDRs (G0 > 105 in units of Habing field, n > 106 cm-3). Aims: Our ultimate goal is to probe the physical and chemical conditions in the PDR around the UC H ii Mon R2. Methods: HIFI observations of the abundant compounds 13CO, C18O, o-H218O, HCO+, CS, CH, and NH have been used to derive the physical and chemical conditions in the PDR, in particular the water abundance. The modeling of the lines has been done with the Meudon PDR code and the non-local radiative transfer model described by Cernicharo et al. Results: The 13CO, C18O, o-H_218O, HCO+ and CS observations are well described assuming that the emission is coming from a dense (n = 5 × 106 cm-3, N(H2)> 1022 cm-2) layer of molecular gas around the H ii region. Based on our o-H_218O observations, we estimate an o-H2O abundance of ≈2 × 10-8. This is the average ortho-water abundance in the PDR. Additional H_218O and/or water lines are required to derive the water abundance profile. A lower density envelope (n ~ 105 cm-3, N(H2) = 2-5 × 1022 cm-2) is responsible for the absorption in the NH 1_1→ 0_2 line. The emission of the CH ground state triplet is coming from both regions with a complex and self-absorbed profile in the main component. The radiative transfer modeling shows that the 13CO and HCO+ line profiles are consistent with an expansion of the molecular gas with a velocity law, ve = 0.5 × (r/Rout)-1 km s-1, although the expansion velocity is poorly constrained by the observations presented here. Conclusions: We determine an ortho-water abundance of ≈2 × 10-8 in Mon R2. Because shocks are unimportant in this region and our estimate is based on H_218O observations that avoids opacity problems, this is probably the most accurate estimate of the water abundance in PDRs thus far. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Figures 1 and 4 (page 5) are only available in electronic form at http://www.aanda.org

HIFI observations of warm gas in DR21: Shock versus radiative heating

Context. The molecular gas in the DR21 massive star formation region is known to be affected by the strong UV field from the central star cluster and by a fast outflow creating a bright shock. The relative contribution of both heating mechanisms is the matter of a long debate. Aims. By better sampling the excitation ladder of various tracers we provide a quantitative distinction between the different heating mechanisms. Methods. HIFI observations of mid-J transitions of CO and HCO^+ isotopes allow us to bridge the gap in excitation energies between observations from the ground, characterizing the cooler gas, and existing ISO LWS spectra, constraining the properties of the hot gas. Comparing the detailed line profiles allows to identify the physical structure of the different components. Results. In spite of the known shock-excitation of H_2 and the clearly visible strong outflow, we find that the emission of all lines up to ≳2 THz can be explained by purely radiative heating of the material. However, the new Herschel/HIFI observations reveal two types of excitation conditions. We find hot and dense clumps close to the central cluster, probably dynamically affected by the outflow, and a more widespread distribution of cooler, but nevertheless dense, molecular clumps.