% http://www.oan.es/preprints % \documentstyle{proceed} \documentstyle[epsfig]{proceed} %----------------- User defined macros --------------------------- %\newcommand{\sub}[1]{_{\rm #1}} \def\E#1 {\ifmmode {\times 10^{#1}\;} \else \hbox{$\times 10^{#1}\;$}\fi} % i sin punto, i acentuada \chardef\isp="10 \def\i{\'\isp} \def\formal{\hbox{H$_2$CO}} \def\nht {\hbox{${\rm NH}_{3}$}} %NH3 \def\kmsns{~km~s$^{-1}$} \def\apro{$\sim$} %aproximado \def\eg{{\rm e.g.\/\ }} % example (e.g.) \def\methanol{\hbox{CH$_3$OH}\ } \def\hdos {\hbox{${\rm H}_2$}} %H2 \def\T#1 {\ifmmode \,10^{#1}\; \else {${\rm\,10^{#1}}\;$}\fi} % 10^ \def\percc {$\hbox{{\rm cm}}^{-3}$} %cm-3 \def\TK{\hbox{T$_{\rm k}$}} \def\as {\ifmmode {^{\scriptscriptstyle\prime\prime}} % arcsec \else $^{\scriptscriptstyle\prime\prime}$\fi} \def\lsim {\ifmmode {\buildrel<\over\sim} % less or similar \else {\lower.6ex\hbox{$\buildrel<\over\sim$}}\fi} \def\pc{\ifmmode {\,\hbox{\rm pc}} \else $\,{\rm pc}$\fi} % pc \def\solarns{\ifmmode_{\mathord\odot} \else $_{\mathord\odot}$\fi}\def \lons{\ifmmode {\,{\it L}\solarns} \else $\,L$\solarns\fi} %------------------ Article header ------------------------------- % \title[Short title to appear in the page header]{Full title} \title[Hot molecular shells ]{Hot Expanding Shells in the Envelope of the Sagittarius B2 Molecular Cloud} % \author[Short name list to appear in the page header]{ % Name1$^1$\email{addvips ress}, Name2$^2$ \and{} Name3$^2$} \author[Mart{\i}n-Pintado et al.]{J. Mart{\i}n-Pintado$^1$\email{martin@oan.es }, R. A. Gaume$^2$\email{rgaume@usno.navy.mil}, N. Rodr{\i}guez-Fern\'andez $^1$\email{nemesio@oan.es }, P. de Vicente$^1$\email{vicente@oan.es}, and T. L. Wilson$^{3,4}$\email{twilson@as.arizona.edu}} % \institute{$^1$Institute1 \\ % $^2$Institute2} \institute{ $^1$ Observatorio Astron\'omico Nacional, Apartado 1143, E-28800 Alcal\'a de Henares, Spain \\ $^2$ U.S. Naval Observatory, 3450 Massachusetts Ave., NW, Washington, DC 20392-5420 \\ $^3$ Max Planck Institut f\"{u}r Radioastronomie, Postfach 2024, D-53010 Bonn, Germany \\ $^4$Sub-mm Telescope Observatory, Steward Observatory, The University of Arizona, Tucson, Az, 85721 \\} % ----------------------------- \begin{document} \maketitle \abstract{We present high resolution images of the warm gas in the envelope of the Sagittarius B2 (Sgr B2) molecular cloud. These images reveal that the morphology of the Sgr B2 molecular envelope is dominated by several shells and incomplete shells expanding at low velocities $\sim$10 \kmsns. The sizes of the shells are between 1 and 2.6 \pc\ and their thicknesses are between 0.2 and 0.4 \pc. The shells are hot with kinetic temperatures, \TK, of 40 -150 K. The large number of masers in H$_2$CO, CH$_3$OH and the newly detected (3,3) \nht\ masers are correlated with the hot molecular shells. Associated with the shells, we have also detected new hot cores, suggesting that massive star formation has taken place very recently in the molecular envelope of Sgr B2. Wind-blown bubbles, driven by typical galactic Wolf-Rayet stars, could account for the kinetic energies and the momenta observed in the hot \nht\ shells. Shocks associated with the expanding bubbles provide the chemistry required to explain the large number of masers, and the heating of the molecular gas in the envelope of Sgr B2. } \section{Introduction} The Sgr B2 molecular cloud is one of the most active regions of star-formation in the Galactic center. Massive star-formation is mainly occurring in two cores (Sgr B2M and Sgr B2N) with \apro $10^4\ {\rm M_\odot}$ embedded in a giant molecular cloud with a total mass of \apro 7\E{6} ${\rm M_\odot }$ (see \eg \cite{vicen97}). In spite of its large mass, little is known about the origin and the detailed properties of the envelope surrounding star-forming cores Sgr B2M/ B2N. In the envelope, recent massive star-formation occurs at lower efficiency than in the Sgr B2M/N cores, but a large number of \methanol and \formal\ masers have been detected (\cite{mehr94}; \cite{mehrmenten97}; \cite{houghtonwhiteoak95}). The envelope contains hot gas ($T_k \geq 100$ K) and moderate \hdos\ densities, \T{3} -- \T{4} \percc, (\cite{wil82}; \cite{hutte93}; \cite{mart90}). The warm gas (\TK$\sim$50-70 K) in the envelope extends up to distances of at least 8 \pc\ from Sgr B2M/N (\cite{vicen97}). The dust temperature in the warm envelope is too low, 10--20 K, to heat the gas by gas-dust collisions (see \eg \cite{wil82}). Shock heating by dissipation of turbulence has been proposed to heat the gas in the molecular clouds in