------------------------------------------------------------------------ ms.tex ApJ, Feb 20, 2004, in press Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-MailScanner-Information: Please contact postmaster@aoc.nrao.edu for more information X-MailScanner: Found to be clean X-MailScanner-SpamCheck: not spam, SpamAssassin (score=-5.4, required 7, BAYES_01 -5.40, USER_AGENT_PINE 0.00) \documentclass{aastex} \slugcomment{to be published in The Astrophysical Journal, vol. 602, Feb 20, 2004} \lefthead{Geballe et al.} \righthead{Bow Shock at IRS~8} \begin{document} \title{A Bow Shock of Heated Dust Surrounding Galactic Center Source IRS~8 } \author{T. R. Geballe\altaffilmark{1}, F. Rigaut\altaffilmark{1}, J.-R. Roy\altaffilmark{1}, and B. T. Draine\altaffilmark{2}} 96720; tgeballe@gemini.edu} \altaffiltext{2}{Princeton University Observatory, Princeton, NJ 08544 USA} \begin{abstract} High resolution images in the $H$ and $K$ bands obtained by the Gemini North Telescope of the peculiar Galactic center source, IRS~8, reveal a central pointlike object enveloped in a remarkable bow-shock, whose apex is located ~0.2'' to the northeast. The $H-K'$ color of the bow shock is considerably redder than that of the central star. A UKIRT $K$ band spectrum reveals that the combined spectrum of the point source and bow shock is nearly featureless and that no shocked line emission (e.g., from H$_2$) is physically associated with the bow. We interpret the bow as resulting from the interaction of the envelope or wind of the central star of IRS~8 with the extension of the Northern Arm of Sgr~A West and/or the Circumnuclear Disk, and its emission as coming from radiatively- and possibly shock-heated dust. \end{abstract} \keywords{shock waves -- stars: winds, outflows -- Galaxy: center -- infrared: ISM, stars} \section{Introduction} The Galactic center source, IRS~8, has long been one of the most mysterious objects of the central infrared cluster. Located 30 arcseconds (1.2 pc assuming a galactocentric distance of 8 kpc) due north of Sgr~A* and the IRS~16 cluster (Becklin \& Neugebauer 1975), the source is well removed from the other bright 10~$\mu$m infrared sources, which are all well within the central parsec. However, it is probably still bathed in the intense and high temperature ($\sim$35,000~K) radiation field mainly produced by the cluster of hot stars within the central parsec. IRS~8 also stands out at 2.2~$\mu$m, but like several sources (IRS 1, 2, 5, 10, and 21) located along the Northern Arm of dust and ionized gas and like IRS~3, it is much brighter at 10~$\mu$m than at short infrared wavelengths, even after correcting for extinction. This indicates that its infrared radiation arises predominantly from heated dust. Unlike IRS~3 and the Northern Arm sources that were detected at 2.2~$\mu$m, IRS~8 was spatially resolved at 2.2~$\mu$m in a 2.5'' aperture by Becklin \& Neugebauer (1975). This marked it as a unique object in the Galactic center. The luminosity of the source has been estimated to be ~$\sim$1~$\times$~10$^5~$L$_\odot$ (Becklin et al. 1978a). IRS~8 is located near or at the intersection on the plane of the sky of the Northern Arm, which extends inward toward the nucleus, and the inner edge of the Circumnuclear Disk or Ring of dust and molecular gas. Lacy et al. (1979, 1980) found that the bright 12.8~$\mu$m \ion{Ne}{2} fine structure line at the position of IRS~8 has two velocity components, at LSR velocities of +110 and -10~km~s$^{-1}$. The redshifted component, has been interpreted by Lacy et al. (1980) as a portion of the redshifted Northern Arm. The narrow -10~km~s$^{-1}$ component is more localized at IRS~8. At that location the molecular line emission from the Ring peaks at approximately +60~km~s$^{-1}$ (e.g., Marshall, Lasenby, \& Harris (1995). Recently diffraction-limited 2--25~$\mu$m images obtained with the Keck telescopes have resolved the Northern Arm source IRS~21 (Tanner et al. 2002). The large size of IRS~21 and its spectral energy distribution are explained by those authors as due to a combination of scattered circumstellar radiation at near-infrared wavelengths and emission from dense and heated dust at mid-infrared wavelengths. They suggest that the density enhancement occurs as the (hypothesized) wind from IRS~21 encounters and shock-compresses material in the Northern Arm. The morphology of the emission from IRS~21 is roughly circularly symmetric and, in itself, does not suggest this interpretation, nor does the near-infrared spectrum of IRS~21, which is featureless. However, Tanner et al. cite the