CO OBSERVATIONS AND INVESTIGATION OF TRIGGERED STAR FORMATION TOWARD THE N10 INFRARED BUBBLE AND SURROUNDINGS

The observations were carried out with the PMO (Purple Mountain Observatory) 13.7 m radio telescope in 2012 June. We observed the $J=1-0$ transition of ¹²CO (115.27 GHz), ¹³CO (110.20 GHz), and C¹⁸O (109.78 GHz). The On-The-Fly (OTF) observing mode was applied to map a $21^{\prime} \times 25^{\prime}$ region centered at ${\alpha }_{2000}={18}^{{\rm{h}}}{14}^{{\rm{m}}}01\buildrel{\rm{s}}\over{.} 361$ and ${\delta }_{2000}=-17\,\deg \,28^{\prime} 23\buildrel{\prime\prime}\over{.} 14$. For the 13.7 m PMO antenna, we have considered a half-power beam width (HPBW) around $52^{\prime\prime}$.

We used a nine-beam array of SIS receivers at the front end (Shan et al. 2012). The main beam efficiency at the center of the 3 × 3 array is about 0.44 at 115 GHz and 0.48 at 110 GHz. Our spectral resolution was about 61 kHz, corresponding to velocity resolutions of 0.16 km s⁻¹ (at 115 GHz) and 0.17 km s⁻¹ (at 110 and 109 GHz).

The cloudy weather condition during our observations led to system temperatures reaching 550 K and 350 K at 115 GHz and 110 GHz, respectively. This resulted in rms noises of 1.7 and 1.2 K in the brightness temperature for ¹²CO $J=1-0$ and ¹³CO $J=1-0$, respectively. Such large noise would make relatively weak signals undetectable. However, regions with strong line emission can be validly probed. The velocity information provided by these data convincingly reveal the kinematics of the bubble and molecular conditions in some subregions.

Public data from infrared to centimeter surveys was used to analyze the bubble N10 at other wavelengths. The GLIMPSE survey (Benjamin et al. 2003) mapped parts of the inner Galactic plane, with the Infrared Array Camera (IRAC; Fazio et al. 2004) on the Spitzer Space Telescope. We obtained images of 4.5, 5.8, and 8.0 μm IRAC bands (see Figure 3). The 24 μm image of N10 was obtained from another survey of the inner Galactic plane, MIPSGAL, using the MIPS instrument (Multiband Imaging Photometer for the Spitzer; Rieke et al. 2004). In the panel at 24 μm of Figure 3, we can see that this emission fills the whole area indicated by the red ellipse. This emission, typical of galactic bubbles, is caused by warm dust present in the region of ionized gas.

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N10 was mapped at sub-millimeter wavelengths with the APEX telescope (Miettinen 2012). The images at 870 μm wavelength were obtained with ATLASGAL (APEX Telescope Large Area Survey of the Galaxy), an observing program using the LABOCA (Large Apex BOlometer CAmera instrument Schuller et al. 2009). The 870 μm cold dust emission is useful to reveal the presence of dense dark clouds. Two of these clouds, reported by Wienen et al. (2012), are seen bordering the bubble, along the upper and left borders of the bubble. The H ii region, which is probably expanding, seems to be interacting with these clouds. We used the MAGPIS website⁶ to obtain the images presented Figure 3.

We also used the all-sky Wide-field Infrared Survey Explorer satellite (WISE; Wright et al. 2010) data to analyze the content of YSOs in N10, in order to reveal the regions where star formation took place recently and possible gradients of the evolutionary stage.

The emission of ¹²CO,¹³CO, and C¹⁸O J = 1-0 was observed at the same time. Strong emission of ¹²CO and ¹³CO was observed; the emission of C¹⁸O is weak and we do not analyze in this work. Figure 4 presents the observed spectral lines.

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Detected ¹²CO and ¹³CO emission allows us to identify three peaks of velocity: at 20, 37, and 52 km s⁻¹, approximately. The central velocities and line widths were determined by Gaussian fits using the CLASS package (GILDAS software⁷ ). In this paper, velocities are referred to the local standard of rest (V_LSR). The upper panel in Figure 5 displays a channel map of ¹²CO emission and the bottom panel shows the channel map of ¹³CO emission. The background in both figures shows 8.0 μm emission. We have fitted the channels by increasing the velocity from 45 to 62 km s⁻¹. There is an strong correlation between ¹²CO and ¹³CO emission, especially in the range of 48–53 km s⁻¹.

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The broad CO component centered at 20 km s⁻¹ has the lower intensity of the three peaks. The component centered at 37 km s⁻¹ presents a narrower profile than the former and lower intensity if compared with the component centered at 52 km s⁻¹.

The velocities found in the literature for different emission lines associated with N10 range from 48.5 to 54.1 km s⁻¹, as shown in Table 2. This leads us to adopt the component with peak at 52.6 km s⁻¹ as the one related to the source.

