5.1 Introduction
Massive stars play an important role in the evolution of galaxies because they are the principle source of heavy elemental in the our Universe. Understanding the formation of massive stars is an important topic in modern astrophysics. Compar-ing with low-mass star formation, our understandCompar-ing toward the birth of massive stars is fairly unclear (Zinnecker & Yorke, 2007). Within 2 kpc, 70
− 90% of stars
have been found to form in the clusters (Lada & Lada, 2003) and are embedded deeply in dense molecular cloud. There exists various difficulties in identifying sources and deriving physical parameters, such as stellar mass, age, slope of initial mass function (IMF) et al. The development of new sensitive equipment, such as the large mosaic IR array, together multi-wavelengths data enables us to observe embedded stellar clusters in greater detail than in previous efforts.G173.58+2.45 (also known as IRAS 05361+3539) is an ultracompact HII region located at α
2000
= 5h
39m
27.7s
, δ2000
= +35◦
40′
43′′
, about 1.5′
SSE of Sh2-233IR (Yan et al., 2010, see Figure 1). The distance to G173.58+2.45 is about 1.8 kpc from the Earth (Shepherd & Watson, 2002). H
2
O maser emission, as a tracer of the star-formation activity, was detected towards the IRAS source by Wouterloot et al. (1988) and Palagi et al. (1993). G173.58+2.45 is associated with a large molecular cloud and was first identified as an outflow source by Shepherd& Churchwell (1996), who found the IRAS source to be precisely located between a blue and a red outflow lobe in
12
CO maps, and centered in a cloud core. The core is also mapped in CS (J = 2-1) by Zinchenko et al. (1998). A more detailed study of the molecular outflow was presented by Shepherd & Watson (2002, SW02 hereafter), who used continuum emission maps at 2.7 mm to identify two sources around the center of the outflow at the location of the IRAS source; their spectral types are likely to be between late B and mid A. Near-infrared images have revealed the presence of a small cluster of embedded Class I and Class II sources around the IRAS source (Chakraborty et al., 2000; Shepherd & Watson, 2002; Varricatt et al., 2005). Molecular hydrogen images have revealed a rich group of infrared shocks in the region of the cluster (Chakraborty et al., 2000; Varricatt et al., 2005).In this paper, we revisit this G173.58+2.45 region with a wider field of view, higher resolution, and better sensitivity data in near-infrared and radio wave-lengths, including large scale mapping and archived interferometry data. Our goal is to understand the ambient environment of this GMC complex in a whole picture. Observations both in radio and NIR will provide important constraints to understand the possible driving force of massive star formation. We summarize our observations in the radio and near-infrared in § 5.2, and the observed results in § 5.3. A discussion is given in § 5.4, and the summary is given in Section 5.5.
5.2 Observations and Data Analysis
5.2.1 CFHT observations
WIRCam data were obtained on November 18, 19 and December 20, 2005 for the H
2
and KS
bands, and February 4, 2007 for the J, H, and K continuum bands.The seeing was 0.5
′′ − 0.7 ′′
during the observations. The total integration times for each filter were 420 seconds for J and H, 235 seconds for KS
, 1755 seconds for H2
, and 2400 seconds for K-continuum. Fig. 5.1 is a color image, composed by J(blue), H(green) and KS
(red) filters, of star forming regions G173.58+2.45 and Sh 2-233IR. In Fig. 5.1(a), the separation of two star forming regions is about 6.5′
away from each other. The G173.58+2.45 region is shown in Fig. 5.1(b) and marked as white box in Fig. 5.1(a).The CFHT WIRCam standard pipeline (the IDL Interpretor of the WIRCam Images, aka i’iwi) was used for basic image processing, such as, the standard bias subtraction, flat-fielding and bad pixel masking. Sky background of each image was subtracted using the comoving-averaged frame taken during the observations.
The standard stars were not observed during observation runs. Therefore, as-trometry and photometry corrections were performed against the 2MASS catalog in order to correct the image distortion and flux level. Fig. 5.2 shows the result of photometry calibration of three bands. The scattering of the data points in bright ends were caused by saturation. Saturation levels of WIRCam detector is 44000 ADUs, corresponding to 13.4 (J), 13.5 (H) and 12.8 (K
S
) magnitude. The pho-tometry for saturated stars were adopted from the 2MASS catalog directly. The observed magnitude (mobs
) is corrected against 2MASS magnitude (m2MASS
), so that the magnitude offsets (mobs
-m2MASS
) are zero as indicated as red solid lines.The errors of photometry offsets against 2MASS catalog in J, H and K
S
are 0.07, 0.10 and 0.10 magnitude, respectively. The 3σ errors are plotted as black-dashed lines in Fig. 5.2.Images were analyzed using IDL, IDL Astronomy Library and SExtractor, a aperture photometry software (Bertin & Arnouts, 1996). The completeness
40.8 31.2 21.6 5:39:12.0 02.4
Figure 5.1 The Near-infrared color-composite image of the star forming regions G173.58+2.45 and Sh 2-233IR of J(blue), H(green) and Ks(red) bands. The white box indicates the G173.58+2.45 region, and the Sh 2-233IR region is located at the north-west direction from the G173.58+2.45.
Figure 5.2 The photometry calibration of G173.58+2.45 region in J (left), H (mid-dle) and K
S
(right) bands. The observed magnitude (mobs
) were calibrated against 2MASS magnitude (m2MASS
). the error bars indicate the photometry error of in-dividual data point. The errors of magnitude offsets are 0.07 (J), 0.10 (H) and 0.10 (KS
). The black-dashed lines represented the 3σ errors in three bands.and limiting magnitude were estimated by adding artificial stars in the observed images. The limiting magnitudes were estimated to be 19.2, 18.7 and 18.2 for the J, H, and K
S
bands, respectively, at a 90% completeness level. We identified 296, 377 and 516 stars, in a 4′ ××4 ′
marked as white box in Fig.5.1, on the J, H and Ks band images, respectively. The H2
(2.12µm) and K-continuum image were both calibrated against 2MASS point source catalog. Then, the continuum emission was subtracted from H2
image.5.2.2 Radio observations
The CO line (J = 3