Methanol Emission in Distant Protoplanetary Disks I

Astronomy Reports, Vol. 47, No. 10, 2003, pp. 797–808. Translated from Astronomicheski˘ı Zhurnal, Vol. 80, No. 10, 2003, pp. 867–878. Original Russian Text Copyright c 2003 by Val’tts, Lyubchenko. Methanol Emission in Distant Protoplanetary Disks I. E. Val’tts and S. Yu. Lyubchenko Astro Space Center, Lebedev Physical Institute, Russian Academy of Sciences, Profsoyuznaya ul. 84/32, Moscow, 117997 Russia Received April 28, 2003; in final form, May 8, 2003 Abstract—Thirty four-frequency line profiles of Class II methanol masers have been analyzed to investi- gate carefully the coincidences of various spectral features. Data at 6.7, 12.2, 107, and 156.6 GHz have been analyzed. Two clusters of Class II methanol maser lines at 6.7 and 12.2 GHz are observed in the spectra of many sources. These maser-line clusters are located on either side of the thermal methanol lines at 107 and 156.6 GHz. To avoid the effect of amplification in these thermal methanol lines, a similar analysis was performed for 80 sources having both maser emission at 6.7 GHz and thermal CS emission. The relative distributions of the methanol maser lines and the thermal CS line confirm on the basis of richer statistics that the maser lines are located in two clusters on either side of the thermal feature. It is proposed that the two maser-line clusters correspond to two edges of a Keplerian disk. The thermal methanol and CS emission is formed in dense molecular cores, whose centers are coincident with the disk centers. c 2003 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION 2. OBSERVATIONAL DATA Figure 1 shows a level diagram for methanol. Maser emission of the complex organic molecule The arrows denote transitions and frequencies cor- CH3OH (methanol) is observed in star-forming responding to Class II masers. Note that the A and regions. The mechanism giving rise to these masers is E states of methanol differ in the mutual orientations not fully understood. However, it is obvious that stud- of the spins of the hydrogen nuclei with respect to ies of these methanol lines can yield information about the rotational axis of the molecule, and consequently the evolution of the molecular-cloud material and have different systems of rotational levels. However, the physical conditions in the interstellar medium, astrophysical observations of these molecules can be since the masing process is extremely sensitive to interpreted jointly, since such differences simply en- changes of these conditions. The methanol molecule rich the overall methanol spectrum. Class II methanol has a multitude of allowed transitions, and studies masers were first discovered in the 20–3−1E tran- of methanol masers can be carried out at many dif- sition at 12.2 GHz on the 43-m telescope of the ferent frequencies corresponding to these transitions. National Radio Astronomy Observatory by Batrla Multifrequency studies are very profitable from the et al. [1]. Their intensity typically reaches 3000 Jy. viewpoint of obtaining homogeneous information The brightest and most widespread Class II masers + about the molecular-cloud fragments in which the radiate in the 51–60A transition at 6.7 GHz, and masers are formed. Methanol masers are divided were discovered by Menten [6] using the same radio into two classes according to the dominant pumping telescope. At present, more than 400 such masers mechanism [1, 2]. The levels of the methanol molecule are known [6–11]. The brightest of them, 9.62+0.20, responsible for Class Imaser emission are excited has a peak flux density of about 5000 Jy [6]. Many of by collisional interactions between the molecules. these masers radiate at 12.2 GHz (see, e.g., [12]), but Class II emission requires the action of an external maser emission in the J0–J−1E series of transitions source of radiation on the ensemble of molecules. at 157 GHz has been detected in only four [13]. The relationship between Class Imethanol masers Twenty-five 107-GHz masers have been detected + and specific astronomical objects is unclear, whereas in transitions of A methanol in the J1–(J +1)0A Class II methanol masers are associated with the ladder [14–16]; the methanol emission at 156.6 GHz early stages of the formation of protoplanetary sys- is mainly thermal [16], and only four masers have been tems (see, e.g., [3–5]). In this paper, we will present discovered to the present [13, 16]. additional evidence for this hypothesis. Let us proceed These observations have been published in plots to an analysis of the currently available observational made on various scales and in tables given in various data. formats, hindering direct comparisons and analyses. 