3.1. Morphology of the region
The image of AFGL 961 shown in Fig. 1 possesses several interesting features. Present are numerous nebulous regions, some are locate relatively close to the binary while some are at a considerable distance from it. These nebulous objects are mostly compact although in a few cases, particularly the regions directly north and east of the stars, they are extended and show distinct morphological structure. These compact nebulae are seen only in H2; a 2.1µm narrow band continuum images of the central 22´ of the H2 image shows little structure other than the stellar components in the field and some faint nebulosity in the region of AFGL 961 W. For this reason the continuum image is not presented here. The H2/continuum images therefore confirm the spectroscopic observations in that bright pure H2 emission nebulae possessing no significant continuum flux are present around AFGL 961. The top spectrum in Fig. 3 presents a representative region of this nebulosity located 15" south of AFGL 961b.
The numbers in Fig. 1 identifies the nine nebulous regions found in the vicinity of AFGL 961. We herein referred to these regions as CA objects in agreement with the suggestion of the IAU Nomenclature Committee whereby NIR nebulous objects similar to morphology to optical HH objects should be designated by the first letters of the discovery authors names. Details of the CA objects in AFGL 961 are given in Table 1 where we present accurate spatial offset positions and position angles for all nine objects. Also given are morphological description of their appearance in H2. Next, we comment on each of these objects and where possible, relate them to their optical counterparts.
Table 1. Positions and Descriptions of Nebulous Objects
CA 1 is located to the north of the binary and has the morphological appearance similar to that of an optical HH bow shock structure. This IR bow shock, shown in close-up in Fig. 4, is pure H2 emission and is oriented with its axis of symmetry at position angle (p.a.) 354 east of north. The eastern edge of the bow shock is somewhat brighter than its western counterpart. It is uncertain which of the two stars forming the binary is the originator of this structure, although, as we shall see below (Sect. 3.2), only AFGL 961b currently shows signs of activity generally associated with outflowing gas, specifically, weak 12CO bandhead emission. Tracing the axis of symmetry back from the tip of CA 1 towards the binary suggests that the identification of AFGL 961b as its originator is likely correct.
CA 2 is located to the west of the binary and is cavity- or shell-like in morphology. This source is associated with the stellar object, AFGL 961 W and briefly discussed by Hodapp (1994). AFGL 961 W is located between the two linear cavity/shell walls. A detailed view of CA 2 is shown in Fig. 4. CA 2 may be physically related to AFGL 961 W rather than the AFGL 961 binary since AFGL 961 W exhibits nebulous extensions itself (to both west and south). However, the cavity/shell walls do extend away from the approximate location of the AFGL 961 binary and there is fainter nebulous extensions joining the cavity/shell structure to the immediate vicinity of the binary itself (see Figs. 1 and 2). On the Digitized Sky Survey (SS) 2 (Fig. 7) CA 2 appears as a diffuse/nebulous regions suggesting that this is a distinct and separate PMS star unrelated to AFGL 961; being visible in the optical implies that it is either foreground to AFGL 961 (itself highly obscured in the optical) or seen through a low extinction path in the molecular cloud.
CA 3 is located to the south of the binary and, as in CA 1, exhibits a structure reminiscent of an optical bow shock. In this case however, the bow shock structure is not as definitive as in CA 1 and is confused by the close association of a second component to the west. In fact, in Fig. 2, both of these components bear much similarity to bow shock like structures. Fig. 4 shows this region in detail although the bow shock nature of these objects is not as apparent in this lower contrast image. If these two components are in fact bow shock then they are directed along axes that extend from the vicinity of the binary at p.a. 178 and 157 , respectively. The eastern bow shocks appears to lie on an axis from AFGL 961a while the western bow shock lies on an axis from AFGL 961b.
CA 4 is approximately an arcminute to the south of the binary and also has a morphology reminiscent of an optical bow shock. It can be seen more clearly in Fig. 4. The outflow axis associated with this object is 164 . This is close to halfway between the the two axes defined for CA 3 and hence, coupled with the sources physical distance from the binary, it is impossible to determine from which binary components the object originated.
CA 5 is a diffuse region of H2 emission south-west of the binary. This object, shown in Fig. 5, possesses several distinct clumps or knots superimposed on a more diffuse nebula.
CA 6 is on the north-west edge of our image and is the most distant nebulous object detected in our images at 110" from the binary. At the assumed distance of AFGL 961 of 1.6kpc, this corresponds to a physical separation of 0.85pc. This object, shown in Fig. 5, also consists of several bright knots with a fainter diffuse component. No identifiable morphological structure can be seen in CA 6.
CA 7 lies almost exactly halfway between the western binary component and CA 6 and along the identical axis (p.a. 36 ). CA 7 is faint but shows a definite morphological structure similar to an optical bow shock (see Fig. 1 and Fig. 5).
CA 8 also lies approximately along the axis to CA 6 and CA 7 although this object is more knotty/diffuse than CA 7, more like CA 6 in appearance (see Fig. 1 and Fig. 5). There is a hint that the fainter regions of CA 8 (see Fig. 1 a few arcseconds to the north of the peak flux) form a bow shock-like structure although deeper data is required to confirm this.