In our observation, velocities along the emission with peaks at 52.6 km s⁻¹ range from 48 to 53 km s⁻¹. In order to verify the correspondence between the physical distribution of molecular gas and the bubble seen in infrared, ¹²CO and ¹³CO contours of narrow-velocity emission were superposed over a Spitzer 8.0 μm image in Figure 6. The spatial distribution of ¹²CO shows two main structures that seem to be related to the 8.0 μm emission, and ¹³CO presents two denser clumps in the border of the ring morphology of N10, at the same position of ¹²CO structures.

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We have studied the bubble N10 through the emission of the CO and the cold dust, which is useful to reveal the densest and coldest regions of N10. However, it is necessary to explore other tracers.

4.2.1. Ionized Gas

Ionized gas associated with N10 can be traced by VLA 20 cm emission. The presence of emission at $\nu =1.5\,\mathrm{GHz}$ implies that the H ii region in the inner part of the bubble is created by UV photons. Using the Greg/GILDAS software, we estimated the 20 cm total flux ${F}_{20\mathrm{cm}}=1.17$ Jy inside the bubble and we calculated the electron density (n_e) according to Panagia & Walmsley (1978):

$$\begin{eqnarray}\displaystyle \frac{{n}_{e}}{{\mathrm{cm}}^{-3}} & = & 3.113\times {10}^{2}{\left(\displaystyle \frac{{T}_{e}}{{10}^{4}{\rm{K}}}\right)}^{0.25}{\left(\displaystyle \frac{{S}_{\nu }}{\mathrm{Jy}}\right)}^{0.5}{\left(\displaystyle \frac{D}{\mathrm{kpc}}\right)}^{-0.5}\\ & & \times b{(\nu ,T)}^{-0.5}\times {\theta }_{R}^{-1.5},\end{eqnarray} \tag{ 1 }$$

where ${T}_{e}$ in K is the electron temperature, ${S}_{\nu }$ is the measured total flux density in Jy, $D$ is the distance in kpc, and ${\theta }_{R}$ is the angular radius in arcmin. The function $b(\nu ,T)$ is defined as

$$\begin{eqnarray}b(\nu ,T) & = & 1+0.3195\,\mathrm{log}\left(\displaystyle \frac{{T}_{e}}{{10}^{4}\,{\rm{K}}}\right)\\ & & -0.2130\,\mathrm{log}\left(\displaystyle \frac{\nu }{1\,\mathrm{GHz}}\right).\end{eqnarray} \tag{ 2 }$$

Assuming ${T}_{e}={10}^{4}\,{\rm{K}}$ for the free–free emission region, the electron density is ${n}_{e}=129.71$ cm⁻³.

Figure 7 displays two peaks of radio continuum emission in grayscale and black contours in the left panel. In the same figure, the right panel shows one of the peaks coinciding with an O-type star.

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The number of Lyman continuum photons that are absorbed by the gas in the region H ii was calculated using the radio continuum map, following the relation given by Matsakis et al. (1976):

$$\begin{eqnarray}&&{N}_{{uv}}=7.5\times {10}^{46}{\left(\displaystyle \frac{\nu }{\mathrm{GHz}}\right)}^{0.1}{\left(\displaystyle \frac{{T}_{e}}{{10}^{4}{\rm{K}}}\right)}^{-0.45}\left(\displaystyle \frac{{S}_{\nu }}{\mathrm{Jy}}\right){\left(\displaystyle \frac{D}{\mathrm{kpc}}\right)}^{2}.\end{eqnarray} \tag{ 3 }$$

We estimate ${N}_{{uv}}=1.86\times {10}^{49}$ ionizing photons s⁻¹ in the Lyman continuum, equivalent to a single star type O (Watson et al. 2008). Considering a model of  the H ii region in expansion, the neutral material accumulates between the ionization front and the shock front (Deharveng et al. 2010) and the ionized gas is surrounded by a shell of dense, neutral material hosting PAHs, the main material responsible for 8.0 μm emission in infrared wavelengths. Therefore, ionized gas appears  to be confined by the bubble shell exhibited by 8.0 μm emission in red color.

4.2.2. Cold and Warm Dust

In Figure 3, the set of emission maps at 4.5, 5.8, 8.0, and 24 μm has been displayed. These emissions, corresponding to warm dust, allow us to identify clearly the expanding H ii region around the bubble N10. The upper panels, maps at 4.5, 5.8, and 8.0 μm, reveal the arc-shaped boarder of N10.

The thermal emission from cold dust is mainly responsible for the continuum 870 μm distribution toward N10, while the emission at 8.0 μm originates from polyciclic aromatic hydrocarbons (PAHs) excited by UV photons. Figure 8 displays the emission at 870 μm in grayscale and contours for N10 (left panel), and the same contours of the cold dust emission are superimposed on a Spitzer 8.0 μm image (right panel).

