The Molecular Cloud S242: Physical Environment and Star-formation Activities

We retrieved a narrow-band Hα image at 0.6563 μm from the Isaac Newton Telescope (INT) Photometric Hα Survey of the Northern Galactic Plane (IPHAS; Drew et al. 2005) survey database. The IPHAS imaging survey was carried out using the Wide-Field Camera (WFC) at the 2.5 m INT, located at La Palma. The WFC contains four 4k × 2k CCDs, in an L-shape configuration. The pixel scale is $0\buildrel{\prime\prime}\over{.} 33$ (see Drew et al. 2005 for more details).

We utilized the NIR photometric magnitudes of point-like sources extracted from the UKIDSS DR10PLUS Galactic Plane Survey (GPS; Lawrence et al. 2007) and the Two Micron All Sky Survey (2MASS; Skrutskie et al. 2006; hereafter GPS-2MASS). The UKIDSS observations (resolution ∼$0\buildrel{\prime\prime}\over{.} 8$) were made with the WFCAM mounted on UKIRT. The 2MASS photometric data were utilized to calibrate the UKIDSS fluxes. In this work, we extracted only a reliable NIR photometric catalog. More information about the selection procedures of the GPS photometry can be obtained in Dewangan et al. (2015). To obtain reliable 2MASS photometric data, we selected only those sources with photometric magnitude errors of 0.1 or less in each band.

Warm-Spitzer-IRAC 3.6 and 4.5 μm photometric images (resolution ∼2'') and magnitudes of point sources are downloaded from the Glimpse360⁴ (Whitney et al. 2011) survey. The photometric magnitudes were extracted from the Glimpse360 highly reliable catalog. To obtain further reliable Glimpse360 photometric data, we retrieved only those sources having photometric magnitude errors of 0.2 or less in each band.

MIR images at 12 μm (spatial resolution ∼6'') and 22 μm (spatial resolution $\sim 12^{\prime\prime}$) were obtained from the publicly available archival WISE⁵ (Wright et al. 2010) database.

We utilized the Herschel Space Observatory data archives to obtain FIR and sub-mm continuum images. The processed level2${}_{-}5$ images at 70, 160, 250, 350, and 500 μm were downloaded using the Herschel Interactive Processing Environment (HIPE; Ott 2010). The beam sizes of the Herschel images at 70, 160, 250, 350, and 500 μm are $5\buildrel{\prime\prime}\over{.} 8$, 12'', 18'', 25'', and 37'', respectively (Griffin et al. 2010; Poglitsch et al. 2010). In this work, Herschel temperature and column density maps are produced using the Herschel continuum images, following the methods described in Mallick et al. (2015). The Herschel temperature and column density maps are obtained from a pixel-by-pixel spectral energy distribution (SED) fit with a modified blackbody to the cold dust emission at Herschel 160–500 μm (also see Dewangan et al. 2015). The Herschel 70 μm data are not included in the analysis, because the 70 μm emission is dominated by the ultraviolet (UV) heated warm dust. Here we provide a brief step-by-step explanation of the procedures.

The Herschel 160 μm image is in units of Jy pixel⁻¹, while the images at 250–500 μm are in units of surface brightness, MJy sr⁻¹. The plate scales of 160, 250, 350, and 500 μm images are 32, $6^{\prime\prime}$, $10^{\prime\prime}$, and $14^{\prime\prime}$ pixel⁻¹, respectively. Prior to the SED fit, the 160–350 μm images were convolved to the lowest angular resolution of the 500 μm image (∼37'') and were converted into the same flux unit (i.e., Jy pixel⁻¹). Furthermore, we regridded these images to the pixel size of the 500 μm image (∼14''). These steps were performed using the convolution kernels available in the HIPE software. Next, the sky background flux level was estimated to be 0.060, 0.133, 0.198, and −0.095 Jy pixel⁻¹ for the 500, 350, 250, and 160 μm images (size of the selected region ∼134 × 148; centered at $l=183\buildrel{\circ}\over{.} 181;$ b = −0354), respectively. The negative flux value at 160 μm is obtained due to the arbitrary scaling of the Herschel 160 μm image. To avoid diffuse emission linked with the S242 site, the featureless dark field away from the selected target was carefully chosen for the background estimation.