likelihood that IRS~21 is actually embedded in the Northern Arm together with the large population of similarly luminous mass-losing stars that have been found nearby but outside the line of sight to the Northern Arm as strong circumstantial evidence for their interpretation. By analogy the Northern Arm sources with similar spectral energy distributions, including IRS~8, might be interpreted in the same way. Although near-diffraction-limited and diffraction-limited images of the central few arcseconds of the Galaxy have been obtained by a number of research teams, the fields that have been imaged have not included IRS~8. Thus, the large area $H$-band (1.49--1.76~$\mu$m) and $K'$-band (1.95--2.30~$\mu$m) mapping of the Galactic center carried out at the Gemini North telescope in 2000 using the University of Hawaii's Hokupa'a adaptive optics system (Graves et al. 1998) and QUIRC camera (Hodapp et al. 1996), part of the Gemini science verification observations (see Rigaut et al. 2003), contain the first near-diffraction-limited observations of the IRS~8 region obtained by any of the new 8-10~m class telescopes. \section{Observations and Results} \subsection{AO imaging} Figure 1a is a portion of one of the images, obtained during the above-mentioned Gemini science verification campaign, showing the IRS~8 region in a narrow band filter centered at 2.3~$\mu$m. The resolution of the image is 0.10\arcsec\ (FWHM) and individual pixels are 0.020\arcsec\ across. A bright arc of emission, presumably a bow shock, is seen nearly surrounding a pointlike object, presumably a luminous star. In Fig.~1b, a contour plot of Fig.~1a, a fainter arc of emission can be seen just to the east of the bright bow shock and an additional region of emission is visible approximately 1.5\arcsec\ to the west of IRS~8. The pointlike source is 1.24\arcsec\ east and 23.66\arcsec\ north of IRS~7 (uncertainties in both offsets are 0.05\arcsec). Using the offset of IRS~7 with respect to Sgr~A* (Menten et al. 1997) and the absolute position of Sgr~A* (Rogers et al. 1994), the position of the point source is \centerline{RA($J$2000) = 17$^{h}$~45$^{m}$~40\fs 14 $\pm$ 0\fs 02;} \centerline{Dec($J$2000) = -28\degr~59\arcmin~58\farcs 7 $\pm$ 0\farcs 3.} Cross sections through central source in the NS and EW axes at $K'$ are shown in Fig. 2; the EW cut shows that the central star is well resolved from the bow shock as well as showing the previously mentioned faint extended emission well to the west of IRS~8. Broad band images at $H$ and $K'$, at slightly lower angular resolution also show the bow shock, with the emission appearing much brighter at $K'$ than at $H$. As seen in Fig.~3, an $HK$ composite image, the $H-K'$ color of the bow is much redder than both the central star of IRS~8 and other nearby stars, indicating that emission from the bow is from much cooler material than in stellar atmospheres. Aperture photometry of the IRS~8 region was performed in order to estimate the fluxes in the $H$ and $K'$ bands from both the bow shock and the central point source. Faint nearby standards from the lists of Hunt et al. (1998) and Hawarden et al. (2001) served as calibrators. Because of the difficulty of accurately determining the flux from a point source superposed on extended emission that varies rapidly with position, the uncertainties in the magnitudes of the IRS~8 point source are approximately 0.1~mag at $K'$ and 0.2~mag at $H$. At $K'$ the extended component, covering 2$\times$2 square arcsecond area centered on the IRS~8 point source has magnitude 10.4. The IRS~8 point source has $K'$=14.1; other discernible point sources in the region are 1-3 mag fainter. The total flux is consistent with Blum et al.'s (1996) result that $K$=10.5 in a 2'' diameter aperture and with the measurement by Becklin et al (1978b) of $K$=10.1 in a 3.8'' diameter aperture (the present images show that almost all of the flux lies within 1'' of the point source). At $H$ the nebula is fainter, the image quality is worse, and it is more difficult to separate the point source from the extended emission, even though relative to the extended emission the central point source is much more prominent at $H$ than at $K'$. The extended emission has $H=13.4$ over the same 4 square arcsecond area of the sky as was measured at $K'$. The $H$ magnitude of the point source probably lies between 15.5 and 16.0. The apex of the bright bow is located 0.22'' (0.009 pc) to the northeast of the central object. The central pointlike object accounts for about four percent of the flux at $K'$ from the IRS~8 region. Radio