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The two 870 μm clumps coincide with ¹³CO molecular condensations detected at the peak velocity of 52 km s⁻¹ shown in the upper right panel of Figure 6. According to Dewangan et al. (2015), the coincidence of distribution of the molecular gas, PAH and cold dust emission are evidence of star-forming material around the bubble. Following the example of Duronea et al. (2015) we calculated the physical parameters for the denser condensation in the right panel of Figure 8. We consider that the 870 μm radiation originates from the thermal radiation from dust grains. The total mass (dust and gas) in grams can be estimated following the relation by Hildebrand (1983):

$$\begin{eqnarray}&&{M}_{\mathrm{tot}}=100\displaystyle \frac{{S}_{870\mu {\rm{m}}}\ {D}^{2}}{{\kappa }_{870\mu {\rm{m}}}\ {B}_{870\mu {\rm{m}}}({T}_{\mathrm{dust}})},\end{eqnarray} \tag{ 4 }$$

where ${S}_{870\mu {\rm{m}}}$ is the flux density of 870 μm emission in Jy, ${\kappa }_{870\mu {\rm{m}}}$ is the dust opacity per unity mass at 870 μm, and ${B}_{870\mu {\rm{m}}}$ is the Planck function for a given dust temperature ${T}_{\mathrm{dust}}$ in Jy. We assumed ${T}_{\mathrm{dust}}=20$ K and ${\kappa }_{870\mu {\rm{m}}}=1.8$ cm² g⁻¹. We assumed a gas-to-dust ratio of 100 (Mathis et al. 1977; Draine & Anderson 1985).

The H₂ column density $N({{\rm{H}}}_{2})$ can be estimated using the following formula, by Deharveng et al. (2010):

$$\begin{eqnarray}&&N({{\rm{H}}}_{2})=\displaystyle \frac{100\ {F}_{870\mu {\rm{m}}}}{{\kappa }_{870\mu {\rm{m}}}\ {B}_{870\mu {\rm{m}}}({T}_{\mathrm{dust}})\ 2.8\ {m}_{{\rm{H}}}\ {{\rm{\Omega }}}_{\mathrm{beam}}},\end{eqnarray} \tag{ 5 }$$

where $N({{\rm{H}}}_{2})$ is in cm⁻², the surface brightness ${F}_{870\mu {\rm{m}}}$ is in Jy beam⁻¹, the beam solid angle ${{\rm{\Omega }}}_{\mathrm{beam}}$ is in steradians and the hydrogen mass m_H is in grams. We estimated a column density of $N({{\rm{H}}}_{2})=6.3\times {10}^{22}$ cm⁻².

We calculate the effective radius of the clumps as

$$\begin{eqnarray}&&{R}_{D}={({R}_{\mathrm{maj},D}\times {R}_{\min ,D})}^{0.5}={\left(\displaystyle \frac{{\theta }_{\mathrm{maj},D}}{2}\times \displaystyle \frac{{\theta }_{\min ,D}}{2}\right)}^{0.5}\end{eqnarray} \tag{ 6 }$$

where the major and minor deconvolved FWHM of the condensation (${\theta }_{\mathrm{maj},D}$ and ${\theta }_{\min ,D}$) was calculated as

$$\begin{eqnarray}&&{\theta }_{\mathrm{maj},D}=\sqrt{{\theta }_{\mathrm{maj}}^{2}-{\theta }_{\mathrm{HPBW}}^{2}}\quad \mathrm{and}\quad {\theta }_{\min ,D}=\sqrt{{\theta }_{\min }^{2}-{\theta }_{\mathrm{HPBW}}^{2}},\end{eqnarray} \tag{ 7 }$$

where ${\theta }_{\mathrm{maj}}$ and ${\theta }_{\min }$ are the major and the minor FWHM sizes, respectively, was obtained by using the GreG/GILDAS fitting. We found that ${\theta }_{\mathrm{maj}}=44$ arcsec and ${\theta }_{\min }=32$ arcsec. The half-power beam width for 870 μm is ${\theta }_{\mathrm{HPBW}}=19.2$ arcsec. Therefore, ${R}_{\min ,D}=25.6$ arcsec and ${R}_{\mathrm{maj},D}=39.6$ arcsec (0.6 pc and 0.9 pc, respectively, at a distance of 4.7 kpc). According to Equation (6), the mean radius deconvolved is R_D =31.8 arcsec, or 0.72 pc, at a distance of 4.7 kpc.