Finally, to generate the temperature and column density maps, a modified blackbody was fitted to the observed fluxes on a pixel-by-pixel basis (see Equations (8) and (9) in Mallick et al. 2015). The fitting was performed using the four data points for each pixel, maintaining the dust temperature (T_d) and the column density ($N({{\rm{H}}}_{2})$) as free parameters. In the analysis, we used a mean molecular weight per hydrogen molecule (${\mu }_{{\rm{H}}2}$) of 2.8 (Kauffmann et al. 2008) and an absorption coefficient (${\kappa }_{\nu }$) of 0.1 ${(\nu /1000\mathrm{GHz})}^{\beta }$ cm² g⁻¹, including a gas-to-dust ratio (R_t) of 100, with a dust spectral index (β) of 2 (see Hildebrand 1983). We considered flux uncertainties of the order of ∼15% in all Herschel images, based on the previously reported work by Launhardt et al. (2013). We describe the Herschel temperature and column density maps in Section 3.1.1.

We also obtained Bolocam dust continuum sources at 1.1 mm (v2.1; Ginsburg et al. 2013) from Bolocam Galactic Plane Survey (BGPS). The effective full width at half maximum (FWHM) of the 1.1 mm map (Aguirre et al. 2011) is ∼33''.

To trace the molecular cloud associated with the S242 site, the 2.6 mm ¹²CO data (beam size ∼8') were obtained from the 1.2 m CfA telescope (Dame et al. 2001). The line data have a velocity resolution of 0.65 km s⁻¹ and a typical rms value of 0.22 K km s⁻¹. One can find more details about the ¹²CO data in Dame et al. (2001).

We utilized the archival radio continuum data at 1.28 GHz and 1.4 GHz. The radio continuum map at 1.4 GHz (21 cm; beam size ∼45'') was extracted from the NRAO VLA Sky Survey (NVSS; Condon et al. 1998) archive, while the radio continuum data of S242 at 1.28 GHz were retrieved from the GMRT archive.⁶ The GMRT observations were obtained on 2005 November 30 (Project Code: 09SKG01). The GMRT radio data reduction was carried out using the AIPS software, following similar procedures as those described in Mallick et al. (2013). The 1.28 GHz map has an rms noise of 0.15 mJy/beam and a beam size of ∼204 × 204.

We retrieved 21 cm H i line data from the Canadian Galactic Plane Survey (CGPS; Taylor et al 2003). The velocity resolution of H i line data is 1.32 km s⁻¹, sampled every 0.82 km s⁻¹. The data have a spatial resolution of 1' × 1' cscδ. The line data have a brightness–temperature sensitivity of ΔT_B = 3.5 sinδ K. One can find more details about the CGPS data in Taylor et al (2003).

In this section, we study the distribution of dense materials, molecular gas, and ionized emission toward S242, enabling us to probe the physical environment around the target region.

The study of molecular line data is very essential to trace the physical association of different subregions seen in the large-scale map of a given star-forming region. Based on the CfA ¹²CO line profile, the molecular cloud linked with the S242 site (hereafter S242 molecular cloud) is traced in a velocity range from −3.25 to 4.55 km s⁻¹ (see Figure 1). Figure 1 shows the spatial distribution of the molecular gas in our selected region around S242, revealing an elongated molecular cloud. We have also shown the positions of two IRAS sources (i.e., IRAS 05488+2657 and IRAS 05483+2728) in our selected region. Based on the distribution of molecular emission, we selected a region of ∼105 × $0\buildrel{\circ}\over{.} 56$ containing the S242 site for our present study, where the majority of molecular gas has been found (see the dotted–dashed box in Figure 1). In Figure 2, the integrated ¹²CO intensity map (see Figure 2(a)) and the position–velocity maps (see Figures 2(b) and (d)) are presented. In Figure 2(a), we show the CfA ¹²CO intensity map integrated over −3.25 to 4.55 km s⁻¹. The peak positions of 10 dust continuum sources at Bolocam 1.1 mm are also marked in Figure 2(a). Bolocam 1.1 mm dust emission contours are presented in Figure 2(c) and trace the cold dust. In Figures 2(b) and (d), we show galactic position–velocity diagrams of the ¹²CO emission, indicating the presence of a single velocity component (at peak velocity ∼1.5 km s⁻¹) in the direction of our selected target region.