images of the same region (e.g., Yusef-Zadeh, Morris, \& Ekers 1990) show a bright and extended clump of ionized gas at IRS~8, strongly suggesting that IRS~8 contains a hot star emitting ionizing radiation. \subsection{Spectroscopy} We have obtained a 1.9-2.5~$\mu$m spectrum, of IRS~8 and its surroundings at the United Kingdom Infrared Telescope (UKIRT), in order to search for line emission specifically associated with the bow and the central point source. The data were acquired on UT 2002 July 14 with the facility spectrograph CGS4 using a 0.61'' wide slit (oriented EW) and 0.61'' square pixels, both of which which are too large to isolate the point source from the bow. The velocity resolution of the spectrum is 350~km~s$^{-1}$ and the 1$\sigma$ uncertainty in the wavelength calibration corresponds to 30 km~s$^{-1}$. Spectra of IRS~8 (the point source and the bow shock) from 1.95 to 2.42~$\mu$m are shown in Fig.~4. The raw sky-subtracted spectrum contains considerable extended continuum and line emission. When the mean spectrum immediately adjacent to IRS~8 is subtracted to better isolate the combined spectrum of the bow and stellar object, the result is a nearly featureless and very red continuum as shown. No obvious indicators of shocked gas, such as the lines of H$_2$ that are prominent in shocked molecular clouds, are present in the subtracted spectrum. Weak Br~$\gamma$ (2.166~$\mu$m, $\sim$8$\times$10$^{-18}$~W~m$^{-2}$) and He I (2.058~$\mu$m, $\sim$4$\times$10$^{-18}$~W~m$^{-2}$) emission lines are present in the subtracted spectrum, suggesting a slight enhancement of them at the location of the bow shock and point source (compared to a strong enhancement of the continuum there). The ratio of the He~I to Br~$\gamma$ intensities is greater in the subtracted spectrum. As ionization of helium requires higher energy UV photons than ionization of hydrogen, this enhancement suggests the presence of an internal source of UV photons. The above line strengths are not unusual for an emission line star with the $K$ magnitude of IRS~8. It is also possible, however, that these lines are residuals from the subtraction process described above, due to small nonuniformities in the extended emission. The LSR radial velocities at the line peaks are 0 $\pm$ 50 and +30 $\pm$ 50 km~s$^{-1}$, most consistent with a value intermediate in velocity between the two components observed at IRS~8 in the Ne~II line at higher resolution by Lacy et al. (1980). \section{Discussion} The characteristic morphology of the bright $H$ and $K'$ band emission surrounding IRS~8 immediately demonstrates that this emission originates in a bow shock. As IRS~8 is superposed on both the extension of the Northern Arm and the inner edge Circumnuclear Disk, the cause of the shock could be the collision of the wind or envelope of the starlike object in IRS~8 with material in one of those structures. The enhancement of the Ne II component due to the Northern Arm at IRS~8 suggests that the collision is with material in the Arm, at a speed equal to or exceeding the difference in radial velocities of the two \ion{Ne}{2} components, 120 km~s$^{-1}$ (see Fig.~5). In such a collision the gas and dust densities in the post-shock material would be highly enhanced. Initially both would be collisionally heated as well. The dust also would be warmed by the radiation from the central star of IRS~8 and by the ambient Galactic center radiation field. One might therefore expect such a bow shock to be a prominent feature in infrared images. The location and dimensions of such a shocked region will depend on the densities of the wind and ambient material as well as on the wind speed and stellar motion, as discussed in Section 3.1, and the appearance of the bow shock on the sky will obviously depend on its orientation relative to the observer. As noted previously, in addition to the obvious bow-like structure there are arcs or strips of emission further to the east and west of IRS~8 (Fig. 1b) that may also result from the IRS~8 wind. If due to IRS~8, this would require non-uniformities in the ambient medium, allowing the wind to penetrate further in some directions than in others. A different bow-shock-like structure, associated with the Galactic center source IRS~7, seen in \ion{Ne}{2} and radio continuum observations (Yusef-Zadeh \& Morris 1991, Serabyn, Lacy \& Achtermann 1991), has been interpreted by Yusef-Zadeh and Melia (1992) as due to the collision between a wind from the IRS~16 cluster and a wind from IRS~7. The IRS~7 bow shock is not observed in the Gemini images, due