The average volume density for this condensation, assuming a spherical geometry, was calculated as the following.

$$\begin{eqnarray}&&n({{\rm{H}}}_{2})=\displaystyle \frac{{M}_{\mathrm{tot}}}{4/3\pi \ {R}_{D}^{3}\mu {m}_{{\rm{H}}}},\end{eqnarray} \tag{ 8 }$$

where μ is the mean molecular weight and m_H is the mass of the hydrogen atom. We assumed $\mu =2.33$ g and ${m}_{{\rm{H}}}=1.67\ \times {10}^{-24}$ g, what leads to an average volume density $n({{\rm{H}}}_{2})\ =1.17\times {10}^{4}$ cm⁻³.

In short, for the denser cold dust condensation, we found a column density of $N({{\rm{H}}}_{2})=6.3\times {10}^{22}$ cm⁻², a total mass of ${M}_{\mathrm{tot}}=240$ ${M}_{\odot }$, a deconvolved  mean radius of ${R}_{D}=0.36$ pc, and an average volume density of $n({{\rm{H}}}_{2})=1.17\times {10}^{4}$ cm⁻³.

Warm dust can also be traced in this region by the distribution of 24 μm emission in grayscale. VLA 20 cm emission is shown in contours. Warm dust and ionized emission appears to be quite correlated, as Figure 9 displays, which is expected for H ii regions (e.g., Paladini et al. 2012).

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We notice that the IRAC 24 μm emission appears to be saturated close to the VLA 20 cm emission. This wavelength band concerns dust and gas ionized emission, which is shown in our results too. From Figure 3, we can see that in the saturated region both the IRAC and 20 cm emissions are strong.

In resume, the map at 8.0 μm is very useful, since it traces the expanding arc in the upper left part of N10, this region is associated with ionized gas, excited PAHs, and warm dust.

On the other hand, the emission map at 20 cm exposes the central part of the H ii region, which is surrounded by the N10 boarder; as would be expected, this region contains dust warmer than the expanding arcs. Furthermore, Figure 7 shows clearly the central part of the H ii region.

Adopting a distance of 4.6 kpc from Pandian et al. (2008; see also Deharveng et al. 2010), the physical size of the ring is about 4.7 × 2.5 pc. Using this distance, N10 is found to be close to the near extremity of the Galactic bar, a region of intensive star formation (see, e.g., the maps of the Galactic arms structure by Hou & Han 2014).

For this work, using circular Galactic rotation models (e.g., Brand & Blitz 1993) is possible to compute near and far kinematic distances of the source; we have analyzed the kinematic distance ambiguity and the results show that the near kinematic one may be more reasonable.

Churchwell et al. (2006) argued that infrared bubbles are more likely located at their near kinematic distances, since objects on the far side of the Galactic disk would be obscured by interstellar extinction and contamination of other structures.

Based on our CO observations and using the velocity of 52.6 km s⁻¹ (see Section 4.4), we obtained near and far kinematic distances of 4.7 kpc and 11.3 kpc, respectively. This value is compatible with the near distance estimated by Szymczak et al. (2000) d = 4.4 kpc, using the methanol maser emission (see Table 2). Since N10 is located in the inner disk of the Galaxy, we adopt a 10% uncertainty for the kinematic distance (Yuan et al. 2014), resulting in a value of 4.7 ± 0.5 kpc for N10.

In the following discussion, we adopt the labels given in Figure 10 to refer to identified condensations. Due to the poor spatial resolution of PMO (the beam size is ∼0.9 pc in our observations), the sizes of these molecular condensations could be smaller than 0.9 pc.

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We used CLASS to calculate the parameters performing Gaussian fits to the average spectra obtained for the whole observed region. We obtained the centroid velocity (V_LSR), the antenna temperature (${T}_{A}^{* }$), and the full width at half maximum (ΔV_FWHM). These observed parameters are shown in Table 3.

Table 3.  Observed Line Parameters Obtained Over the Integrated Area Described in Figure 4, Where V_lsr Is the Centroid Velocity of the Main Peak, the ${T}_{A}^{* }$ is the Antenna Temperature, and ΔV_FWHM is the Full Width at Half Maximum of Lines

	Vlsr	${T}_{A}^{* }$	ΔVFWHM
	(km ${{\rm{s}}}^{-1}$ )	(K)	(km ${{\rm{s}}}^{-1}$ )
12CO	52.6	8.3	9.2
13CO	52.6	2.9	6.1

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In order to understand the evolutionary status of molecular clumps, we derived the physical properties for the two clumps we identified in our CO observations. The masses of the clumps were estimated by using the Miriad software package (Sault et al. 1995).

