ABSTRACT
We present infrared observations of the ultracompact H ii region W3(OH) made by the FORCAST instrument aboard the Stratospheric Observatory for Infrared Astronomy (SOFIA) and by the Spitzer/Infrared Array Camera. We contribute new wavelength data to the spectral energy distribution (SED), which constrains the optical depth, grain size distribution, and temperature gradient of the dusty shell surrounding the H ii region. We model the dust component as a spherical shell containing an inner cavity with radius ∼600
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1. INTRODUCTION
Ultracompact H ii regions (UCHs) can be found throughout the Galaxy surrounding newly formed massive stars just completing the accretion stage and entering their main-sequence lifetimes. UCHs are small (<0.4 pc), hot (>100 K), and massive (>103 M☉), and emit 104–106 L☉ (Churchwell 2002). These regions of ionized gas are surrounded by molecular and dust clouds, out to as far as 10 times the radius of the UCHs themselves (Conti et al. 2008). This causes attenuation of much of the emitted luminosity, as it is absorbed and reradiated in the infrared.
W3(OH) is one of the largest and best-studied UCH ii regions known in the Galaxy. It is at a distance of approximately 2.04 kpc in the Perseus arm (HaChisuka et al. 2006). Like most UCH ii regions, it is surrounded by its natal dust and molecular gas envelope as well as a larger giant molecular cloud encompassing W3(OH) and W3(H2O), W3 Main, and AFGL 333; these are all regions of potentially triggered star formation, based on their positions on the dense outskirts of a much less dense cavity in the W3 GMC (Ruch et al. 2007; Moore et al. 2007). A further review of star formation in the W3 GMC is presented by Megeath et al. (2008).
Line emission from the molecular gas surrounding W3(OH) has been mapped at submillimeter and radio wavelengths in the transitions of OH and H2O (Mader et al. 1978), HCN (Turner & Welch 1984), NH3 (Wilson et al. 1978; Zeng et al. 1984; Tieftrunk et al. 1998), CH3OH (Menten et al. 1992), C18O (Wink et al. 1994), and C17O (Wyrowski et al. 1997). This molecular emission is found out to a 1' diameter region. These studies have particularly focused on further characterizing the molecular gas and the OH and methanol masers within and around W3(OH), as well as the hot core/water maser region W3(H2O) approximately 6'' to the east. In continuum emission, however, the dust cocoon surrounding the UCH ii region comes into primary focus.
The dust component to the UCH ii W3(OH) was detected by Wynn-Williams et al. (1972) at ∼1–20
In this work, we present new, high spatial resolution observations of the W3(OH) region in the wavelength range ∼3.6–40
2. OBSERVATIONS
2.1. SOFIA/FORCAST Observations
Observations were conducted on 2010 December 8 aboard the SOFIA, using the Cornell-built FORCAST (Adams et al. 2010). FORCAST uses an Si:As BIB detector array at wavelengths 7638 pixel scale (rectified) over a total field of view of 3
4 × 3
2. Four filters were utilized for these observations, with central wavelengths (and bandpasses) at 19.7 (5.5), 24.2 (2.9), 31.4 (5.7), and 37.1 (3.3)
2.2. Spitzer/IRAC Observations
We used data obtained with IRAC on Spitzer. The data were taken from observations that were obtained in high dynamic range mode, whereby two images are taken in succession at 0.4 s and 10.4 s integration times. The brightest objects in the field, including W3(OH) were saturated in the longer frames, so we used the 0.4 s exposure time frames to construct the mosaic that was used for the bright source photometry. The following AORIDs were used: 5050624, 19305728, 20590592, 38744064, 38757632, 38763776, 38769408, 38770944, 38790656, and 38801408. We utilized the Basic Calibrated Data (BCD) version S18.5 products from the Spitzer Science Center (SSC) standard data pipeline. For the 3.6 and 4.5 863 pixel−1.
3. RESULTS
3.1. Imagery and SOFIA/FORCAST Detections
Reduced, multi-wavelength IRAC and FORCAST images are shown in Figures 1 and 2. The measured image quality in the FORCAST images was consistent with the overall image quality achieved by SOFIA during the Early Science period (Herter et al. 2012). W3(OH) is spatially resolved in the FORCAST images. Within W3(OH), the dual peaks in flux correspond to a bright cometary southwestern H ii region and a dimmer, more eliptically shaped northeastern H ii region (Stecklum et al. 2002).