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Figure 3(a) shows a three-color composite map made using the Herschel 250 μm in red, WISE 22 μm in green, and WISE 12 μm in blue. The images at wavelengths longer than 150 μm trace the cold dust emission, while the 12–70 μm emission is sensitive for the warm dust components. Figure 3(b) shows the Herschel sub-mm images overlaid with the NVSS 1.4 GHz continuum emission. The NVSS map allows us to infer the spatial distribution of ionized emission, which is only found toward the S242 site. In Figures 3(a) and (b), an extended filamentary structure (EFS; extension ∼25 pc; average width ∼1.3 pc), containing the S242 site, is revealed and is prominently observed in the Herschel sub-mm images at 250–500 μm. Note that the Bolocam 1.1 mm dust emission map does not trace the entire EFS as seen in the Herschel sub-mm images; however, the Bolocam clumps are detected toward the EFS (see Figure 2(c)). In Figure 1, with the help of the CfA ¹²CO gas distribution, we find a continuous velocity structure in the direction of S242 and the EFS is embedded within the S242 molecular cloud (see Figures 3(a) and (b)). In the velocity space, there is only one velocity component observed in the direction of the EFS (see Figures 2(b) and (d)). This implies the existence of a single EFS. The peak positions of 10 dust continuum sources at 1.1 mm are also shown in Figures 3(a) and (b) and are mainly distributed toward the EFS (also see Section 3.1.1 for a quantitative estimate). This particular result gives a hint about the fragmentation of the filamentary cloud. To further infer this signature, we estimated the virial mass (M_vir) and the virial parameter (M_vir/M_cloud) of the S242 molecular cloud using the observed physical parameters. Using the NANTEN ¹³CO (1-0) line data (beam size ∼27), Kawamura et al. (1998) reported M_cloud (∼7000 ${M}_{\odot }$), radius (R_c ∼ 7 pc), and line width (${\rm{\Delta }}V$ = 2.1 km s⁻¹) for the S242 molecular cloud. The virial mass of a cloud of radius R_c (in pc) and line-width ${\rm{\Delta }}V$ (in km s⁻¹) is defined as M_vir (${M}_{\odot }$) = k R_c ${\rm{\Delta }}{V}^{2}$ (MacLaren et al. 1988), where the geometrical parameter, k = 126, for a density profile ρ ∝ 1/r². A virial parameter less than 1 indicates that the cloud is prone to collapse, and a parameter greater than 1 indicates that it is resistant to collapse. In the present case, we obtain M_vir ∼ 3890 ${M}_{\odot }$, which is less than M_cloud. This implies that the virial parameter is less than 1, suggesting that the cloud is unstable against gravitational collapse.

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In Figures 3(a) and (b), we find that the S242 site has a shell-like appearance, where noticeable warm dust emission at MIR is seen. In general, the ionized gas and the warm dust emissions are seen systematically correlated within H ii regions (e.g., Deharveng et al. 2010). With the knowledge of the presence and absence of the radio continuum emission, we find two distinct ends of the EFS. One EFS end contains the S242 H ii region, while the other EFS end is seen without noticeable ionized emission (see Figure 3(b)). Note that the majority of the dust continuum sources at 1.1 mm are seen at both ends of the EFS.

Together, Figures 1 and 3 allow us to probe the S242 molecular cloud, EFS, dust continuum sources at 1.1 mm, and H ii region in our selected site probed in this paper.

3.1.1. Herschel Temperature and Column Density Maps

In this section, the temperature and column density maps derived using the Herschel continuum images are discussed. The final temperature and column density maps (resolution ∼37'') are shown in Figures 4(a) and (b), respectively.