the GC (\cite{wil82}), but the origin of the shocks is unclear. We have used the VLA \footnote[5]{The VLA is a telescope of the National Radio Astronomy Observatory (NRAO), a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc} in the DnC configuration to image the (3,3) and the (4,4) lines of \nht\ toward the southern part of the Sgr B2 envelope. These data show, for the first time, that the morphology of the hot gas at small scales is dominated by ring-like structures. This might allow us to determine the heating mechanisms acting in the Sgr B2 envelope. \section{Results and discussion} Images of the line and continuum emission with angular resolutions of 3\as\ and 1\as\ have been obtained. The line maps, in particular those of the (3,3) line, show that the hot gas in the envelope of Sgr B2 presents a very complex morphology that changes substantially as a function of radial velocity (see Fig. 1). The main features found from the (3,3) and the (4,4) \nht\ line maps can be classified as: a) hot expanding shells and filaments, b) masers in the (3,3) line and c) unresolved hot cores. In the following sections we discuss these features in detail. \begin{figure} \epsfig{file=f1.eps} \caption{ Summary of the main features detected in the (3,3) \nht\ line maps towards the envelope of Sgr B2. The main rings and arcs found in our data are outlined by ellipses and partial ellipses which are named, as a function of radial velocity, after capital letters. The radial velocity interval used to obtain the \nht\ (3,3) averaged intensity line maps are shown in the upper left corner of the panels. The contour levels are 4.0 mJy/beam times 1, 3, 5, 7, 9. The rotational temperatures derived from the (3,3) and the (4,4) line ratios at selected positions are also shown. } \end{figure} \subsection{Hot expanding molecular shells} Figure 1 summarizes the main features observed in the (3,3) line maps and the distribution of the \TK\ derived from the ratios between the (3,3) and the (4,4) lines. The morphology of the hot gas in the envelope of Sgr B2 is mainly dominated by six elliptical rings (A, B, D, F, G and H), two arcs (C and E), and some straight filaments (see Fig. 1). The sizes of the rings are between 1 and 2.6 \pc\ and their thicknesses between 0.2 to 0.4 \pc. Most of the gas in the rings is warm with \TK\ between 40 and 70 K. Kinetic temperatures larger than 100 K are found only in the region where two rings at low radial velocity (40--50\kmsns) appear to overlap. The derived \nht\ column densities towards these features show that the rings and arcs represent regions of enhanced \hdos\ density and/or enhanced \nht\ abundance. Two of the hot rings (F and D) show the systematic change of size as a function of radial velocity as expected if the \nht\ emission arises in three dimensional shells expanding at velocities of 6--10\kmsns. \subsection{Masers in the (3,3) line of NH$_3$} The high-resolution maps of the (3,3) line reveal the presence of six unresolved sources with sizes of \lsim 1\as\ which do not appear in the (4,4) line maps. For all these sources, the peak brightness temperatures of the (3,3) line are larger than the kinetic temperatures, suggesting that the anomalous intensity of the (3,3) line is caused by maser amplification. As shown in Figure 2, most of the \nht\ (3,3) masers are located in the hot rings with similar radial velocities. Remarkably, we also find that other masers in the region, such as the Class II \methanol and \formal\ masers (\cite{houghtonwhiteoak95}; \cite{mehrmenten97}) are not associated with newly formed stars, but they are also located in the hot \nht\ rings. From this, we conclude that the origin of the large number of masers in the envelope of Sgr B2 is related to the hot expanding molecular shells. The combination of a large abundance of volatile molecules like \nht, CH$_3$OH, and \formal\ in the hot shells and the good velocity coherence found at the edges of the expanding shells can explain the high number of masers observed in the envelope of Sgr B2. The large abundance of fragile molecules which are easily photodissociated like \nht\ suggests that the material in the shells has been processed by shocks which heat the gas and drive shock chemistry. \begin{figure} \epsfig{file=f2.eps} \caption{Spatial distribution of the various types of masers (\nht\ --filled squares--, \methanol class I (\cite{mehrmenten97}) and II (\cite{houghtonwhiteoak95}) -x symbol and filled circle, respectively-- and \formal\ (\cite{mehr94}) --filled triangles--) found in the envelope of the Sgr