Figure 1. False-color IRAC image of the W3(OH) region at 3.6
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Standard image High-resolution imageFigure 2. False-color image of the W3(OH) region at 3.6
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Standard image High-resolution imageWe detect four mid-infrared point sources in the SOFIA field. These sources are designated in the 2MASS catalog as J02270352+6152357, J02270743+6152281, J02270824+6152281, and J02270887+6152344. There are also 2
Table 1. Positions and Flux Densities in Janskies for W3(OH), W3(H2O), and Four Mid-IR Sources in the SOFIA Field
J02270352+6152357 | W3(OH) | Northeastern H ii | W3(H2O) | J02270743+6152281 | J02270824+6152281 | J02270887+6152344 | |
---|---|---|---|---|---|---|---|
R.A. (2000) | 02 27 03.52 | 02 27 03.83 | 02:27:07.43 | 02:27:08.24 | 02:27:08.87 | ||
decl. (2000) | +61 52 35.7 | +61 52 24.8 | +61:52:28.1 | +61:52:28.1 | +61:52:34.4 | ||
F2.16 | ... | 1.76 × 10−3 ± 2.36 × 10−4 | ... | ... | ... | ... | ... |
F3.6 | 0.0262 ± 0.00013 | 0.619 ± 0.00027 | ... | ... | 0.0691 ± 0.026 | 0.120 ± 0.00018 | 1.96 ± 0.00027 |
F4.5 | 0.0470 ± 0.000088 | 3.74 ± 0.00018 | ... | ... | 0.0759 ± 0.026 | 0.137 ± 0.00012 | 2.97 ± 0.00018 |
F5.8 | 0.237 ± 0.00054 | 11.8 ± 0.0011 | ... | ... | 0.364 ± 0.15 | 0.613 ± 0.00072 | 4.76 ± 0.0011 |
F8.0 | 0.584 ± 0.00030 | 24.9 ± 0.00060 | ... | ... | 1.08 ± 0.41 | 1.66 ± 0.00040 | 9.43 ± 0.00060 |
F19.7 | 1.53 ± 0.31 | 144 ± 37 | 19.7 ± 3.9 | 9.41 ± 1.9 | 1.28 ± 0.26 | 4.86 ± 0.97 | 12.8 ± 2.6 |
F24.2 | 4.68 ± 0.94 | 683 ± 148 | 79.1 ± 16 | 76.7 ± 15 | 2.70 ± 0.54 | 10.5 ± 2.1 | 23.1 ± 4.6 |
F31.4 | 10.6 ± 2.1 | 1626 ± 332 | 170 ± 34 | 147 ± 29 | 3.32 ± 0.66 | 15.0 ± 3.0 | 29.0 ± 5.8 |
F37.1 | 13.0 ± 2.6 | 3232 ± 646 | 223 ± 45 | 225 ± 45 | ... | 16.4 ± 3.3 | 40.9 ± 8.2 |
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3.2. Deconvolved FORCAST Images
We performed beam deconvolution on each of the FORCAST images in order to search for the hot core W3(H2O) and to measure the size of W3(OH). The images were deconvolved using the maximum likelihood method (Richardson 1972; Lucy 1974). Like all deconvolution methods, knowledge of the point-spread function (PSF) of an unresolved source is needed at each wavelength. The delivered PSF can change due to different wind loads on the secondary mirror and differences in the telescope flexure as a function of telescope position. To mitigate these effects on ground-based telescopes, high signal-to-noise (S/N) observations of mid-infrared bright stars are usually taken immediately before and/or after each science target observation and as close to the science target as possible (<1° away) so as to get the best PSF calibration for use in the deconvolution procedure. However, in our case, finding a PSF star that is bright enough at wavelengths out to 40
We claim to detect unresolved emission from the hot core W3(H2O) at 31.4 and 37.1
Figure 3. Deconvolved SOFIA/FORCAST image of W3(OH) at 31.4 and decl. = +61°52'24
6 (J2000). The core of W3(OH) has been subtracted using a two-dimensional Gaussian fit to its radial brightness profile. The locations of sources A, B, and C in W3(H2O) from Wyrowski et al. (1999) are shown. The green contours represent 8.4 GHz VLA observations from Wilner et al. (1999). The contours levels are 0.00004, 0.0001, 0.00018, 0.0003, 0.0005, 0.001, 0.005, 0.01, 0.015, 0.016, 0.02, 0.022, and 0.03 Jy beam−1. An assumption is that the peak of the 8.4 GHz contours coincides with the peak of the 19.7–37.1
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Standard image High-resolution imageFigure 4. Same as Figure 3 for 37.1
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Standard image High-resolution imageFlux extraction for W3(OH), W3(H2O), the northeast H ii region, and 2MASS J02270352+6152357 was performed using the deconvolved images. The resolved diameter of W3(OH) in the deconvolved images (Figure 5) is approximately 40 pixels, corresponding to ∼63,000
Figure 5. SOFIA/FORCAST deconvolved images at 19.7, 24.2, 31.4, and 37.1
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Standard image High-resolution imageFigure 5 shows boxed regions where the flux profiles for W3(OH) and W3(H2O) were extracted (integrated vertically) and fit to Gaussian functions (Figure 6). Each of the profiles shows a secondary peak coincident with the location of W3(H2O). The area under the profile fit to W3(H2O) yields its flux. The extracted flux densities for W3(H2O) are also listed in Table 1. We discuss our detection of this source further in Section 4.2. The flux densities for W3(OH) measured in this fashion agree to within ∼10% of the flux density measured from large aperture photometry, which is listed in Table 1.
