From menten@sunkmm.HARVARD.EDU Fri Nov 8 09:50:01 1996 Date: Fri, 8 Nov 96 09:50:44 EST From: menten@sunkmm.HARVARD.EDU (Karl Menten) To: hfalcke@astro.umd.edu Subject: Menten et al. paper (LaTeX) X-Status: \documentstyle[11pt,aaspp4,tighten,flushrt]{article} \slugcomment{To appear in The Astrophysical Journal (Letters)} % %------------------------------------------------------------------------------ % Definitions follow: % \def\hzo {H$_2$O} \def\kbw {\hbox{$6_{16}\to5_{23}$}} \def\kms {\ifmmode{{\rm km~s}^{-1}}\else{km~s$^{-1}$}\fi} \def\Mdot {\ifmmode {\dot M} \else $\dot M$\fi} \def\Mspy {\ifmmode {M_{\odot} {\rm yr}^{-1}} \else $M_{\odot}$~yr$^{-1}$\fi} \def\Msun {$M_{\odot}$} \def\mum {\ifmmode{\mu{\rm m}}\else{$\mu{\rm m}$}\fi} \def\Rstar {$R_{\star}$} % \newbox\grsign \setbox\grsign=\hbox{$>$} \newdimen\grdimen \grdimen=\ht\grsign \newbox\simgreatbox \setbox\simgreatbox=\hbox{\raise.5ex\hbox{$>$}\llap {\lower.5ex\hbox{$\sim$}}}\ht1=\grdimen\dp1=0pt \newbox\simlessbox \setbox\simlessbox =\hbox{\raise.5ex\hbox{$<$}\llap {\lower.5ex\hbox{$\sim$}}}\ht2=\grdimen\dp2=0pt \def\simgreat{\mathrel{\copy\simgreatbox}} \def\simless {\mathrel{\copy\simlessbox }} % \def\sgras {Sgr~A$^\star$} \def\jotz {$J = 1\to0$} % % Definitions end here %------------------------------------------------------------------------------ % % Authors may supply running head information, if they wish to do so, although % this may be modified by the editorial offices. The left head contains a % list of authors, usually three allowed---otherwise use et al. The right % head is a modified title of up to roughly 44 characters. Running heads % are not printed. \lefthead{Menten et al.} \righthead{Position of Sgr~A$^\star$} % This is the end of the "preamble". Now we wish to start with the % real material for the paper, which we indicate with \begin{document}. % Following the \begin{document} command is the front matter for the % paper, viz., the title, author and address data, the abstract, and % any keywords or subject headings that are relevant. \begin{document} \title{~~ \\ The Position of Sagittarius~A$^\star$:\\ Accurate Alignment of the Radio and Infrared Reference Frames\\ at the Galactic Center} \author{Karl M. Menten \&\ Mark J. Reid} \affil{Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138\\ kmenten@cfa.harvard.edu, mreid@cfa.harvard.edu} \author{and} \author{Andreas Eckart \&\ Reinhard Genzel} \affil{Max-Planck-Institut f\"ur extraterrestrische Physik, Postfach 1603, D-85740 Garching, Germany\\ eckart@mpe.mpe-garching.mpg.de, genzel@mpe.mpe-garching.mpg.de} \begin{abstract} We present a novel approach to the long-standing problem of locating the position of the compact nonthermal radio source \sgras\ on infrared images of the Galactic center region. Using the VLA, we have detected SiO and \hzo\ maser emission toward several sources within the central parsec of our Galaxy. These masers arise from the innermost parts of circumstellar envelopes of giant and supergiant stars that are members of the nuclear star cluster and appear as compact infrared sources in a diffraction-limited 2.2 \mum\ infrared image. One of the SiO masers is associated with the M-type supergiant IRS 7, the most prominent 2.2 \mum\ point source in the Galactic center region. The radio data allow measurements of the maser positions relative to the compact non-thermal radio continuum source \sgras\ with milliarcsecond accuracy. %errors $\simless 0{\rlap.}{''}02$. Because stellar SiO masers near the Galactic center trace their host stars to within a few milliarcseconds, these relative positions can be used to calibrate the plate scale and rotation of the infrared image. Our method allows registration of the radio relative to the infrared reference frame with an estimated accuracy of $0{\rlap.