to saturation. IRS~7 is located only 0.3~pc from Sgr~A* and the IRS~16 cluster and the bow shock is located on the side of IRS~7 facing the center. Both the apparent distance from the center and the orientation of the bow shock are quite different for IRS~8. \subsection{Shape and Dynamics} Wind bow shocks are formed when stars with highly supersonic space velocities and powerful winds interact with the interstellar medium through which they are travelling (Baranov et al. 1971; Comer\'on \& Kaper 1998). The structure and size of a wind bow shock are determined by the balance between the ram pressure of the moving wind bubble and that of the ambient medium, according to \begin{equation} \label{eq:eq} \rho_a v_*^2 = \dot{M}_w v_w / 4\pi d_s^2 \end{equation} \noindent where $\rho_a$ is the mass density of the ambient medium, $v_*$ is the space velocity of the star, $d_s$ the distance between the star and the apex (or standoff point) of the bow shock, and $\dot{M}_w$ and $v_w$ the mass loss rate and terminal velocity of the wind. This may be reexpressed as follows (Huthoff \& Kaper 2002). \begin{equation} \label{eq:eq} (v_*/170{\rm km~s}^{-1}) = (\dot{M}_{w,-6} v_{w,8}/n_3)^{1/2} (.009 {\rm pc}/d_s) \end{equation} \noindent where $\dot{M}_{w,-6}=\dot{M}_w/10^{-6}M_\odot {\rm yr}^{-1}$, $v_{w,8}=v_w/1000$ km~s$^{-1}$, and $n_3=n_{\rm H}/10^3$ cm$^{-3}$. In the case of IRS~8, the apex of the bow shock is 0.009~pc from the star. The density of the ambient medium is inaccurately known. Lacy et al. (1979) estimated that the densities of typical cloud clumps in the galactic center are 10$^4$ cm$^{-3}$, and one might expect that intercloud densities are $\sim$10$^3$ cm$^{-3}$. Using $n_3\approx 1$ (the intercloud density) and assuming normal wind properties for the presumed hot star in IRS~8 (e.g., $M_{w,-6}\approx 1$, $v_{w,8}\approx 1$), we find $v_* \approx 150~{\rm km~s}^{-1}$. For a similar $v_*$ and a bow shock formed in a collision with a clump, the product of the mass loss rate and the wind speed would need to be an order of magnitude larger to produce the same stand-off distance; such values are not unusual. A relative velocity of 150~km~s$^{-1}$ would only result in the destruction of a small fraction of the dust (Draine \& Salpeter 1979), so the shocked material would emit continuum radiation. Since, as discussed earlier, it seems most plausible that IRS~8 is associated with the narrow \ion{Ne}{2} velocity component seen at -10 km~s$^{-1}$ (see Fig.~5), most of the motion of IRS~8 is in the plane of the sky, which is consistent with the apparent geometry of the bow shock. The orientation of the bow shock indicates that IRS~8 is moving away from the Galactic center. \subsection{Continuum and Line Emission} Typical grains of sizes $\sim$0.1~$\mu$m located at the distance of the bow shock from IRS~8 and heated solely by its radiation field would achieve steady state temperatures of a few hundred Kelvins. Smaller particles ($\sim$0.01$\mu$m) would be heated occasionally by single photons to higher temperatures. The shock-heated gas will have electrons and ions with kinetic energies of about 100~eV, and stochastic collisional heating can raise grains with radii $\approx$~0.001$\mu$m to temperatures approaching 1000~K (Dwek 1986). The population of grains with temperatures of 500--1000~K (some heated by photons, others by the shock) could account for some of the spatially extended emission seen in the $H$ and $K$ bands, while lower temperature grains, warmed by the radiation field, would account for virtually all of the mid-infared continuum emission. Shock-heating in molecular clouds usually results in strong line emission from H$_{2}$, which has either been collisionally excited to high vibrational states or has reformed in excited states following dissociation. The lack of such line emission in the IRS~8 bow shock is probably due to the lack of molecular gas owing either to its destruction in the shock or to the generally hostile conditions for molecules close to the Galactic center or exposed to UV radiation from IRS 8. \subsection{The IRS~8 Point Source} Assuming the canonical 2.7~mag of extinction toward the Galactic center in the $K$ band (e.g., Becklin et al. 1978b), the dereddened $K'$ magnitude of the IRS~8 point source is approximately 10.4, which at the 8~kpc distance of the Galactic Center, indicates that if IRS~8 is a main sequence star, its spectral classification is $\sim$O9. This classification is consistent with the low ionization state of the surrounding gas and of the galactic center as a whole (e.g., Lacy