Figure 6. One-dimensional integrated and normalized line profiles (points) and best Gaussian fits for W3(OH) (magenta line), W3(H2O) (blue line), the northeastern H ii region (green line), and the point-source 2MASS J02270352+6152357 (red line). The one-dimensional integrations were performed along the short directions of the boxes in Figure 5. The black lines represent the sum of the fits. The extracted flux densities for all sources are given in Table 1.
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Standard image High-resolution imageFigure 5 also shows the regions selected for line profiles of the northeastern H ii region (integrated horizontally) and 2MASS J02270352+6152357 (integrated vertically). The latter source is flagged in the 2MASS catalog as confused with neighboring objects; thus we do not consider its 2
4. DISCUSSION
4.1. W3 (OH)
In Figure 7, we show the SED for W3 (OH) with additional data taken from the literature: 2MASS (2.2
Figure 7. SED of W3(OH) and model fits. Filled circles: combined data points (see the text for references). The IRAS fluxes were taken as upper limits, indicated as downward arrows at ∼60 and 100
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Standard image High-resolution imageWe model the dust component as an optically thick, dusty shell around the H ii region, irradiated at the inner boundary by the central star with surface temperature 3.11 × 104 K. The emergent SED of the model was computed using the DUSTY radiative transfer code.7 The density distribution of this model (
Table 2. DUSTY Parameters for Best-fit Model of the W3(OH) Dust Component
Parameter | Value |
---|---|
T* | 31145 K |
Rin | 576 |
Rout | 29942 |
Tin | 400 K |
Tout | 26 K |
Composition fraction, silicates | 53% |
Composition fraction, graphite | 47% |
p | 1.5 |
q | 3.5 |
2.8 |
Notes. The parameters are stellar temperature T*, inner shell radius Rin, outer shell radius Rout, inner shell boundary temperature Tin, outer shell boundary temperature Tout, grain composition, density distribution parameter p, grain size distribution parameter q, and optical depth
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Although they both use an optically thick, free-fall density shell in the IR, this model differs quantitatively from the model presented in Stecklum et al. (2002). The central star is of later O-type and cooler surface temperature. The inner dust shell radius is nearly four times smaller and the outer radius nearly twice as compact as in Stecklum et al. These parameters are necessary in order to explain the mid-IR emission in the range 17–37
Note this model cannot account for the excess emission at 2.2–5.8
An alternative explanation for the excess at 2.2–5.8
4.2. W3(H2O)
In the radio continuum, the hot core W3(H2O) consists of three clumps designated as A, B, and C (Wyrowski et al. 1999; Wilner et al. 1999). An early claim to a detection of W3(H2O) in the infrared came from Keto et al. (1992) who presented ground-based observations at 12.2 0 at 37.1
4 at 31.4
4.3. Protostars and Young Stars with Disks in the SOFIA Field
We construct SEDs for the four point sources in the FORCAST field and compare them with model SEDs of protostellar and young stellar objects using the online SED fitting tool of Robitaille et al. (2006, 2007). The SEDs and those of the best-fit models are shown in Figure 8. We provide comments on each object in Sections 4.3.1–4.3.4.
Figure 8. SEDs of the four point sources in the SOFIA field (filled circles) and the best-fit protostellar SED model (solid line) from the online SED fitting tool of Robitaille et al. (2006, 2007). For 2MASS J02270743+6152281 and 2MASS J02270887+6152344, the best-fitting alternative, disk-dominated model SEDs are also shown (dashed lines).