}{''}03$. Using the improved position accuracy we put a stringent upper limit on \sgras's 2.2 \mum\ flux density that is significantly lower than values predicted by recent theoretical model calculations. \end{abstract} % The different journals have different requirements for keywords. The % keywords.apj file, found on aas.org in the pubs/aastex-misc directory, % contains a list of keywords used with the ApJ and Letters. These are % usually assigned by the editor, but authors may include them in their % manuscripts if they wish. \keywords{galaxies: nuclei --- Galaxy: center --- stars: AGB and post-AGB --- circumstellar matter --- stars: mass-loss --- stars: variables: other --- masers} \clearpage \section{Introduction} In 1974, Balick and Brown reported the detection of a compact, high-brightness, radio continuum source toward the Galactic center region. This unique radio source, now called \sgras, appears to be at the dynamical center of the Galaxy (Backer \&\ Sramek 1987; Backer 1995). There is mounting evidence for a compact ``dark'' mass of $\approx 2$ -- $3 \times 10^6$ solar masses within the central parsec of our Galaxy, which suggests the presence of a supermassive black hole (Serabyn \&\ Lacy 1985; Sellgren et al. 1987, 1990; Krabbe et al. 1995; Haller et al. 1996; Genzel et al. 1996; see Genzel, Hollenbach, \&\ Townes 1994 for a recent review). The recent detection of stellar proper motions (Eckart \&\ Genzel 1996) indicates that this mass is concentrated within $\approx 0{\rlap.}{''}5$ of \sgras, which corresponds to 0.02 pc at the distance of the Galactic center (8 kpc, Reid 1993). Synchrotron emission produced by accretion onto this black hole may power \sgras\ (e.g., Lynden-Bell \&\ Rees 1971; Falcke et al. 1993; Melia 1994; Narayan et al. 1995). Previous determinations of the spectral energy distribution of \sgras\ %which places strong constraints on the physical processes involved, at infrared wavelengths have been complicated by considerable confusion, from multiple sources within the rich central star cluster, and by uncertainties in the registration of the radio and infrared reference frames needed to assign an infrared source to \sgras. The radio and infrared frames can be registered by comparing {\it absolute} positions determined at infrared wavelengths with absolute positions in the radio frame. Initially this absolute registration was accurate to $\approx 1''$ and focussed attention on the infrared source IRS~16 (Becklin \&\ Neugebauer 1975). More recent efforts (e.g, Forrest, Pipher, \&\ Stein 1986; Tollestrup et al. 1989) resulted in a $\approx \pm0{\rlap.}{''}3$ uncertainty in the radio/infrared registration, and suggested that \sgras\ was South-West of IRS~16, which with increasing angular resolution breaks up into separate components. Another method to register the radio and infrared frames utilizes the fact that some infrared sources are surrounded by thermal radio emission, which in the case of IRS 7 results from external ionization of this M-type supergiant's outer envelope. However, this technique results in uncertainties of similar magnitude as the absolute registrations (Yusef-Zadeh \&\ Morris 1991; Yusef-Zadeh \&\ Melia 1992), since the radio emission is asymmetric and diffuse. Recent dramatic progress involving diffraction-limited $0{\rlap.}{''}15$ resolution 2.2 \mum\ images of the Galactic center region have resolved the infrared emission from the immediate vicinity of \sgras\ into many relatively faint compact sources that form a small cluster covering an area of radius $\approx 0{\rlap.}{''}5$ or 0.02 pc (Eckart et al. 1993, 1995). While this increase in resolution yields precise relative positions among compact infrared sources, the problem of accurately registering the position of the \sgras\ radio source on the infrared image remains, since there exists no compact radio continuum source within the central parsec that has a bona fide infrared counterpart. Indeed, given the $\approx 0{\rlap.