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Water in the Envelopes and Disks Around Young High-Mass Stars

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Water in the Envelopes and Disks Around Young High-Mass Stars Astronomy & Astrophysics manuscript no. 3937 December 17, 2018 (DOI: will be inserted by hand later) Water in the envelopes and disks around young high-mass stars F. F. S. van der Tak1, C. M. Walmsley2, F. Herpin3, and C. Ceccarelli4 1 Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, 53121 Bonn, Germany; e-mail: [email protected] 2 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy 3 Observatoire de Bordeaux, L3AB, UMR 5804, B.P. 89, 33270 Floirac, France 4 Laboratoire Astrophysique de l’Observatoire de Grenoble, BP 53, 38041 Grenoble, France Received 28 July 2005 / Accepted 21 October 2005 18 Abstract. Single-dish spectra and interferometric maps of (sub-)millimeter lines of H2 O and HDO are used to study the chemistry of water in eight regions of high-mass star formation. The spectra indicate HDO excitation temperatures of ∼110 K and column densities in an 11′′ beam of ∼2×1014 cm−2 for HDO and ∼2×1017 cm−2 for H2O, with the N(HDO)/N(H2O) ratio increasing with decreasing temperature. Simultaneous observations of CH3OH and SO2 indicate that 20 – 50% of the single-dish line flux arises in the molecular outflows of these 18 objects. The outflow contribution to the H2 O and HDO emission is estimated to be 10 – 20%. Radiative transfer models indicate that the water abundance is low (∼10−6) outside a critical radius corresponding to a temperature −4 in the protostellar envelope of ≈100 K, and ‘jumps’ to H2O/H2∼10 inside this radius. This value corresponds −3 to the observed abundance of solid water and together with the derived HDO/H2O abundance ratios of ∼10 suggests that the origin of the observed water is evaporation of grain mantles. This idea is confirmed in the case 18 of AFGL 2591 by interferometer observations of the HDO 110–111, H2 O 313–220 and SO2 120,12–111,11 lines, which reveal compact (Ø∼800 AU) emission with a systematic velocity gradient. This size is similar to that of the 1.3 mm continuum towards AFGL 2591, from which we estimate a mass of ≈0.8 M⊙, or ∼5% of the mass of the central star. We speculate that we may be observing a circumstellar disk in an almost face-on orientation. Key words. ISM: molecules – Molecular processes – Stars: formation – Astrochemistry 1. Introduction portunity to study the relative importance of each of these types of chemistry in the protostellar environment. Water is a cornerstone molecule in the oxygen chem- istry of dense interstellar clouds and a major coolant of warm molecular gas1. In the surroundings of embed- There has been considerable controversy about arXiv:astro-ph/0510640v1 21 Oct 2005 ded protostars, water can be formed by three very differ- the water abundance around high-mass protostars. ent mechanisms. In cold (∼10 K) protostellar envelopes, Observations of the H2O 6 µm bending mode with ISO- −5 water may be formed in the gas phase by ion-molecule SWS have revealed abundant water (H2O/H2 ∼10 – + −4 chemistry, through dissociative recombination of H3O . 10 ) in absorption toward several high-mass protostars Simultaneously, on the surfaces of cold dust grains, O (Boonman & van Dishoeck 2003). The absorption data and H atoms may combine to form water-rich ice mantles. do not tell us the location of the H2O along the line These mantles will evaporate when the grains are heated of sight, except that the high excitation temperatures to ∼100 K, either by protostellar radiation or by grain (∼300–500 K) imply an origin in warm gas. In contrast, sputtering in outflow shocks. Third, in gas with temper- observations of the o-H2O ground state line at 557 GHz > atures ∼ 250 K, reactions of O and OH with H2 drive all with SWAS of the same sources indicate much lower abun- −7 −6 gas-phase oxygen into water. Such high temperatures may dances (H2O/H2 ∼10 –10 ; Snell et al. 2000). The nar- occur very close to the star due to radiation, or further out row line width indicates an origin in the envelopes rather in outflow shocks. The water molecule thus offers the op- than the outflows of the sources, but the data have too low angular resolution (several arcminutes) for more detailed statements. Boonman et al. (2003) performed a simulta- 1 This paper uses the word ‘water’ to denote the chemical neous analysis of ISO-SWS, ISO-LWS and SWAS data and 18 species, and the notations