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Standard image High-resolution image4.3.1. 2MASS J02270352+6152357
The best-fitting model for J02270352+6152357 corresponds to a protostellar object with a mass of 3.96 M☉ and a luminosity of 236 L☉. The model indicates that this object is young and cool, with an age of just under 5000 yr and a temperature in the low 4000 s of K. It is undergoing active envelope accretion from a massive envelope (11.7 M☉) onto a low-mass disk (3.92 × 10−3). The self-consistency of the top ten models with this best-fitting model suggest that this object is indeed a young, intermediate-luminosity protostar.
4.3.2. 2MASS J02270743+6152281
The SED fitting tool produced two families of best-fit models for J02270743+6152281, with the first family headed by the best-fitting SED and the second by the second-best fit. The families differ in the object's stage of envelope accretion and disk formation, with one family representing an object still actively undergoing envelope accretion and the other corresponding to a more highly developed disk with a depleted envelope.
The best-fitting (total
The second-best fit (total
The two families of models differ primarily in the presence or absence of an amorphous silicate absorption feature at 9.7
4.3.3. 2MASS J02270824+6152281
Model fitting for J02270824+6152281 presents a convergent set of parameters. The best-fit model corresponds to a 6.11 M☉ protostar with a luminosity of 432 L☉. This model suggests the object is a young protostar (approximately 8000 yr) with a temperature of around 4000 K. The object is embedded in a large 35.1 M☉ envelope with a disk of mass 1.48 × 10−2 M☉, indicating that this object is likely experiencing ongoing disk formation. The relatively high envelope accretion rate displayed by this model (1.39 × 10−3 M☉ yr−1) supports this conclusion. The models predict substantial far-IR emission arising from the envelope, meaning follow-up observations in the far-IR range could confirm our assessment.
4.3.4. 2MASS J02270887+6152344
For J02270887+6152344, the data again result in two possible families of models, representing either an intermediate- to high-mass protostellar object with a large envelope or an older intermediate-mass young star with a clearly defined disk and minimal envelope. The top three fits all corresponded to the former of these models, indicating its greater likelihood for accuracy.
The best-fitting (total
The second group of well-fitting models is represented by the fourth-best-fitting (total
These families of models diverge at far-infrared wavelengths. With only present data, we therefore cannot rule out either the higher-mass, younger protostellar object or the intermediate-mass, more evolved young star. Further observations of this object at far-IR and/or submillimeter wavelengths are required to resolve the degeneracy in the model parameters.
5. CONCLUSIONS
We present SOFIA/FORCAST and Spitzer/IRAC observations of the UCH ii region W3(OH) in the wavelength range 3.6–37.1
We detect the hot core W3(H2O) at 31.4 and 37.1
In addition, SEDs have been constructed for four young stellar or protostellar objects which lie in the SOFIA/FORCAST field. The model SED fitting tool of Robitaille et al. (2006) was used to determine the nature of these objects. 2MASS J02270352+6152357 is an intermediate-luminosity protostar undergoing envelope accretion; 2MASS J02270824+6152281 is most likely a very young intermediate-mass protostar with a large natal envelope; 2MASS J02270887+6152344 is a high-luminosity object which is either a protostar with ongoing envelope accretion onto a young disk or a young star with a circumstellar disk and a depleted envelope; and 2MASS J02270743+6152281 could be an intermediate-luminosity protostar or potentially a young star with a developed disk and an almost entirely depleted envelope. Further observations in the mid-IR, far-IR, and/or submillimeter range(s) are required to definitively characterize 2MASS J02270887+6152344 and 2MASS J02270743+6152281.
We thank R. Grashius, S. Adams, H. Jakob, A. Reinacher, and U. Lampeter for their SOFIA telescope engineering and operations support. We also thank the SOFIA flight crews and mission operations team (A. Meyer, N. McKown, C. Kaminski) for their SOFIA flight planning and flight support. We are grateful to an anonymous referee for his or her comments which have improved this manuscript. This work is based on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA science mission operations are conducted jointly by the Universities Space Research Association, Inc. (USRA), under NASA contract NAS2-97001, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901. Financial support for FORCAST was provided to Cornell by NASA through award 8500-98-014 issued by USRA. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has made use of the NASA/IPAC Infrared Science ArChive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made use of NASA's Astrophysics Data System Abstract Service.
Facilities: Spitzer - Spitzer Space Telescope satellite, SOFIA - Stratospheric Observatory For Infrared Astronomy, IRAS - InfraRed Astronomical Satellite
Footnotes
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Ivezic, Z., Nenkova, M., and Elitzur, M. 1996, University of Kentucky, http://www.pa.uky.edu/m~oshe/dusty/