Hot Molecular Cores and High-Mass Star Formation

This review covers hot cores in the context of high-mass star formation. After giving an overview of chemical processes and diversity during high-mass star formation, it reviews the `warm envelope' phase which probably precedes the formation of hot cores. Some recent determinations of the cosmic-ray ionization rate are discussed, as well as recent evidence for hot cores around low-mass stars. Routes for future hot core research are outlined.Comment: 8 pages, 1 figure; to appear in the Proceeding of IAU Symposium 221, Star Formation at High Angular Resolution, Editors M. Burton, R. Jayawardhana & T. Bourke, Astronomical Society of the Pacifi

Energetic radiation and the sulfur chemistry of protostellar envelopes: Submillimeter interferometry of AFGL 2591

CONTEXT: The chemistry in the inner few thousand AU of accreting envelopes around young stellar objects is predicted to vary greatly with far-UV and X-ray irradiation by the central star. Aim We search for molecular tracers of high-energy irradiation by the protostar in the hot inner envelope. METHODS: The Submillimeter Array (SMA) has observed the high-mass star forming region AFGL 2591 in lines of CS, SO, HCN, HCN(v2=1), and HC15N with 0.6" resolution at 350 GHz probing radial scales of 600-3500 AU for an assumed distance of 1 kpc. The SMA observations are compared with the predictions of a chemical model fitted to previous single-dish observations. RESULTS: The CS and SO main peaks are extended in space at the FWHM level, as predicted in the model assuming protostellar X-rays. However, the main peak sizes are found smaller than modeled by nearly a factor of 2. On the other hand, the lines of CS, HCN, and HC15N, but not SO and HCN(v2=1), show pedestal emissions at radii of about 3500 AU that are not predicted. All lines except SO show a secondary peak within the approaching outflow cone. A dip or null in the visibilities caused by a sharp decrease in abundance with increasing radius is not observed in CS and only tentatively in SO. CONCLUSIONS: The emission of protostellar X-rays is supported by the good fit of the modeled SO and CS amplitude visibilities including an extended main peak in CS. The broad pedestals can be interpreted by far-UV irradiation in a spherically non-symmetric geometry, possibly comprising outflow walls on scales of 3500 -- 7000 AU. The extended CS and SO main peaks suggest sulfur evaporation near the 100 K temperature radius.Comment: Astronomy and Astrophysics, in pres

Limits on the cosmic-ray ionization rate toward massive young stars

Recent models of the envelopes of seven massive protostars are used to analyze observations of H3+ infrared absorption and H13CO+ submillimeter emission lines toward these stars, and to constrain the cosmic-ray ionization rate zeta. The H13CO+ gives best-fit values of zeta=(2.6+/- 1.8) x 10^-17 s^-1, in good agreement with diffuse cloud models and with recent Voyager/Pioneer data but factors of up to 7 lower than found from the H3+ data. No relation of zeta with luminosity or total column density is found, so that local (X-ray) ionization and shielding against cosmic rays appear unimportant for these sources. The difference between the H3+ and H13CO+ results and the correlation of N(H3+) with heliocentric distance suggest that intervening clouds contribute significantly to the H3+ absorptions in the more distant regions. The most likely absorbers are low-density (<~10^4 cm^-3) clouds with most carbon in neutral form or in CO.Comment: To be published in A&A 358 (Letters); 4 pages including 3 figure

Water abundance variations around high-mass protostars: HIFI observations of the DR21 region

Context. Water is a key molecule in the star formation process, but its spatial distribution in star-forming regions is not well known. Aims. We study the distribution of dust continuum and H_(2)O and ^(13)CO line emission in DR21, a luminous star-forming region with a powerful outflow and a compact H ii region. Methods. Herschel-HIFI spectra near 1100 GHz show narrow ^(13)CO 10–9 emission and H_(2)O 1_(11)–0_(00) absorption from the dense core and broad emission from the outflow in both lines. The H_(2)O line also shows absorption by a foreground cloud known from ground-based observations of low-J CO lines. Results. The dust continuum emission is extended over 36” FWHM, while the ^(13)CO and H_(2)O lines are confined to ≈24” or less. The foreground absorption appears to peak further North than the other components. Radiative transfer models indicate very low abundances of ~2×10^(-10) for H_(2)O and ~8×10^(-7) for ^(13)CO in the dense core, and higher H_(2)O abundances of ~4×10^(-9) in the foreground cloud and ~7×10^(-7) in the outflow. Conclusions. The high H_(2)O abundance in the warm outflow is probably due to the evaporation of water-rich icy grain mantles, while the H_(2)O abundance is kept down by freeze-out in the dense core and by photodissociation in the foreground cloud

