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Unusual $\rm {H}_{2}$ Emission near a Compact Source in Orion

M. Lindqvist and J.H. Black Onsala Space Observatory, S-439 92 Onsala, Sweden, Email: michael@oso.chalmers.se, jblack@oso.chalmers.se

   

 

Abstract:

Preliminary results are reported for high-resolution imaging of parts of the Orion Bar. Data have been obtained through a variety of filters with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) on Hubble Space Telescope. One striking feature is a small source of almost pure H₂ line emission, which is located near one of the compact sources in Orion reported by O'Dell and collaborators (HST-18 = 203-504). We suggest how some physical properties may be inferred from narrow-band fluxes.

H-H objects, Proplyds, Orion Bar, H₂

Introduction

Molecular hydrogen ($\rm {H}_{2}$) emits strong near-infrared lines, both in regions where it is thermally excited at high temperatures (T>1000 K, e.g. in molecular shocks), and in the illuminated boundaries of molecular clouds (photon-dominated regions or PDRs) where it is excited by UV pumping. In the case of UV fluorescence, when the illuminating radiation is fairly uniform, observed fluctuations in the $\rm {H}_{2}$ line intensity will trace fluctuations in density and column density in the boundary layer of the molecular cloud. This was one of our motivations in obtaining high-resolution images of H₂ emission in the Orion Bar with NICMOS. The Orion Bar is the visible ionization boundary of the Orion Nebula, southeast of the Trapezium. It is a well studied example of a Photon-Dominated Region (PDR) where the boundary of a molecular cloud is illuminated by intense ultraviolet radiation from hot stars (Hogerheijde et al. 1995, Jansen et al. 1995). Direct observations of the excited H₂ emission with $\approx 0\farcs1$ resolution offer the possibility to trace the small-scale structure of such a boundary region.

The Orion Bar region also harbors Herbig-Haro objects, notably H-H 203 and H-H 204 (also known as M42 H-H 3 and H-H 4, respectively) and some of the compact sources and proplyds identified by O'Dell and collaborators (O'Dell, Wen & Hu 1993; O'Dell & Wen 1994).

Observations

We obtained a series of images across the bright rim of the Orion Bar near the peaks of mm-wave molecular line emission with NICMOS cameras 1-3 (NIC1, NIC2, NIC3) in several filters on 1998 February 18. Only preliminary results from NIC2 are discussed here. The NIC2 camera has a field of view of $19\farcs2\times19\farcs2$ and a plate scale of $0\farcs076$ pixel^-1 (Thompson et al. 1998). Images were obtained in two narrow-band (1% resolution) filters, F212N (centered on the H₂ v=1-0 S(1) line) and F215N (line-free continuum for thermal emission), as well as in two medium-band filters, F171M, and F180M.

The data have been reduced through the pipeline processing with the CALNICA software, which carries out the subtraction of dark current, flat-fielding, and linearity corrections. Multiple exposures have been combined in a mosaic by further processing with CALNICB software.

The full mosaic in filters F212N and F215N shows that the Bar region is threaded by a complicated pattern of thin filaments, clearly highlighted by their H₂ line emission. When the excitation of the H₂ vibration-rotation spectrum is near-thermal at high temperatures ( $T\geq 1000$ K), the contrast in flux between F212N and F215N images should be quite high. When the excitation is caused by ultraviolet fluorescence at relatively low densities ( $n({\rm H}_2)\leq 10^6$ cm^-3), strong lines appear in both bands and the contrast is not so large (see §3 below). Where the extended emission shows up in both bands, it is likely that fluorescence makes a large contribution to the observed emission. The appearance and interpretation of the extended emission will be discussed in more detail elsewhere.

We focus here on a very striking feature of the narrow-band images. There is a very compact source which shows up conspicuously in the F212N image but is nearly undetected in the F215N image. It is located $1\farcs6$ from a much brighter stellar source, which has equal brightness in the two narrow-band filters. This stellar source coincides with the compact source HST 18 discovered by O'Dell, Wen & Hu (1993) in their WF/PC survey of the Orion Nebula region. Although many of the compact sources in that work were identified as ``proplyds'' (= protoplanetary disks), this particular object was considered unlikely to be of the proplyd type, possibly an H-H object instead. In the subsequent study of proplyds (O'Dell & Wen 1994), this source was given the designation 203-504 and was listed as associated with a variable star, number 644 (far-red magnitude I=13.5), in the catalogue of Jones and Walker (1988). The image containing this star and our H₂-emitting condensation is shown as a contour diagram in Fig. 1. The infrared image of the star is evidently unresolved ($0\farcs2$ diameter FWHM), while the H₂ condensation is extended ( $\approx 0\farcs46$ FWHM) in F212N. Close inspection of a WF/PC narrow-band image in H$\alpha$ shows that the H₂ condensation coincides with a small, dark area in the bright emission, but the contrast is not high. The fluxes of the star in the two narrow-band filters are equal, approximately 7.0 mJy, which corresponds to a K magnitude of approximately 12.5. In F212N, the integrated flux corresponds to a surface brightness (over $0\farcs46$) of $I\approx 2.4\times 10^{-3}$ erg s^-1 cm^-2 sr^-1. The marginally detected source at the position of the H₂ condensation in F215N is consistent with a point source at K=18.0 mag. The ratio of integrated fluxes f₂₁₂/f₂₁₅=24. In F171M, we measure a weak source at the H₂ position, which is not obviously extended. The ratio of integrated fluxes, corrected for bandwidth, is f₂₁₂/f₁₇₁=4.6.