}{''}3$ uncertainty in the radio/infrared registration, many of the weak infrared sources mentioned above (or none of them) could be the infrared counterpart of \sgras. In this {\it Letter} we present a novel approach to the Galactic center radio/infrared registration problem that involves accurate radio measurements of the positions of circumstellar masers that are associated with some of the stellar infrared sources {\it relative} to the continuum emission of \sgras. Many giant and supergiant stars exhibit strong hydroxyl (OH), water (\hzo), and/or silicon monoxide (SiO) maser emission. Recent Very Long Baseline Interferometry (VLBI) observations (Diamond et al. 1994; Miyoshi et al. 1994; Greenhill et al. 1995) indicate that SiO masers arise from the innermost regions ($\approx$ 4 -- 8~AU radius) of the circumstellar envelopes. Therefore, if stellar SiO masers could be detected near the Galactic center, they would trace the positions of their host stars to within a few milliarcseconds In the past, the most sensitive searches for stellar SiO and \hzo\ masers in the Galactic center region were made toward OH/IR stars detected in VLA surveys of the 1612 MHz OH maser line (Winnberg et al. 1985; Lindqvist et al. 1992). OH/IR stars are highly obscured red giants going through a short-lived, high-mass-loss phase and, thus, are quite rare. %Consequently, during these searches, which employed single-dish telescopes, and only relatively few masers were detected in the inner few parsecs (Lindqvist et al. 1987, 1990, 1991). On the other hand, it is well known that the central star cluster contains numerous M-type giant stars (e.g., Mira variables), with less extreme mass loss than OH/IR stars, and several M supergiants, such as IRS 7. Mira variables generally have weak OH maser emission. However, the luminosities of some SiO and \hzo\ maser lines emitted from Miras can be very high. For example, if the archetypical Mira variable $o$ Cet, which is at a distance of $\approx100$~pc, was placed at the Galactic center, its strongest SiO lines (see, e.g., Mart\'\i nez, Bujarrabal, \&\ Alcolea 1988) would have flux densities of $\approx 0.1$ Jy and, thus, be detectable with sensitive radio telescopes. The same holds for \hzo\ masers and, indeed, Levine et al. (1995) report the detection of \hzo\ maser emission from an M-type supergiant at a projected distance of 1.9 pc from \sgras. Motivated by the analysis above, we conducted a very deep search of the central few parsecs for emission in the 43.1 GHz $v = 1$, $J = 1\to0$ SiO and the 22 GHz \kbw\ \hzo\ maser lines using the VLA. We observed a field centered near \sgras, used \sgras's strong ($\approx1$~Jy) continuum emission to calibrate the interferometer data, and searched for maser emission over the primary beam ($\approx 1'$ FWHM at 43.1 GHz) of the individual VLA antennas. We discovered several SiO and \hzo\ maser sources within the central 0.5 pc of \sgras\ that can be identified with stellar infrared sources, including IRS 7, and we have measured their positions relative to \sgras\ with nearly milliarcsecond accuracy. Combining our maser positions with recent high resolution infrared images allows relative registration of the radio and 2.2 \mum\ infrared reference frames to within $0{\rlap.}{''}03$. This represents an order of magnitude improvement over earlier efforts and tightly constrains the infrared emission from \sgras. \section{Observations and Data Analysis} \subsection{VLA Observations} %\section{VLA Observations and Data Analysis} Our Galactic center maser search was conducted with the NRAO\footnote{The National Radio Astronomy Observatory (NRAO) is operated by Associated Universities, Inc. under a cooperative agreement with the National Science Foundation.} Very Large Array. A first set of observations was made on 1995 February 9 when the VLA was in its most compact (D) configuration. At that time and during the A-array observations described below, the 10 antennas equipped with new 40--50 GHz receivers were located in the inner part of the array. These were used to observe the $v = 1$, $J = 1\to0$ transition of SiO, which has a rest frequency of 43122.08 MHz. Two independently tunable intermediate frequency (IF) channels of 12.5 MHz bandwidth each were centered at different LSR velocities. Each IF band was subdivided into 32 channels, resulting in a velocity resolution of 2.72 \kms. The antennas not equipped with 40--50 GHz receivers were used to observe the \kbw\ transition of \hzo, which has a rest frequency of 22235.08 MHz. A dual-IF setup analogous to that used at 43.1 GHz was