CA 9 is closer to the western edge of our image and consists of two clumps, one relatively stellar, the other diffuse with a prominent knot (see Fig. 6). Since this object was not included in the narrow band continuum image obtained, the star-like eastern component of CA 9 could possible be a field star of a young star with associated nebulosity.
3.2. NIR spectroscopy: shock-excited H2 emission
Fig. 3 shows the 2µm spectra of both AFGL 961a, AFGL 961b and of the nebulous region located 15" south of AFGL 961b. By far the reddest source of the binary is AFGL 961a which shows a strong rising red continuum over the wavelength regime observed. Superimposed on this is a strong Br- line in emission. The small-scale `ripple' on the spectra is an artifact of the data acquisition. No other features are detected in the spectrum of AFGL 961a. Porter et al. (1998) suggested that the strong Br emission may be temporally variable. AFGL 961b shows a modestly rising red continuum compared to that of its companion superimposed on this spectra is also a relatively strong Br- emission line and also weak, but certainly identifiable, emission bands of 12CO (specifically, the 2-0 12CO at 2.294µm and the 3-1 12CO at 2.323µm). The nebulosity south of AFGL 961b exhibits a spectrum of pure H2 emission with the line emission being dominated by the v=1-0 S(1) line at 2.122µm and the Q-branch at 2.4-2.45µm. Several additional (fainter) H2 lines are also present. No Br- emission is detected at this location. The spectra obtained along the long-slit used in the observations (for both binary components) show similar spectra with little to no continuum emission present.
The H2 spectrum shown in Fig. 3 exhibits a large flux ratio (specifically 10.6) between the v=1-0 S(1) line (2.122µm) and the v=2-1 S(1) line at 2.248µm. This generally implies that the H2 gas is shock-excited although with high enough densities (n105 cm-3) fluorescent emission (excitation by UV absorption) can produce a similar line ratio. For low density regions fluorescent excitation gives a significantly lower flux ratio (more like 2). Burton (1992) discusses the excitation of H2 in more details. Given a number density 105, we are certainly observing shock-excited H2 emission. The CA object corresponding to this location along the long-slit is the eastern region of CA 3 which in the deep S(1) images shown in Fig. 1 resembles a small bow shock (and would be most likely shock-excited). At the other positions along the CGS4 long-slit showing H2 emission, the flux ratio of these two lines is similar varying only between 8 and 11. Hence, even in the presence of the high-luminosity hot binary components, the H2 in the vicinity of the stars is probably shock-excited.
3.3. The extinction towards the H2 emission
We can obtain an estimate of the overlying extinction towards the H2 emission from the ratio of the two H2 emission lines the v=1-0 Q(3) line at 2.424 µm and the v=1-0 S(1) line at 2.122 µm. Both these lines are emitted from the same level and their unreddened ratio has been calculated to be 0.7 (Turner et al. 1977). NIR extinction has additionally been shown to be representable by a power law of slope -1.8 (Martin & Whittet 1990). Using this we can estimate the visual extinction, AV from the differential extinction between the two H2 lines. At the location of the H2 emission shown in Fig. 3, e.g. 15" south of AFGL 961b, we find a line ratio Q(3)/S(1) of 1.160.2. This corresponds to a differential extinction between the two lines of 0.550.1 magnitudes. Extrapolating to the optical implies an overlying AV of 295 magnitudes. This value is somewhat larger than the extinction derived towards AFGL 961a and AFGL 961b by Castelaz et al. (1985) e.g. 17 magnitudes. However, Alonso-Costa & Kwan (1989) and Porter et al. (1998) derived larger values of AV, specifically, 284 and 20-25 magnitudes, respectively, from both NIR H line ratios (both Alonso-Costa & Kwan, and Porter et al.) and NIR continuum ( 1.5µm) fitting (Porter et al.). Our value is therefore consistent with other recent determinations.
3.4. The stellar population: evidence for a PMS cluster
If one compares Fig. 1, our 2.122µm H2 image, with the Sky Survey (SS) image shown in Fig. 7, it is obvious that the stellar population in the immediate vicinity of AFGL 961 is considerably larger in the NIR than the optical. The region centred on the AFGL 961 binary is clearly heavily obscured by overlying molecular material; few stars are found at this location in the SS image. The brightest source in the SS image is AFGL 961 W with its associated nebulosity located to the west of the centre of the image. In total, 17 stellar sources are visible in the SS image covering the region imaged in the NIR. In the H2 image (Fig. 1) we see over 80 stellar objects. A large number of these sources are concentrated towards the centre of the image i.e. around the AFGL 961 binary. The overall impression from Fig. 1 is that there is a stellar cluster centred close to AFGL 961, in particular, on the region 30" north of the binary. We confirm therefore that a small cluster, consisting of perhaps 40-50 stars, is present around the high-mass binary as originally suggested by Hodapp (1994). Further optical/NIR photometric observations in standard (broad-band) passbands are required to better investigate the cluster membership and size.
© European Southern Observatory (ESO) 1998
Online publication: June 26, 1998