Detection of extragalactic H3O+

The H3O+ molecule probes the oxygen chemistry and the ionization rate of dense circumnuclear gas in galaxies. In particular, recent H3O+ observations show variations in the cosmic-ray ionization rate by factors of $>$10 within our Galaxy. Using the JCMT, we have observed the 364 GHz line of p-H3O+ in the centers of M82 and Arp 220. In Arp 220, the line profile suggests that the emission originates in the Western nucleus. In M82, both the eastern molecular peak and the circumnuclear region contribute to the emission. The derived column densities, abundances, and H3O+ / H2O ratios indicate ionization rates similar to or even exceeding that in the Galactic Center. Model calculations of the chemistry of irradiated molecular gas indicate a likely origin of this high ionization rate in the extended, evolved starburst of M82. In contrast, irradiation by X-rays from the AGN disk is the most likely model for Arp 220.Comment: Accepted by A&A Letters; 4-5 pages depending on paper format; two b/w figure

Very compact radio emission from high-mass protostars. II. Dust disks and ionized accretion flows

This paper reports 43 GHz imaging of the high-mass protostars W33A, AFGL 2591 and NGC 7538 IRS9 at 0.04'' and 0.6'' resolution. In each case, weak (~mJy), compact (~100 AU) emission is detected, which has an elongated shape (axis ratio ~3). For AFGL 2591 and NGC 7538 IRS9, the emission is single-peaked, while for the highest luminosity source, W33A, a `mini-cluster' of three sources is detected. The derived sizes, flux densities, and broad-band radio spectra of the sources support recent models where the initial expansion of HII regions around very young O-type stars is prevented by stellar gravity. In these models, accretion flows onto high-mass stars originate in large-scale molecular envelopes and become ionized close to the star. These models reproduce our observations of ionized gas as well as the structure of the molecular envelopes of these sources on 10^3--10^4 AU scales derived previously from single-dish submillimeter continuum and line mapping. For AFGL 2591, the 43 GHz flux density is also consistent with dust emission from a disk seen in near-infrared `speckle' images. However, the alignment of the 43 GHz emission with the large-scale molecular outflow argues against an origin in a disk for AFGL 2591 and NGC 7538 IRS9. In contrast, the outflow from W33A does not appear to be collimated. Together with previously presented case studies of W3 IRS5 and AFGL 2136, our results indicate that the formation of stars and stellar clusters with luminosities up to ~10^5 L0 proceeds through accretion and produces collimated outflows as in the solar-type case, with the `additional feature' that the accretion flow becomes ionized close to the star. Above ~10^5 L0, clusters of HII regions appear, and outflows are no longer collimated, possibly as the result of mergers of protostars or pre-stellar cores.Comment: Accepted by A&A; 11 pages, 4 b/w figure

Tracing early evolutionary stages of high-mass star formation with molecular lines

Despite its major role in the evolution of the interstellar medium, the formation of high-mass stars (M > 10 Msol) is still poorly understood. Two types of massive star cluster precursors, the so-called Massive Dense Cores (MDCs), have been observed, which differ in their mid-infrared brightness. The origin of this difference is not established and could be the result of evolution, density, geometry differences, or a combination of these. We compare several molecular tracers of physical conditions (hot cores, shocks) observed in a sample of mid-IR weak emitting MDCs with previous results obtained in a sample of exclusively mid-IR bright MDCs. The aim is to understand the differences between these two types of object. We present single-dish observations of HDO, H2O-18, SO2 and CH3OH lines at lambda = 1.3 - 3.5 mm. We study line profiles and estimate abundances of these molecules, and use a partial correlation method to search for trends in the results. The detection rates of thermal emission lines are found to be very similar between mid-IR quiet and bright objects. The abundances of H2O, HDO (1E-13 to 1E-9 in the cold outer envelopes), SO2 and CH3OH differ from source to source but independently of their mid-IR flux. In contrast, the methanol class I maser emission, a tracer of outflow shocks, is found to be strongly anti-correlated with the 12 micron source brightnesses. The enhancement of the methanol maser emission in mid-IR quiet MDCs may indicate a more embedded nature. Since total masses are similar between the two samples, we suggest that the matter distribution is spherical around mid-IR quiet sources but flattened around mid-IR bright ones. In contrast, water emission is associated with objects containing a hot molecular core, irrespective of their mid-IR brightness. These results indicate that the mid-IR brightness of MDCs is an indicator of their evolutionary stage.Comment: 15 pages, 6 figures, 11 tables, accepted for publication in A&A the 11/06/201

Observations and models of the embedded phase of high-mass star formation

This paper is a review and an update on recent work on the physical and chemical structure of the envelopes of newly born massive stars, at the stages preceding ultracompact H II regions. It discusses methods and results to determine total mass, temperature and density structure, ionization rate, and depth-dependent chemical composition.Comment: 8 pages incl 4 figures, to appear in "Hot Star Workshop III: The Earliest Phases of Massive Star Birth" (ed. P.A. Crowther) (ASP). Uses newpasp.sty (included