employed. Each IF channel had a bandwidth of 6.25 MHz and was divided in 64 spectral channels, resulting in a velocity resolution of 1.32 \kms. At both the SiO and the \hzo\ frequencies, bands centered at different velocities were observed alternately to cover a wide velocity range. Details of our observing procedures and the complete set of results will be presented in a future publication. The second phase of our observing program took place on 1995 June 26 when the VLA was in its most extended (A) array, which provided $\approx 20$ times higher spatial resolution yielding very accurate positions. The general setup was similar to that used for the D-array observations, except that some SiO data were taken at 4 times higher velocity resolution. % For all observations the phase center was R.A. = $17^{\rm h}45^{\rm m}40{\rlap.}{^{\rm s}}131$, Decl. = $-29^\circ 00'27{\rlap.}{''}50$ (J2000), which was purposely offset by ($\Delta\theta_{\rm x}$, $\Delta\theta_{\rm y}$) $\approx (+1{\rlap.}{''}1, +0{\rlap.}{''}4$) from the position of the \sgras\ radio source determined by Rogers et al. (1994) to avoid possible correlator offsets which would show up at the pointing center. Observations of 1733$-$130 (NRAO~530) were interspersed at hourly intervals to provide initial phase and bandpass calibration. The flux density scale was established from an observation of 1331$+$305 (3C286), which was assumed to have flux densities of 1.86 Jy and 2.55 Jy at 43.1 and 22.2 GHz, respectively. The data were edited and calibrated in the ``standard'' manner using the NRAO AIPS system. Continuum images were made from broad-band data, obtained by %pseudo-continuum $uv$-database, which was obtained by %online- averaging the inner 75\%\ of the passband. Several self-calibration iterations produced high quality maps, which for the A-array data contain, at our sensitivity level, only emission from \sgras. The phase and amplitude corrections determined by self-calibration of the broad-band data were applied to the spectral-line $uv$-data, which were then %After continuum subtraction, %using the AIPS task UVLIN (Cornwell, Uson, \&\ Haddad 1992), mapped and CLEANed channel by channel. For the A-array data the synthesized beams were elongated approximately in north--south direction, with FWHM sizes of $\approx 0{\rlap.}{''}31\times0{\rlap.}{''}20$ and $\approx 0{\rlap.}{''}13\times0{\rlap.}{''}06$ for the SiO and \hzo\ maps, respectively. %Inspection of the maps revealed maser sources. Accurate maser positions were determined by two-dimensional Gaussian fits to the emission in all spectral line channels containing signal. Position errors ($\sigma_\theta$) were calculated from the noise in the maps, assuming $\sigma_\theta \approx 0.5~\theta_b / {\rm SNR}$, where $\theta_b$ is the FWHM beam size and SNR the peak signal-to-noise ratio. In no case did the maser positions for a given source show significant deviations from one channel to the other and the positions listed in Table 1 are weighted averages of the fit results obtained for the individual channels. Given the nature of our continuum/line ``cross-calibration'' the uncertainties in the positions of the masers relative to \sgras\ are dominated by statistical, not systematic, uncertainties. Maser spectra % observed toward the positions listed in Table 1 are shown in Fig. 1. % \subsection{Infrared Observations} %\section{Infrared Observations and Data Reduction} Broad-band 2.2 \mum\ ($K$-band) observations were made on 1995 July 6--15, very close in time to our VLA A-array observations, with the MPE infrared high-resolution SHARP camera at the 3.5 m New Technology Telescope (NTT) of the European Southern Observatory (ESO) in La Silla, Chile. Similar datasets obtained at earlier epochs resulted in the images presented by Eckart et al. (1993, 1995). %and since the data aquisition and reduction procedures are described in those %papers we only summarize a few facts. The raw data consisted of a large number of $256\times 256$ pixel frames, with exposure times of 0.3 -- 1 s and a scale of $\approx0{\rlap.