Discussion

Proplyds are seen as dark silhouettes in projection against the Orion Nebula and are believed to be circumstellar disks. NICMOS observations of several proplyds in Orion have recently been discussed in several papers (Chen et al. 1998; McCaughrean et al. 1998; Stolovy et al. 1998). Although O'Dell et al. 1993 suggested that HST 18 is probably not of the proplyd form, it is interesting that it appears to be associated with the compact source of H₂ emission that we have found. The evidence of an association consists of (1) the small angular offset and (2) the extension of H₂ emission contours along an axis directed away from the star HST 18. At the distance of Orion KL, d=480 pc (Genzel et al. 1981), the angular distance between the star and the H₂ condensation corresponds to 770 AU in projection on the sky. The angular extent of the H₂ emission, $0\farcs46$, corresponds to 220 AU ( $3.3\times 10^{15}$ cm).

In fact, O'Dell and Wen (1994) proposed that HST 18 is an Herbig-Haro (H-H) object. H-H objects are collisionally excited nebulae produced by supersonic outflows ejected by young stellar objects (YSOs). Although HST 18 shows H$\alpha$ and [N II] line emission based on the WF/PC images, our infrared images of that source appear stellar. The suggestion of an H-H object does not seem compelling. It is impossible to decide whether the compact source of H₂ emission nearby is excited by a shock or by some other thermal process without a spectrum. However, the flux ratios in the narrow-band images can distinguish between pure fluorescent emission and some form of thermal excitation. We have computed thermal emission spectra of H₂ at temperatures T=1000, 2000, and 3000 K, and convolved these with the NICMOS filter response functions. The predicted flux ratios at these three temperatures are f₂₁₂/f₂₁₅ = 950, 43, and 15, respectively. The flux ratio in a typical fluorescence spectrum (cf. Black & van Dishoeck 1987) is f₂₁₂/f₂₁₅ = 2.7. The observed ratio of integrated fluxes, $f_{212}/f_{215}\geq 24$, is thus inconsistent with pure fluorescence; it would imply a temperature $T\leq 2500$ K for fully thermal emission. There is a large number of H₂ lines within the bandpass of the F171M filter. The observed, bandwidth-corrected ratio of integrated fluxes f₂₁₂/f₁₇₁=4.6 is in harmony with thermal emission at 1500 < T < 2400 K. The corresponding value of the ratio predicted for pure fluorescence is 0.84. As shown by Sternberg & Dalgarno (1989), UV-pumping of H₂ emission produces a spectrum more like the thermal spectrum, when the density is high enough that the excited H₂ is partly de-excited by collisions (i.e. partly thermalized). The position of our H₂ condensation is close to one of the peaks of emission in the C 65$\alpha$ and C 91$\alpha$ recombination lines, which trace the density of the PDR that is maintained by the light of the Trapezium stars (Wyrowski et al. 1997). The ambient interstellar density in this direction is moderately high, $(0.5- 2.5)\times 10^5$ cm^-5. Thus it is conceivable that the H₂ emission arises in an even denser condensation in the PDR through partly thermalized UV fluorescence. Spectroscopy will be needed to decide definitively the nature of this source. Chen et al. 1998 suggested that the H₂ emission from their circumstellar disks (proplyds) cannot be excited by a mechanism internal to the source; they concluded that it is more likely that the $\rm {H}_{2}$ is excited by UV fluorescence from one of the hot stars nearby.

The observed flux ratios for our source are consistent with thermal emission at $T\approx 2000$ K. The observed surface brightness of the H₂ condensation requires a column density of molecules $N({\rm H}_2)=7.5\times 10^{18}$ cm^-2 thermalized at this temperature. Such a column density spread over the small measured extent of the source corresponds to a total hydrogen mass of only $2.7\times 10^{26}$ g or 0.046 earth-masses. If nothing else, this is an indication of the remarkable sensitivity of NICMOS to small amounts of excited hydrogen.

  

Figure: The top panel shows the $\rm {H}_{2}$ emission (F212N) taken with NIC2. The bottom panel shows the adjacent ``continuum'' band (F215N). Note the large contrast between the $\rm {H}_{2}$ emission and the continuum emission in the source at offsets (-24.8,-16.0).

We gratefully acknowledge the efforts of the NICMOS Science Team over the years and of the NICMOS project staff at NASA Goddard Space Flight Center and STScI. Our work is a part of the NICMOS/GTO science and has been generously supported by a contract through the University of Arizona.