1063-7729/03/4710-0797$24.00 c 2003 MAIK “Nauka/Interperiodica” 798 VAL’TTS, LYUBCHENKO 150 AE 10 11 11 10 – 10 + ± 9 11 10 10 9 10 9 23 10 100 – ± 8 + 8 10 ± 10 ± 9 8 10 8 28 7 9 ± 9 9 – 7 + ± 9 6 7 –1 9 ± 8 9 7 ± 8 8 8 6 ± ± 157 , cm 5 8 6 E 8 38 5 ± 7 ± 8 ± 4 7 7 8 7 ± 5 4 7 157 ± 5 ± 3 6 50 6 6 4 6± 3 7 ± ± 7 4 ± 157 5 133 6 3 5 3 ± 5 6.7 5 ± ± 6 157 6 2 ± 4 5 4 4 4 ± ± 3 5 157 5 4 3 3 3 107 ± 2 19 157 2 ± 4 3 2 156.6 ± 12.2 2 4 157 ± 3 1 ± 2 157 3 1 1 ± 2 108 2 ± 1 0 0 1 ± 0 0 1 2 3 –3 –2 –1 0 1 2 3 K Fig. 1. Level diagram for the methanol molecule and transitions yielding Class II emission. The numbers at the base of the arrows indicate the transition frequencies in GHz. Therefore, we have taken the most homogeneous 3. ANALYSIS of FOUR-FREQUENCY data from two surveys, at 6.7 and 12.2 GHz [12] METHANOL-LINE PROFILES and 107 and 156.6 GHz [16], and replotted the It is generally accepted that the thermal lines are formed in the quiescent gas of the parent molecular available spectra using a single radial-velocity scale cloud, and consequently have the same velocity as for all 30 sources with line profiles published in the unperturbed surrounding medium. It is also [16]. The table lists these 30 sources together with known that the velocities of the Class II methanol parameters of their thermal 107-GHz methanol maser lines differ from the molecular-cloud velocity. This fact is illustrated by our set of spectra. The lines and CS(2–1) lines. The 107-GHz masers in thermal lines at 107 and 156.6 GHz do not coincide the table are marked with asterisks. All the four- visually with the maser lines at 6.7 and 12.2 GHz, frequency spectra are presented in electronic form at if we consider the line peak velocities. In some ftp://ftp.asc.rssi.ru/outgoing/stella; Fig. 2 presents cases, the maser lines are located on one side of the thermal lines (at lower velocities in 328.808+0.633, typical examples. In this figure, the horizontal axis 340.054−0.244, and 9.621+0.196, and at higher plots the line-of-sight velocity in km/s and the verti- velocities in 188.946+0.886, 351.775−0.536, and 12.909−0.260); in some sources (approximately cal axis the flux in Janskys. The transition frequency twice as many as display maser lines to either the is given in the upper left corner of each panel. This right or the left), the thermal lines are located be- means of presenting the spectra is fairly common, tween groups of maser features (322.158+0.636, − − − and is most convenient for careful analyses of the 323.740 0.263, 337.404 0.402, 340.785 0.096, 344.227−0.569, 345.010−1.792, 348.703−1.043, coincidences of individual spectral features at various 351.417+0.645, 10.47+0.03, 23.010−0.410, and frequencies. 35.201−1.736). For the remaining sources in the ASTRONOMY REPORTS Vol. 47 No. 10 2003 METHANOL EMISSION 799 188.946+0.886 322.158+0.636 500 200 400 6.7 GHz 150 6.7 GHz 300 200 100 100 50 0 0 200 12.2 GHz 15 12.2 GHz 150 10 100 50 5 0 0 10 Flux density, Jy 107 GHz 10 107 GHz Flux density, Jy 0 0 30 5 156.6 GHz 20 156.6 GHz 10 0 0 –80 –40 8 CS(2–1) 6 Radial velocity, km/s , K mb 4 T 2 0 020 Radial velocity, km/s 323.740–0.263 327.120+0.511 3000 2500 6.7 GHz 80 6.7 GHz 2000 60 1500 1000 40 500 20 0 0 5 400 12.2 GHz 4 12.2 GHz 300 3 200 2 100 1 0 0 10 107 GHz 107 GHz 10 0 0 Flux density, Jy Flux density, Jy 156.6 GHz 4 156.6 GHz 5 2 0 0 –2 10 1 CS(2–1) CS(2–1) , K , K mb mb T T 0 0 –60 –40 –100 –80 Radial velocity, km/s Radial velocity, km/s Fig. 2. Four-frequency spectra of the methanol emission. A conditional CS line profile is shown when this line was observed (see text for explanation). Two CS lines are observed toward 345.003−0.223 [17]; the velocity of the weaker line is closer to those of the thermal methanol lines.

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