}{''}05$ pixel$^{-1}$. Deep, diffraction-limited images were produced by combining a large number of these individual frames (after initial corrections) by using a shift-and-add algorithm, deconvolving the summed images with a Lucy algorithm, and restoring them with a Gaussian to achieve $0{\rlap.}{''}15$ FWHM resolution. Several of the resulting $12{\rlap.}{''}8\times12{\rlap.}{''}8$ size images, using different pointing centers, but with some overlap, were combined to form the larger-field ``mosaic'' shown in Fig. 2. Positions of compact infrared sources were determined by two-dimensional Gaussian fits. %Performing these fits on raw (undeconvolved) images, %Lucy-deconvolved images, and the final diffraction-limited %restored maps yielded %the same position estimates within $\approx 5$ milliarcseconds for %bright isolated sources and $\approx 10$ milliarcseconds for close %multiple or very faint isolated sources. To estimate their accuracy, we compared relative positions of some infrared sources appearing on overlapping portions of the images and found differences from one image to the other ranging up to about 20 milliarcseconds (mas), which are most likely due to very small deviations in the ``plate'' scale across an image. While these relative position deviations can be accounted for in a time sequence of images designed to extract proper motions (Eckart \&\ Genzel 1996), they cannot be ``calibrated out'' in an absolute sense without a large ``grid'' of positions measured independently with an accuracy $\ll 20$ mas. \section{Results and Discussion} \subsection{Stellar Masers in the Inner Parsec of the Galaxy} We detected several SiO and \hzo\ maser sources within one parsec of the Galactic center. Spectra of the five sources discussed here are presented in Fig. 1, which also shows blow-ups of the areas of the infrared mosaic (Fig. 2) that contain the maser positions. It is clear from Fig. 1 that at least four of the five masers have 2.2 \mum\ counterparts. In Table 1 we list their infrared designations, as well as velocities determined from infrared absorption data (Genzel et al. 1996). In all cases maser and infrared velocities agree within the uncertainties. Detailed discussions of the masers and their emitting stars will be presented in a future paper. One of the SiO masers, ($+0.0,+5.6$), is coincident with the M-type supergiant IRS 7, the dominant 2.2 \mum\ source in the central cluster. Another SiO maser, ($+7.7,+4.2$), is coincident within the errors with the OH/IR star OH359.946$-$0.047 identified in VLA OH maser surveys as the OH/IR star with the shortest known projected distance to \sgras\ (Winnberg et al. 1985; Lindqvist et al. 1992) . The SiO velocity centroid is in excellent agreement (to within 1 \kms) with the centroid velocity of the 1612 MHz OH spectrum. To assess the accuracy with which masers probe the positions of their host stars, we note that recent VLBI observations of M-type giant and supergiant stars in the solar neighborhood have shown that SiO maser emission arises from a region within 2 -- 4 stellar radii (\Rstar), or 4 -- 8 AU, from the center of a Mira-like star (Diamond et al. 1994; Miyoshi et al. 1994) and from within 1.5 \Rstar, corresponding to 30 AU, of the center of the M-type supergiant VX Sgr (Greenhill et al. 1995). Placed at the distance of the Galactic center, the SiO maser spots for the Miras observed by Diamond et al. would be spread over an area of radius $< 1$ mas, while the VX Sgr maser spots would lie within a 3 mas radius. \hzo\ masers, on the other hand, arise from parts of the circumstellar envelope further away from the star, ranging from $\approx 6$ \Rstar, corresponding to 12 AU, in the case of Mira-like stars to a few tens of \Rstar, corresponding to several hundred AU, for supergiants (e.g, Reid \&\ Menten 1990; Bowers, Claussen, \&\ Johnston 1993; Bowers \&\ Johnston 1994). The \hzo\ shells studied by these authors would, if placed at the Galactic center, have radii between 1 and 10 mas in the case of Mira-like stars and between 20 and 50 mas in the case of supergiants. \subsection{Registration of Radio and Infrared Positions} ----- End Included Message -----