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In the Herschel temperature map, the S242 H ii region is associated with the considerably warmer gas (T_d ∼ 22–32 K; see Figure 4(a)). The map clearly traces the spatial extent of warm dust emission linked with the S242 site, where the ionized emission is observed. The temperature map reveals temperature variations toward the EFS (see areas near both the EFS ends). The EFS is traced in a temperature range of about 10–12 K away from the S242 H ii region. In Figures 4(b), 5(a), and 5(b), the EFS (length ∼25 pc) is evident in the Herschel column density map at a contour level of 1.5 × 10²¹ cm⁻² and several condensations are also seen toward this feature (also see Figure 5(c)). The S242 site is located in the highest column density region (peak $N({{\rm{H}}}_{2})$ ∼ 2.7 × 10²² cm⁻²; A_V ∼ 29 mag). Here, we used a relation (${A}_{V}=1.07\times {10}^{-21}\,N({{\rm{H}}}_{2});$ Bohlin et al. 1978) between optical extinction and hydrogen column density. In the column density map (see Figure 5(a)), the "clumpfind" IDL program (Williams et al. 1994) is employed to trace the clumps and to find their total column densities. We used several column density contour levels as an input parameter for the "clumpfind" and the lowest contour level was assigned at 3σ. Eighteen clumps are identified in the map and are labeled in Figure 5(c). Furthermore, the boundary of each clump is also shown in Figure 5(c). Eleven out of eighteen clumps (e.g., 1–11) are found toward the EFS. We have also determined the mass of each clump using its total column density. The mass of a single Herschel clump is estimated using the following formula:

$$\begin{eqnarray}&&{M}_{\mathrm{clump}}={\mu }_{{{\rm{H}}}_{2}}{m}_{{\rm{H}}}{\mathrm{Area}}_{\mathrm{pix}}{\rm{\Sigma }}N({{\rm{H}}}_{2}),\end{eqnarray} \tag{ 1 }$$

where ${\mu }_{{{\rm{H}}}_{2}}$ is assumed to be 2.8, Area_pix is the area subtended by one pixel, and ${\rm{\Sigma }}N({{\rm{H}}}_{2})$ is the total column density. The mass of each Herschel clump is tabulated in Table 1. The table also lists the effective radius, the peak column density, the peak temperature, and the mean central number density (n_c) of each clump. The clump masses vary between 25 ${M}_{\odot }$ and 1020 ${M}_{\odot }$. We also find peak temperatures, mean central number densities, and peak column densities (corresponding extinction) of the clumps ranging from 10–26 K, 505–2575 cm⁻³, and (1.9–27) × 10²¹ cm⁻² (A_V = 2–29 mag), respectively. The mean central number density of each clump refers to the average number density along the line of sight and is obtained from the peak column density divided by the size of each clump. Eleven clumps, which are distributed toward the EFS, have masses varying between 150 and 1020 ${M}_{\odot }$. Interestingly, massive clumps (M_clump ∼ 260, 700, 700, and 1020 ${M}_{\odot }$) are spatially seen at both the EFS ends (also see Table 1). The virial mass analysis of these clumps is not performed in this paper, due to the non-availability of optically thin line data (such as NH₃ and CS).

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Table 1.  Physical Parameters of the Herschel Clumps Identified in the Region around S242 (see Figures 5(a) and (c))