The mass of the molecular gas in a clump is calculated from the intensity of the emission line, under the local thermal equilibrium (LTE) assumption. With the radiation transfer equation Garden et al. (1991), we derived the mass of the clumps under LTE condition, as follows.

$$\begin{eqnarray}&&{M}_{\mathrm{LTE}}=\displaystyle \frac{4}{3}\pi {R}^{3}\,{m}_{{{\rm{H}}}_{2}}\ {\mu }_{g}\ {n}_{{{\rm{H}}}_{2}}\end{eqnarray} \tag{ 9 }$$

where M_LTE is given in solar masses M_⊙, ${m}_{{{\rm{H}}}_{2}}$ is the mass of hydrogen molecule, ${\mu }_{g}=1.36$ is the mean atomic weight of the gas, and ${n}_{{{\rm{H}}}_{2}}=N/2R$ is the volume density. The mean radius of the clump is obtained by the relation $R\ =\sqrt{{b}_{\mathrm{maj}}\times {b}_{\min }}/2$, where b_maj and b_min are the sizes of the minor and major axes of the ellipse, respectively, obtained using the Miriad software.

We first estimated the value of the column density ${{\rm{H}}}_{2}$ through the CO column density. According to Garden et al. (1991), the column density N of a rigid, asymmetric linear molecule, under LTE condition, can be expressed by

$$\begin{eqnarray}N & = & \displaystyle \frac{3k}{8{\pi }^{3}B{\mu }^{2}}\times \displaystyle \frac{\exp [h\,B\,J(J+1)/k\,{T}_{\mathrm{ex}}]}{(J+1)}\\ & & \times \displaystyle \frac{({T}_{\mathrm{ex}}+{hB}/3k)}{[1-\exp (-h\nu /{{kT}}_{\mathrm{ex}}]}\times \displaystyle \int \tau \,d\nu \end{eqnarray} \tag{ 10 }$$

where B is the rotational constant, J is the rotational quantum number of the lower state of the observed transition, and μ is the electric dipole of the molecule. T_ex is the excitation temperature and τ is the optical depth from 48 to 53 km s⁻¹.

Since the excitation temperature (T_ex) is measured as a function of brightness temperature (T_r), we estimate (Garden et al. 1991)

$$\begin{eqnarray}{T}_{r} & = & \displaystyle \frac{h\nu }{k}\times \left[\displaystyle \frac{1}{\exp (h\nu /{{kT}}_{\mathrm{ex}})-1}-\displaystyle \frac{1}{\exp (h\nu /{{kT}}_{\mathrm{bg}})-1}\right][1-\exp (-\tau )]f,\end{eqnarray} \tag{ 11 }$$

where T_r is the brightness temperature and the temperature of the cosmic background radiation T_bg = 2.73 K. Here we assume a filling factor of f = 1.

Assuming that T_ex is the same for ¹²CO and for ¹³CO, the optical depth for both lines can be obtained directly comparing the measure of its brightness temperatures T_r (Garden et al. 1991):

$$\begin{eqnarray}&&\displaystyle \frac{{T}_{r}{(}^{12}\mathrm{CO})}{{T}_{r}{(}^{13}\mathrm{CO})}\approx \displaystyle \frac{1-\exp (-{\tau }_{12})}{1-\exp (-{\tau }_{13})}.\end{eqnarray} \tag{ 12 }$$

We adopted an isotope ratio of [¹²CO]/[¹³CO] = 60 (Wilson & Rood 1994; Deharveng et al. 2008), implying ${\tau }_{12}/{\tau }_{13}=60$, and the canonical [CO]/[H₂] abundance ratio of 10⁻⁴.

Thus we can estimate the optical depth from Equation (12), and then, using it in Equation (11), and knowing the brightness temperature, we can estimate T_ex.

Using the estimated value of T_ex for the line ¹³CO, we can finally obtain the hydrogen column density (${N}_{{{\rm{H}}}_{2}}$) using Equation (10). Thus, the intensity of the ¹³CO line traces the column density of the clumps #1 and #2, as listed in Table 4.

Table 4.  Derived Parameters of the Clumps Observed in ¹³CO

Clump	R	Tex	N${}_{{{\rm{H}}}_{2}}$	n${}_{{{\rm{H}}}_{2}}$	MLTE	Mvirial	MJeans
	(pc)	(K)	(1022 cm−2)	(103 cm−3)	(103 ${M}_{\odot }$)	(103 ${M}_{\odot }$)	(103 ${M}_{\odot }$)
#1	1.1	16.8	4.1	4.2	2.6	9.3	7.0
#2	1.2	12.9	3.3	3.9	1.5	7.8	4.7

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Obtaining the velocity dispersion and the mass under the LTE assumption, we can estimate the virial condition, by comparing the gas mass (M_LTE) with the virial mass (M_viral). In a cloud in which the temporal average kinetic energy is equal to half of the temporal average of the potential energy, the system is considered in virial equilibrium. The assumption that a gravitationally bound system is in virial equilibrium is widely used in astrophysics to estimate its mass (Huang 1954). The virial mass ${M}_{\mathrm{viral}}$ is given by Ungerechts et al. (2000) by the expression

$$\begin{eqnarray}&&\displaystyle \frac{{M}_{\mathrm{virial}}}{{M}_{\odot }}=2.10\times {10}^{2}\left(\displaystyle \frac{R}{\mathrm{pc}}\right){\left(\displaystyle \frac{{\rm{\Delta }}{V}_{\mathrm{FWHM}}}{\mathrm{km}{{\rm{s}}}^{-1}}\right)}^{2}\end{eqnarray} \tag{ 13 }$$

where R is the mean radius of the clump and ${\rm{\Delta }}{V}_{\mathrm{FWHM}}$ is the line width of ¹³CO line.

