Br Gamma and Br Alpha Emission
Comparison of Infrared and Radio Emission
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S88B (also known as G61.5+0.1), associated with the S88 molecular cloud, is composed of spherical and cometary ultracompact HII regions (Wood & Churchwell 1989, Garay et al. 1993). Strong radio emission detected at 5 GHz is concentrated in two peaks, S88B-1 and S88B-2 (Garay et al. 1993); S88B-1 is probably identical to the emission peak at 2.7 GHz (Pipher $\etal$ 1977). These radio emission sources, located in a region void of near-infrared emission, fall along the edge of optical nebulosity delineated by H$\alpha$ emission. Pipher et al. and Evans et al. (1981) detect infrared nebulosity and point-like objects to the west of these radio emission sources.
Felli & Harten (1981), mapping the region at 5 GHz at low spatial resolution, report a flux density for the whole region of 6.1 Jy, which is consistent with ionization by a single O5.5 star. This prediction agrees with the results of Garay et al. who deduce an O6 star responsible for the emission from S88B-1.
Phillips & Mampaso (1991) report a north-south elongated
outflow detected in CO at low resolution. The peak of the red (south)
lobe coincides most closely with the peak of the 12.6 um emission (Pipher
et al.), but as can be seen from Figure 1, emission peaks at many
wavelengths are located with ~20" of each other. The blue (north)
lobe velocities range
from 15.8 to -3.7 km s-1 while the red
lobe velocity range is 24.9 to 44.4 km s-1.
The number and location of the sources responsible for the emission in this region remain a question. Proposed models for the region (Pipher et al. 1977, Felli & Harten 1981) require the exciting sources be embedded at the edge of a dense molecular cloud. West of these radio emission peaks, presumably indicating a site of star formation, the molecular cloud becomes less dense allowing observation of infrared point-like sources and finally the optical nebula. The point-like infrared objects reported in this work may be additional sources of excitation.
The near-infrared images of S88B were obtained at the
Mount Lemmon Observing Facility 1.5 meter telescope on 20 and 21 May 1997.
Observations were taken with the University of Rochester Third Generation
256 x 256 InSb array. Individual frames were linearized,
background subtracted, and flat fielded.
| Lambda | Delta Lambda (nm) | Date | Integration Time (minutes) | Calibration Star |
| J (1.25 um) | 230 | 20 May 1997 | 0.17a | HD162208 |
| H (1.65 um) | 320 | 20 May 1997 | 0.17a | HD162208 |
| K (2.23 um) | 410 | 20 May 1997 | 0.17a | HD162208 |
| Br Gamma (2.166 um) | 29.07 | 21 May 1997 | 11.7 | Gamma UMa |
| 3.29 um | 63.25 | 21 May 1997 | 26.7 | HD201941 |
| 3.40 um | 63.50 | 21 May 1997 | 29.9 | HD201941 |
| Br Apha (4.052 um) | 65.31 | 21 May 1997 | 9.0 | Gamma UMa |
We detect Br Gamma and Br Alpha emission in the vicinity
of the 2.2 um emission peak. To compare our Br Gamma and Br Alpha
fluxes with those of Evans et al. (1981), we measured the flux inside
an 11" beam, centered at Evans et al.'s position
P (2.2 um emission peak). We find F(Br Gamma) = 1.4 x 10-15
W m-2 and F(Br Alpha) = 1.0 x 10-14 W m-2,
which agree reasonably well with Evans et al. Using the same
beam, we find ABr Gamma- ABr Alpha ~ 1 mag, which
agrees with the determination by Evans et al. However, in
a region of higher F(Br Alpha) signal-to-noise, we find ABr Gamma-
ABr Alpha ~ 0.6 mag; this region is coincident with the
H-alpha emission and suffers less extinction.
The non-detection of Br Gamma emission in the vicinity
of S88B-1 and S88B-2 implies a lower limit on the visual extinction.