ID	l	b	Rc	Mclump	peak $N({{\rm{H}}}_{2})$	peak Td	nc
	(degree)	(degree)	(pc)	(${M}_{\odot }$)	×1021 (cm−2)	(K)	(cm−3)
1a	182.453	0.197	1.2	260	6.9	16	930
2a	182.403	0.247	1.7	1020	27	26	2575
3	182.341	0.247	1.4	420	13	18	1505
4	182.177	0.239	1.6	480	8.3	14	840
5	182.068	0.263	1.5	380	4.7	14	505
6	181.987	0.290	1.6	405	5.6	13	565
7	181.971	0.302	0.8	150	9	13	1820
8	181.928	0.325	1.2	350	7.2	13	970
9a	181.917	0.364	1.5	700	15.8	16	1705
10a	181.851	0.309	1.5	700	12.7	16	1370
11	181.777	0.329	1.0	150	3.6	13	580
12	182.030	0.364	0.9	100	2.8	12	505
13	182.006	0.368	0.8	125	5.1	16	1030
14	182.057	0.399	1.1	250	6.8	10	1000
15	182.037	0.414	1.1	265	6.7	10	985
16	181.870	0.453	0.5	25	1.9	18	615
17	182.127	0.119	0.5	35	2.1	17	680
18	182.193	−0.021	0.5	45	3.3	15	1070

Note.  Column 1 gives the IDs assigned to the clump. The table also lists positions, deconvolved effective radius (R_c), clump mass (M_clump), peak column density ($N({{\rm{H}}}_{2})$), peak temperature (T_d), and mean central number density (n_c = peak $N({{\rm{H}}}_{2})$/(2 R_c)). The column density value can also be used to obtain the extinction using the relation ${A}_{V}=1.07\times {10}^{-21}\,N({{\rm{H}}}_{2})$. The clump IDs 1–11 are distributed toward the EFS containing the S242 site (see Figure 5(c)).

^aIt is found at the EFS end.

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We have also obtained a total column density inside the contour of $N({{\rm{H}}}_{2})$ = 1.5 × 10²¹ cm⁻² and have computed a total mass of the EFS (M_EFS) to be ∼5000 ${M}_{\odot }$. Adopting a value of M_EFS and length of EFS (∼25 pc), the mass per unit length is calculated to be ∼200 ${M}_{\odot }$ pc⁻¹. Note that there is no knowledge of the inclination angle, i, of the EFS, and we have adopted i = 0 here for reference. Due to the inclination, the line mass can be affected by a factor of cos i (e.g., Kainulainen et al. 2016). Hence, the observed mass per unit length value can be considered as an upper limit.

The infrared excess emission displayed by sources is an extremely powerful utility to probe the embedded young stellar populations. In a given star-forming region, the knowledge of spatial distribution of these young stellar populations helps us to infer the SF activities. In this section, we describe the identification and classification schemes of young stellar objects (YSOs) using the GPS-2MASS and GLIMPSE360 photometric data from 1 to 5 μm. Furthermore, to investigate the young stellar clusters, the distribution of YSOs is also presented in and around S242.

The dereddened color–color space ([K−[3.6]]₀ and [[3.6]−[4.5]]₀) is a very promising tool to identify infrared-excess sources (e.g., Gutermuth et al. 2009). We computed the dereddened color–color plot ([K−[3.6]]₀ and [[3.6]−[4.5]]₀) using the GLIMPSE360 and 2MASS photometric data at 1–5 μm. The dereddened colors were obtained using the color excess ratios listed in Flaherty et al. (2007). Using the dereddened color conditions presented in Gutermuth et al. (2009), we obtain 192 (39 Class I and 153 Class II) YSOs in our selected region probed in this paper. One can also infer possible dim extragalactic contaminants from the selected YSOs with additional conditions (i.e., [3.6]₀ < 15 mag for Class I and [3.6]₀ < 14.5 mag for Class II) (e.g., Gutermuth et al. 2009). The dereddened 3.6 μm magnitudes were obtained using the observed color and the reddening laws (from Flaherty et al. 2007). In Figure 6(a), we show the [K−[3.6]]₀ versus [[3.6]−[4.5]]₀ color–color plot for the sources. The selected Class I and Class II YSOs are highlighted by red circles and blue triangles, respectively.

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We also find some additional YSOs having detections only in the H and K bands using the GPS-2MASS data. Note that the UKIDSS-GPS NIR data have better spatial resolution and are three magnitudes deeper than those of 2MASS data. We employed a color–magnitude (H−K/K) diagram to obtain additional young stellar populations, which are shown in Figure 6(b). The diagram allows us to depict the red sources with H−K > 1.1 mag. We obtained this color criterion based on the color–magnitude analysis of a nearby control field. In Figure 6(b), the selected young stellar populations are highlighted by blue triangles. We identify 101 additional YSOs using the color–magnitude space in our selected region.