et al. 1979, 1980). Although its luminosity and $K$-band spectrum hint at it being one of the population of windy He~I stars in the Galactic center, the type of star embedded in IRS 8 is unknown. To constrain its properties will require near-infrared spectroscopy at much higher angular resolution than provided here, in order that the spectrum of the central point source not be so diluted by emission from dust as in the present spectrum. \section{Conclusion} Among the bright mid-infrared sources in the central cluster, IRS~8 stands out as being the most isolated from the other members of the cluster. Its current motion in the plane of the sky is nearly directly away from the IRS~16 cluster. Assuming a velocity in the plane of the sky of 150 km~s$^{-1}$, IRS~8 could have been in the vicinity of the IRS 16 cluster and Sgr~A* about 10$^4$ years ago. It is possible that IRS~8 was flung out of the central cluster by gravitational interactions at that time or is still bound but on a more eccentric orbit than the hot stars within the central parsec. Another possibility is that it is an interloper from well outside of the center. In any of these scenarios the IRS~8 point source would have a proper motion of 0\farcs 004~yr$^{-1}$, which should be observable. The central 1--2 parsecs of the Galaxy contains a number of objects with infrared characteristics similar to IRS~8. As in the case of IRS~21, their high infrared luminosities may also be the result of shock-compression of dust. Tanner et al. (2003) recently reported that 2.2~$\mu$m morphologies of several such objects in the Northern Arm, although less obvious than IRS~8, are reminiscent of bow shocks. Lower luminosity sources with less energetic winds may also be present in within the Northern Arm and other gaseous structures. Thus, IRS~8 may be the most conspicuous member of a large class of objects in the Galactic center. \begin{acknowledgements} We thank J.H. Lacy for making available a portion of his \ion{Ne}{2} data. The Gemini Observatory is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF, on behalf of the international Gemini partnership: the National Science Foundation (United States), the Particle Physics and Astronomy Research Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), CNPq (Brazil) and CONICET (Argentina). UKIRT is operated by the Joint Astronomy Centre on behalf of the U.K. Particle Physics and Astronomy Research Council. We thank the staffs of both telescopes for their assistance. BTD was supported in part by NSF grant AST-9988126. We thank the referee for several helpful suggestions. \end{acknowledgements} \begin{references} \reference{bar71} Baranov, V. B., Krasnobaev, K. V. \& Kulikovskii, A. G. 1971, Sov. Phys. 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The cross indicates the location of the IRS~8 point source (RA($J$2000) = 17$^{h}$~45$^{m}$~40\fs 14 $\pm$ 0\fs 02; Dec($J$2000) = -28\degr~59\arcmin~58\farcs 7 $\pm$ 0\farcs 3). Contour lines are 0.9, 1.5, 2, 3, 5, 10, 30, 50,and 80 \% of maximum surface brightness.} \label{fig1} \end{figure} \begin{figure} \epsscale{0.9} \plottwo{fig2a.eps} {fig2b.eps} \caption{East-west and north-south cuts through the central point source of IRS~8 at $K'$. The flux is averaged over three array rows and columns (0.06'').} \label{fig2} \end{figure} \begin{figure} \epsscale{.8} \plotone{fig3.eps} \caption{$HK'$ color composite image of the IRS~8 region. The resolution is slightly lower at $H$ than at $K'$. The field of view is 11''$\times$9.5'' (RA$\times$Dec)} \label{fig3} \end{figure} \begin{figure} \epsscale{.6} \plotone{fig4.eps} \caption{ $K$-band spectra of the IRS~8 bowshock and point source, obtained in a 0.6'' $\times$ 1.8'' (NS $\times$ EW) aperture, before and after subtraction of the average spectrum of regions immediately to the east and west (see text). Emission lines of He~I and Br~$\gamma$ are indicated. The $K$-band continuum flux density of the point source is approximately $1 \times 10^{-15}$ W~m$^{-2}$~$\mu$m$^{-1}$. The dip in the unsubtracted spectrum at the location of the H$_{2}$ 1-0 S(1) line is due to line emission in the sky spectrum obtained 2' to the west. } \label{fig4} \end{figure} \begin{figure} \epsscale{.6} \plotone{fig5.eps} \caption{Spectrum of the 12.8~$\mu$m fine structure line of \ion{Ne}{2} at IRS~8 in a 2.4'' $\times$ 4.0'' (EW $\times$ NS) aperture and at a resolution of 33 kms$^{-1}$, showing the Northern Arm (broad redshifted) and localized IRS~8 (-10 km~s$^{-1}$) components. The data are from Lacy et al. (1991).} \label{fig5} \end{figure} \end{document}