The Jeans mass M_J is the mass above which a gas cloud will collapse, for a given density and temperature, when the gravitational attraction overcomes the pressure of the gas. It can be calculated according to Stahler & Palla (2005):

$$\begin{eqnarray}&&\displaystyle \frac{{M}_{\mathrm{Jeans}}}{{M}_{\odot }}={\left(\displaystyle \frac{T}{10{\rm{K}}}\right)}^{3/2}{\left(\displaystyle \frac{{n}_{{{\rm{H}}}_{2}}}{{10}^{4}{\mathrm{cm}}^{-3}}\right)}^{-1/2}.\end{eqnarray} \tag{ 14 }$$

The results are presented in Table 4, where columns 2–7 list the following clump parameters: mean radius (R), excitation temperature (T_ex), column density (${N}_{{{\rm{H}}}_{2}}$), volume density (${n}_{{{\rm{H}}}_{2}}$), gas mass calculated under LTE assumption (M_LTE), virial mass (M_virial), and Jeans mass (M_Jeans). We discuss the clump status in Section 5.1.

The distribution of YSOs plays a major role in the interpretation of the dynamics of star-forming region. To identify the YSOs present in the field of N10, we adopted the method described by Koenig & Leisawitz (2014; hereafter KL), based on the data of the WISE (see Wright et al. 2010). In particular, we used the AllWISE release (Cutri et al. 2011, 2013), which combined the data from the cryogenic and post-cryogenic phases of the survey, resulting in a catalog with enhanced sensitivity.

The catalog was accessed through the VIZIER facility of the Strasbourg Data Center. The catalog contains infrared photometric data at 3.6, 4.9, 5.8 and 22 μm wavelengths, hereafter designated as w1, w2, w3, and w4 bands, respectively. In a first step, we selected all the objects situated in the area around N10 that we explored, in the range of Galactic coordinates $13\buildrel{\circ}\over{.} 11\lt l\lt 13\buildrel{\circ}\over{.} 27$ and $-0\buildrel{\circ}\over{.} 04\lt b\lt 0\buildrel{\circ}\over{.} 12$.

We found 565 WISE sources in this area. We next filtered this list of sources by applying a series of quality criteria defined by KL, which they call the uncertainty/signal-to-noise/chi-squared criteria. The purpose of this is to avoid regions in the space of these parameters with relatively high probabilities of spurious catalog entry. Accordingly, the Class I YSOs are classified as such if their color matches with all the following criteria:

• $w1-w3\gt 2.0;$
• $w1-w2\gt -0.42\times (w2-w3)+2.2;$
• $w1-w2\gt 0.46\times (w1-w3)-0.9;$
• $w2-w3\lt 4.5$.

These conditions reflect the divisions in the SED slope $\alpha \ =d\ \mathrm{log}(\lambda {F}_{\lambda })/d\ \mathrm{log}\lambda$.

The Class II objects were also classified according to KL, whose criteria are

• $w1-w2\gt 0.25;$
• $w1-w2\lt -0.9\times (w2-w3)+0.25;$
• $w1-w2\gt 0.46\times (w2-w3)-0.9;$
• $w2-w3\lt 4.5$.

Similarly, also on the criteria by KL, transition disks were classified as follows:

• $w3-w4\gt 1.5;$
• $0.15\lt w1-w2\lt 0.8;$
• $w1-w2\gt 0.46\times (w2-w3)-0.9;$
• $w1\leqslant 13.0$.

For w3, we kept only the condition of S/N larger than five. It is considered that if a source satisfies the criteria of being a true source in any one of the bands, it has little probability of being a fake one and will be included in the final list. After this filtering, the list of sources was reduced to 407 entries.

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The next step was to separate the sources into Class I, Class II, Transition Disks, and remaining objects. Following KL, the Class I and Class II YSOs are classified as such, based on the $w1-w2$ versus w2 – w3 color–color diagram only. The regions of the diagram that are used to classify the YSOs are defined by a number of frontier lines, shown in Figure 12. The equations of the lines are given by KL (their Equations (12) to (20)). We found 12 Class I stars and 91 Class II sources. The transition disk stars are selected separately by means of the $w1-w2$ versus $w2-w3$ color–color diagram as shown in Figure 13. We found 131 transition disk stars. Note that the selection criteria follow an order of priority: a Class I object will remain Class I even if it also satisfies the criterion for Class II, and next the Class II selection prevails over the following selection. This is why we find many Class II objects in the box defining transition disk stars in Figure 13: they were classified Class II in the previous step, on the basis of the different color–color plot.