Using the GPS-2MASS and GLIMPSE360 data at 1–5 μm, our analysis yields a total of 293 YSOs in our selected region around S242. In Figure 6(c), we have overlaid the positions of all the selected YSOs on the Herschel column density map. A majority of these YSOs are distributed toward the filamentary structure. The Class I YSOs are located only in the areas of high column density.

In this section, we study the individual groups or clusters of YSOs based on their spatial distribution and the statistical surface density utility. In Figures 7(a) and (b), to trace the groups/clusters of YSOs, we have superimposed the surface density contours of YSOs on the Herschel column density and temperature maps. The surface density map of YSOs is produced using the nearest-neighbor (NN) technique (e.g., Gutermuth et al. 2009; Bressert et al. 2010; Dewangan et al. 2015). Using a 5'' grid and 6 NN at a distance of 2.1 kpc, the surface density map of 293 YSOs is computed in a manner similar to that described in Dewangan et al. (2015). The clusters of YSOs are mainly seen at both the EFS ends (see Figures 7(a) and 7(b)); however, there is a noticeable presence of young stellar populations toward the EFS without any clustering away from both of its ends (i.e., its center; see Figure 6(c)). Furthermore, a cluster of YSOs is also evident toward the Herschel clumps, nos. 14 and 15, which are spatially distributed away from the EFS (see Figure 7(a)).

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Together, the SF activities have been found toward the clumps linked with the EFS and other Herschel clumps.

3.3.1. S242 H ii Region

In Figure 8, we present a zoomed-in view of the S242 site using the Spitzer-IRAC ratio map and the radio continuum maps. In Figures 8(a) and (b), the area around the S242 site is chosen based on the spatial extent of the warm dust emission (see Figure 7(b)). In combination with the radio continuum map, the Spitzer-IRAC ratio map of 4.5 μm/3.6 μm emission is used here to infer the signatures of molecular outflows and the impact of a massive star on its surroundings (e.g., Dewangan et al. 2016, 2017). The IRAC 3.6 μm band harbors polycyclic aromatic hydrocarbon (PAH) emission at 3.3 μm as well as a prominent molecular hydrogen feature at 3.234 μm (ν = 1–0 O(5)). The IRAC 4.5 μm band contains a hydrogen recombination line Brα (4.05 μm) and a prominent molecular hydrogen line emission (ν = 0–0 S(9); 4.693 μm), which is produced by outflow shocks. It is known that IRAC 3.6 μm and 4.5 μm images have almost identical point response functions; therefore, the ratio of 4.5 to 3.6 μm images can be used to directly produce a ratio map of 4.5 μm/3.6 μm emission (see Figure 8; e.g., Dewangan et al. 2016). In Figure 8(a), we infer the bright and dark/black regions in the ratio map of 4.5 μm/3.6 μm emission. In the ratio 4.5 μm/3.6 μm map, the bright emission regions trace the excess of 4.5 μm emission, while the black or dark gray regions depict the excess of 3.6 μm emission. In Figures 8(a) and (b), the ionized emissions at 1.28 and 1.4 GHz are distributed within a shell-like morphology. Due to the presence of the 3.3 μm PAH feature in the 3.6 μm band, the dark/black regions surrounding the ionized emission appear to trace photodissociation regions (or photon-dominated regions, or PDRs). Furthermore, the bright emission regions at one end of the EFS containing the S242 site, where the radio continuum emission is absent and three clusters of YSOs are found, appear to trace the outflow activities (see Figure 8(b)). This interpretation can be supported by the presence of the CO and H₂ outflow signatures in the embedded clusters near IRAS 05490+268 (e.g., Snell et al. 1990; Varricatt et al. 2010).