The ionized gas producing the emission at 5 GHz should also be detected
in Br Gamma. Comparing the Br Gamma flux predicted from 5 GHz emission
and the noise level in our Br Gamma images and using the relation AV/AK=
9.29 (Mathis 1990), we find the lower limit of the visual extinction to
be 50 magnitudes.
We detect both 3.29 um and 3.4 um feature emission in
the vicinity of the 2.2 um emission peak, apparently in a shell (PDR)
surrounding the ionized hydrogen emission (see Figure
3). The signal-to-noise ratio is approximately 10 for the 3.29 um
emission and 3 for the 3.4 um emission. In regions of significant
signal-to-noise, the values of the ratio of 3.4 um to 3.29 um fluxes range
from 0.15 to 0.25, excluding star 6, where the value is 0.5. The
independence of the ratio on position most likely indicates a uniform
mixing of large (50 atoms) and small (20 atoms) PAHs (Geballe et al.
1989).
The 8.1 and 2.7 GHz emission detected by Pipher et al. (1977) and the objects S88B-1 and S88B-2 detected at 5 GHz by Garay et al. (1993) appear to be located in a region devoid of infrared emission east of star 6 and the 2.2 um emission peak. Considering the model of Evans et al. (their Figure 7) reproduced in Figure 4, the lack of infrared emission near the radio sources is due to a dense molecular cloud. Star 6, postulated below to be a YSO, could represent the western-most young star of the cluster (see Figures 1, 2, and 7).
a. [H-K] Color of Reflection Nebula
The [H-K] color of the reflection nebula varies from approximately
1.2 magnitudes in the region coincident with the H-alpha emission to approximately
1.7 magnitudes in the area presumably behind the molecular cloud.
Sellgren, Werner & Allamandola (1996) found the [H-K] color of reflection
nebula with no transiently heated grain emission to be 0.03. Using
[H-K]0 = 0.03, we can estimate the visual extinction, AV,
if the reddened color is due to line of sight extinction. From Draine
(1989),
here EH-K is the infrared excess. Our observed
values of [H-K] imply AV ~ 24 magnitudes in regions associated
with
the edge of the molecular cloud and AV ~ 16
magnitudes in the regions coincident with the optical emission.
The coincidence of 3.29 and 3.4 um feature emission with the nebula may indicate the presence of transiently heated grains. Sellgren, Werner & Allamandola quote a [H-K] color of ~1 for NGC 7023, a reflection nebula believed to contain transiently heated grains. Using this value for [H-K]0, we find AV ~ 2.5 magnitudes and ~ 9 magnitudes in the two regions discussed above. On the other hand, the feature emission may be a foreground PDR shell.
b. [H-K] and [J-H] Colors of Stars
We have measured the [J], [H], and [K] magnitudes of selected field stars around S88B, summarized in Table 2, and imaged in Figure 2. Since most stars have [H-K] ~ 0.1 - 0.3, we believe stars 1, 3, 9 and 18 are foreground stars, as their [H-K] color falls within this range. The positions of stars 1 and 3 are obtained from optical observations (Pipher et al. 1977).
The [K] magnitude of star 6, the brightest star at K-band,
is consistent with that of an O6 main sequence star at the distance of
S88B suffering from 25 magnitudes of visual extinction. However,
the [J-H] and [H-K] colors, taken together with the
[K] magnitude, suggest that star 6 is a massive YSO.
Star 6 is located along the edge of the H$\alpha$ emission, and could be
subject to that amount of extinction, as proposed above (see a).
Cluster star 6, possibly a YSO, may be partially responsible for the excitation of S88B.
The [H-K] color of the reflection nebula and the extinction
gradient deduced from the composite J-, H- and
K-band emission image, as well as the non-detection
of the radio sources agree with the extinction pattern deduced by
Evans et al. (1981) for observations of an optical nebula to the
west of the radio sources, with dense molecular cloud material to the east
of star 6.