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This selection of Class I objects is robust, since these objects are well separated from the other classes in the color diagrams. Furthermore, we made experiments with another classification scheme available in the literature (using Spitzer data, e.g., Gutermuth et al. 2009) and the same Class I objects were retrieved. On the other hand, the distinction between Class II and the transition disk is a little arbitrary, as we can see some overlap in Figures 12 and 13. In the samples of objects previously known to belong to given classes, used by KL to decide the position of the frontier lines in Figures 12 and 13, one can see a number of transition disk sources in the locus of Class II and vice versa. So, one must consider that the decision to attribute sources to one or the other classes is only valid in a statistical sense, being correct in about 70% of the cases.

We have compared the position of Class I YSOs, the most embedded young stellar sources, with the molecular distribution and the objects identified from #1 to #9 (see Table 6) are more likely to be physically related to N10 molecular structure. We have fitted their SED by using the online tool developed by Robitaille et al. (2007). Radiation transfer models were fitted to observational data extracted from the WISE catalog based on a ${\chi }^{2}$ test. We selected models for which ${\chi }^{2}-{\chi }_{\min }^{2}\lt 3n$, where ${\chi }_{\min }^{2}$ is the minimum value and n is the number of input data.

The fitting was performed using fluxes from WISE data, distance ranges from 4.23 to 5.17 kpc. Interstellar extinction in the direction of N10 was predict to be approximately 10.7 mag according to model S of Amôres & Lépine (2005), values adopted were from 9.7 to 11.7 mag. The best fit is shown in Figure 11. Resulting values for model parameters are given in Table 5. We found that Class I YSOs have stellar masses ranging from ∼1 to ∼13 ${M}_{\odot }$, stellar temperatures from ∼4000 to 20,000 K, total luminosities from $\sim 3\times {10}^{1}\,-$ $1\times {10}^{3}$ ${L}_{\odot }$, envelope accretion rates from $\sim 9\times {10}^{-8}\,-$ $3\times {10}^{-3}$ ${M}_{\odot }$ yr⁻¹, disk masses from $\sim 7\times {10}^{-3}\,-$ $6\times {10}^{-1}$ ${M}_{\odot }$, and stellar ages from $\sim 2\times {10}^{3}$ to $\sim 1\times {10}^{6}$ years.

Table 5.  Physical Parameters Derived from Robitaille et al. (2007) Model, for Associated Class I Object Candidates

	Class I YSOsa
Parameters	1	2	3	4	5	6	7	8	9
Stellar Mass (${M}_{\odot }$)	1.42	6.53	7.07	6.08	7.42	9.86	12.94	12.89	6.43
Stellar Temperature (K)	4173	10107	4293	4148	20167	4260	12262	7471	19400
Total Luminosity (L${}_{\odot }$)b	3.11 × 101	1.02 × 103	5.45 × 102	5.96 × 102	2.33 × 103	2.47 × 103	1.13 × 104	7.31 × 103	1.19 × 103
${\dot{M}}_{\mathrm{env}}$ (M${}_{\odot }$ yr−1)	2.49 × 10−5	1.20 × 10−5	4.70 × 10−5	1.19 × 10−5	7.60 × 10−5	1.12 × 10−5	3.61 × 10−3	2.25 × 10−4	8.62 × 10−8
Disk Mass (M${}_{\odot }$)c	1.01 × 10−2	3.05 × 10−2	3.38 × 10−2	4.45 × 10−3	1.46 × 10−3	7.71 × 10−3	1.49 × 10−1	6.09 × 10−1	5.25 × 10−5
Stellar Age (year)	1.24 × 104	2.77 × 105	7.07 × 103	1.84 × 103	2.97 × 105	2.41 × 103	2.88 × 104	1.60 × 104	1.02 × 106

Notes.

^aIdentification from Table 6. ^bTotal system luminosity. ^cEnvelope and ambient density mass, in solar mass.

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Channel maps are presented in Figure 5; since ¹²CO is optically thick, it appears to be spread over a large area, whereas ¹³CO traces the denser regions, once it is optically thinner. Figure 10 displays two peaks of molecular emission, i.e., two ¹³CO condensations. Clump #1 is centered at l = 13218, b = 0043 and Clump #2 is located at l = 13169, b = 0072. The two clumps are located precisely on the edge of the ring structure revealed by the 8.0 μm emission and highlighted by an ellipse in Figure 2.