B2 molecular cloud overlaid with the hot \nht\ shells for the radial velocities range between 47.7 and 51.4 \kmsns\ (left panel) and 63.7 and 72.3 \kmsns\ (right panel) . The (3,3) \nht\ masers M1-M3 are shown in the left panel and M4-M6 in the right panel. Though M1 and M4 have radial velocities outside the ranges of the \nht\ maps, they have also been included for completeness. The contour levels of the \nht\ emission are as in Fig. 1. } \end{figure} \subsection{The new hot cores} In the high-resolution maps of the (4,4) line we have also found three unresolved sources (size of \lsim 1\as) which are not very prominent in the (3,3) line maps. For these condensations, the line intensity ratio indicates very high \TK\ of at least 300 K. The characteristics of these condensations, high temperatures, small sizes (\lsim 0.03 \pc), and high densities (10$^{6}$--10$^{7}$ cm$^{-3}$) are similar to those found in the hot cores associated with newly formed massive stars. The lack of radio continuum emission and the large inferred dust luminosity for these hot cores, at least 10$^{5}$ $L_{\solarns }$, indicates that these hot cores are internally heated by massive stars which have not yet developed HII regions. Most likely, the hot cores are associated with a dense circumstellar disk surrounding a (proto)star in a phase of very energetic mass loss. The detection of hot cores indicates that, like in Sgr B2N and Sgr B2M, very recent massive star-formation is also taking place in the envelope of Sgr B2. In this case, massive star-formation might have been triggered by the expanding bubbles which produce the \nht\ hot shells. \section{The origin of the hot expanding shells} The origin of the hot expanding \nht\ shells is so far unknown. The morphology strongly suggests that they have been created by the action of stellar winds or explosive events. From the analysis of several possibilities, we speculate that evolved massive stars, like Wolf-Rayet (WR) stars, could explain the hot \nht\ shells (\cite{mart99}). The momentum and the kinetic energy we measure in the hot shells are consistent with the predictions of models of wind-blown bubbles driven by the stellar winds of typical galactic WR stars. The large concentration of hot shells would indicate that the Sgr B2 envelope contains a cluster of massive evolved stars similar to several others found near the Galactic center, like the Quintuplet and Object 17. Clusters of WR stars with strong stellar winds would provide the widespread turbulent heating required to explain the extended warm envelope and the large extent of the SiO emission in this molecular cloud (\cite{mart97}). In fact, the presence of massive evolved stars driving expanding shells seem to be a general characteristic of the Galactic center (see \cite{hase99} in this volume) which would account for the characteristics of the Galactic center molecular clouds (\cite{gus89}). %\acknowledgements{ This work has been partially supported by %the Spanish DGICYT under grant PB96-104.} % J.M.-P. and R.A.G. have been also partially supported by NATO grant 900440} \begin{thebibliography}{} \bibitem[de Vicente et al. 1997]{vicen97} de Vicente, P., Mart{\i}n-Pintado, J., \& Wilson , T.L. 1997, A\&A, 320, 957 \bibitem[G\"usten 1989]{gus89} G\"usten, R. 1989, in IAU Symposium 136, The Galactic Center of the Galaxy, ed. M. Morris, Kluwer Academic Publisher, p. 89 \bibitem[Hasegawa et al. 1999] {hase99} Hasegawa, T., Oka, T., Sato, F., Tsiboi, M., Miyazaki, A. 1999, in this volume \bibitem[Houghton \& Whiteoak 1995] {houghtonwhiteoak95}Houghton, S., \& Whiteoak, J.B. 1995, MNRAS, 273, 1033 \bibitem[H\"uttemeister et al. 1993] {hutte93} H\"uttemeister S., Wilson T.L., Henkel C., \& Mauersberger R., 1993a, A\&A, 276, 445 \bibitem[Mehringer \& Menten, 1997] {mehrmenten97}Mehringer, D.M., \& Menten, K. 1997, ApJ, 412, 684 \bibitem[Mehringer et al. 1994] {mehr94} Mehringer, D.M., Goss, M.W., \& Palmer, P., 1994, ApJ, 434, 237 \bibitem[Mart{\i}n-Pintado et al. 1990] {mart90} Mart{\i}n-Pintado J., de Vicente, P., Wilson T.L., \& Johnston K.J. 1990, A\&A 236, 193 \bibitem[Mart{\i}n-Pintado et al. 1997] {mart97} Mart{\i}n-Pintado, J., de Vicente, P., Fuente, A., \& Planesas, P. 1997, ApJL, 482, L45 \bibitem[Mart{\i}n-Pintado et al. 1999] {mart99} Mart{\i}n-Pintado, J., Gaume, R., Rodr{\i}guez-Fern\'andez, N., de Vicente, P., \& Wilson, T.L. 1999, ApJ, in press \bibitem[Wilson et al. 1992] {wil82} Wilson T.L., Ruf K., Walmsley, C.M., Martin R.N., Pauls T.A., \& Batrla W. 1982, A\&A 115, 185 \end{thebibliography} \end{document}