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In Figure 9(a), we present an Hα image overlaid with the NVSS 1.4 GHz emission, showing the spatial match between the radio continuum emission and the Hα emission. The position of a previously characterized B0V star (BD+26 980) appears near the peak of radio continuum emission. The surface density contours of the identified YSOs are also highlighted in Figure 9(a). In Figure 9(b), we show a three-color composite map made using the Herschel 70 μm in red, Spitzer 4.5 μm in green, and IPHAS Hα in blue. The color composite map is superimposed with the GMRT 1.28 GHz emission and positions of two 1.2 mm dust continuum peaks (from Beuther et al. 2002). In order to compare the spatial distribution of dust temperature and column density with the ionized emission, the Herschel temperature and column density maps of the S242 site are shown in Figures 9(c) and (d), respectively. We find the YSO clusters toward the high column density materials. Additionally, there is a lack of column density material within the S242 H ii region. Earlier in Section 3.1.1, we mentioned the spatial correlation between the warm dust emission and the ionized emission (also see Figures 9(b) and (c)). In Figure 9, one can infer the zoomed-in view of the S242 site, tracing the spatial location of the ionized emission, warm dust emission, high column density material, and the embedded stellar populations.

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Using the radio continuum maps, we also compute the Lyman continuum photons and the dynamical age (t_dyn) of the S242 H ii region. Using the NVSS 1.4 GHz map and the clumpfind algorithm, the integrated flux density (${S}_{\nu }$) and the radius (R_{H ii}) of the H ii region are determined to be 437 mJy and 1.84 pc, respectively. Following the equation given in Matsakis et al. (1976), the number of Lyman continuum photons (N_uv) is computed to be ∼1.5 × 10⁴⁷ s⁻¹ (log N_uv ∼47.18) for the S242 H ii region (see Dewangan et al. 2016, for more details). In this analysis, we used the integrated flux density value, a distance of 2.1 kpc, and the electron temperature of 10,000 K. The value of N_uv is found to be consistent with a single ionizing star of spectral type B0.5–B0V (see Table 2 in Panagia 1973 and also Smith et al. 2002). The estimate of N_uv at GMRT 1.28 GHz frequency also corresponds to a single ionizing star of B0.5–B0V spectral type. These calculations are also in agreement with the previously reported spectral type of the ionizing source of the S242 site (Hunter & Massey 1990).

The equation of the dynamical age of the H ii region is given below at a radius R_{H ii} (e.g., Dyson & Williams 1980):

$$\begin{eqnarray}&&{t}_{\mathrm{dyn}}=\left(\displaystyle \frac{4\,{R}_{s}}{7\,{c}_{s}}\right)\left[{\left(\displaystyle \frac{{R}_{{\rm{H}}{\rm{II}}}}{{R}_{s}}\right)}^{7/4}-1\right],\end{eqnarray} \tag{ 2 }$$

where c_s is the isothermal sound velocity in the ionized gas (c_s = 11 km s⁻¹; Bisbas et al. 2009), R_{H ii} is mentioned above, and R_s is the radius of the Strömgren sphere (=(3 N_uv/4$\pi {n}_{0}^{2}{\alpha }_{B}$)${}^{1/3}$, where the radiative recombination coefficient ${\alpha }_{B}$ = 2.6 × 10⁻¹³ (10⁴ K/T)${}^{0.7}$ cm³ s⁻¹ (Kwan 1997), N_uv is mentioned above, and "n₀" is the initial particle number density of the ambient neutral gas). Considering a typical value of n₀ (=10³ cm⁻³), we determined the dynamical age of the S242 H ii region to be ∼0.5 Myr.

We have also utilized 21 cm H i line data toward the S242 site. Figure 10 shows 21 cm H i velocity channel maps of the S242 site. We find the black or dark gray regions in the H i channel maps, which trace the H i self-absorption (HISA) features (e.g., Kerton 2005). In a velocity range of 3.48–5.95 km s⁻¹, the shell-like HISA feature is evident in the channel maps. The existence of the HISA features can be explained by the presence of the residual amounts of very cold H i gas in molecular clouds (Burton et al. 1978; Baker & Burton 1979; Burton & Liszt 1981; Liszt et al 1981; Dewangan et al. 2017).

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