H2O, H2 O and HDO to denote inferred a water abundance jump in the inner envelope by specific isotopologues. four orders of magnitude for several high-mass YSOs. 2 Van der Tak et al.: Water in regions of high-mass star formation Table 1. Source sample. a Source R.A. (1950) Dec. (1950) L d N(H2) ◦ ′ ′′ 4 23 −2 (hms) ( ) (10 L⊙) (kpc) (10 cm ) W3 IRS5 02 21 53.1 +61 52 20 17 2.2 2.3 AFGL 490 03 23 38.9 +58 36 33 0.2 1 2.0 W33A 18 11 43.7 –17 53 02 10 4 6.2 AFGL 2136 18 19 36.6 –13 31 40 7 2 1.2 AFGL 2591 20 27 35.8 +40 01 14 2 1 2.3 S140 IRS1 22 17 41.1 +63 03 42 2 0.9 1.4 NGC 7538 IRS1 23 11 36.7 +61 11 51 13 2.8 6.5 NGC 7538 IRS9 23 11 52.8 +61 10 59 4 2.8 3.3 a: Column density in a 15′′ beam. Locating the water around protostars requires obser- 2. Observations vations at high spatial and spectral resolution, which presently can only be done from the ground. Most ground- Table 2 summarizes spectroscopic parameters of the ob- served lines, and gives the relevant telescope and its based observations of H2O have targeted the maser lines FWHM beam size at that frequency. With Eup ≈ 200 K, at 22 and 183 GHz (e.g., Cernicharo et al. 1990). However, 18 the anomalous excitation of these lines makes it hard the 313 –220 line is the lowest-lying transition of H2 O that to derive column densities from such data, which may can be observed from the ground. We use this line to mea- in any case not be representative of the surrounding re- sure the abundance of H2O in the warm inner envelopes of gion. The only thermal water lines that can be studied the sources. The HDO lines cover the range of excitation 18 energies from 20 to 200 K, and are used to constrain the from the ground are the 313–220 line of H2 O at 203 GHz (Phillips et al. 1978; Jacq et al. 1988), and several HDO excitation and chemical state of the gas, in particular its lines. These lines were used by Gensheimer et al. (1996) deuterium fractionation. The SO2 and CH3OH lines have and Helmich et al. (1996) to estimate envelope-averaged comparable excitation requirements, and are used to mea- sure the effects of shock chemistry (SO2) and ice evapora- abundances of H2O and HDO around several young high- tion (CH3OH). The difference in Einstein A−coefficients mass stars. Advances in sensitivity and resolution al- 3 low us to consider lower-luminosity objects closer than of the lines is mostly due to the ν dependence: all the the Sun than before, and also enable us to study abun- lines have transition dipole moments of a few Debye. dance variations with position in the envelope. This pa- per presents new observations of these lines toward sources 2.1. Single-dish observations that have been studied previously with ISO and SWAS, 18 including the first published interferometric observations Observations of lines of H2 O, HDO, SO2 and CH3OH in 18 the 80 – 225 GHz range were made with the 30-m tele- of the H2 O line (and in fact of any non-masing wa- ter line). The sources are eight deeply embedded high- scope of the Institut de Radio Astronomie Millim´etrique 3 5 2 mass stars, with luminosities of 2×10 – 2×10 L⊙, dis- (IRAM) on Pico Veleta, Spain, in May 2003. The tances of 1 – 4 kpc, and H2 column densities of 1 – front ends were the facility receivers A100, B100, A230 7×1023 cm−2, as listed in Table 1. Single-dish mapping and B230, and the backend was the Versatile Spectral of dust continuum and molecular line emission at (sub- Assembly (VESPA) autocorrelator. The five lines were ob- )millimeter wavelengths indicates envelope masses of 30 – served simultaneously with a spectral resolution of 0.1 −1 1100 M⊙ within radii of 0.09 – 0.36 pc (Van der Tak et al. – 0.3 km s . Integration times are 60 – 180 minutes 2000b). The sources drive powerful outflows as revealed by (on+off) using double beam switching with a throw of ′′ mid-infrared and millimeter-wave observations of CO and 180 . System temperatures were 100 – 150 K at 3 mm HCO+ (Mitchell et al. 1991; Hasegawa & Mitchell 1995a). and 300 – 600 K at 1.3 mm wavelength. Data were cali- The unique aspect of this source sample is its high mid- brated onto TMB scale by multiplying by ηf /ηb, where the infrared brightness, which allows us to compare its (sub- forward efficiency ηf is 0.95 at 3 mm and 0.91 at 1.3 mm, )millimeter emission with solid state data for the chem- and the main beam efficiency ηb is 0.78 at 3 mm and 0.57 istry, and with rovibrational absorption lines for the ge- at 1.3 mm wavelength.
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