As highlighted in Section 4.1, CO components at 20 and 37 km s⁻¹ do not seem to be physically related with bubble N10. What could be the origin of this contribution? It is known that CO emission in galaxies is concentrated in spiral arms (Nieten et al. 2006; Schinnerer et al. 2013). The line of sight toward N10 crosses two spiral arms before reaching the distance of 4.7 kpc (see, e.g., Figure 10 of Hou & Han 2014). The velocities of peaks 20 and 37 km s⁻¹ correspond to near kinematic distances of 2.4 and 3.7 kpc, respectively, representing roughly the distance of those arms.

It is therefore reasonable to suppose that these two velocity peaks are associated with foreground gas situated in distinct spiral arms. Furthermore, emission at these two velocities does not seem to be correlated with the geometry of N10; emission appears to be irregularly spread over a studied field. If one examines Figure 4 of the ¹³CO (1-0) survey of the Galaxy by Lee et al. (2001), selecting the panel corresponding to b = 005, one can see at longitude 132 the presence of ¹³CO at about 50, 35, and 20 km s⁻¹. The last two are part of the elongated structures in the longitude-velocity diagram (extending to higher and lower longitudes), which are usually interpreted as spiral arms. In this region of the diagram, lower velocities correspond to the closest arms.

Note that kinematic distances are uncertain at longitudes close to the Galactic center. We have to make use of a ¹³CO survey because in ¹²CO longitude-velocity diagrams, the spiral arms are wider in velocity and are not seen separated.

The densest clump in the 870 μm emission seems to be a candidate region to form stellar clusters, since it has a total mass of ${M}_{\mathrm{tot}}=240$ ${M}_{\odot }$ and a mean radius of R = 0.36 pc. In accordance with Motte et al. (2003), fragments in the range between 0.09 and 0.56 pc and masses covering a range from 20 to 3600 ${M}_{\odot }$ have characteristics of protoclusters.

Elmegreen & Lada (1977) were the first to propose the scenario of "Collect and Collapse," where the radiation of the massive stars of an H ii region creates an ionization front at the interface with the molecular cloud that drives the propagation of a shock front into the neutral material and that accumulates mass and eventually becomes gravitationally unstable. Other scenarios of triggered star formation have been proposed, such as, e.g., the "Radiation-Driven Implosion" model, based on the over pressure exerted by the ionized gas, suggested by Lefloch & Lazareff (1994). While the "Collect and Collapse" model takes place in a large spatial size (∼10 pc) with a longer timescale (a few million years), "Radiation-Driven Implosion" takes place in ∼1 pc with a timescale of 0.5 Myr.

Although we found evidence for active star formation in N10, we are not sure that the formation of these YSOs was triggered by the "Collect and Collapse" mechanism around the infrared bubble. In order to verify if this process is viable, we can apply the analytical model proposed by Whitworth et al. (1994) and compare the fragmentation timescale t_frag with the dynamical age t_dyn of the region. The Whitworth et al. (1994) model describes the fragmentation time as

$$\begin{eqnarray}&&{t}_{\mathrm{frag}}=1.56\ {c}_{s}^{7/11}\ {N}_{{uv}}^{-1/11}{n}_{o}^{-5/11},\end{eqnarray} \tag{ 15 }$$

where c_s is the isothermal sound speed in the ionized gas in the shocked layer in units of 0.2 km s⁻¹, N_uv is the ionizing photon flux in units of 10⁴⁹ photons s⁻¹, and n_o is the initial particle number density of the ambient neutral gas in units of 10³ cm⁻³. Considering ${c}_{s}=0.2$ km s⁻¹ (Liu et al. 2012), ${N}_{{uv}}=1.86\ \times {10}^{49}$ photons s⁻¹ and ${n}_{o}\sim {10}^{3}$ cm⁻³ (Ma et al. 2013), we estimated ${t}_{\mathrm{frag}}\sim 1.5\times {10}^{6}$ years for the region. From Ma et al. (2013), ${t}_{\mathrm{dyn}}=9.17\times {10}^{4}$ years, i.e., the dynamical age is smaller than the fragmentation timescale, which indicates that the region does not support the "Collect and Collapse" mechanism. In this case, the "Radiation-Driven Implosion" could be considered and further investigated.

The position of YSOs compared with CO distribution indicates that stars are forming inside the molecular clumps. Figure 14 shows the spatial distribution of identified YSOs from Table 6. In fact, the Class I YSO candidates to be associated with N10 presented in Table 5 have ages smaller than the fragmentation timescale, suggesting the possibility of triggered star formation by pre-existing condensations compressed by the pressure of the ionized gas, as the "Radiation-Driven Implosion" scenario proposes.

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Infrared images show N11, a bubble that seems to be physically connected with N10 and extends about 3 pc in the up-right direction. Conversely the molecular distribution of N11 does not suggest a physical connection with N10, since the emission of ¹³CO (1-0) between 47 and 53 km s⁻¹ is not coincident with 8.0 μm emission.

It is probable that this object is a remnant of an H ii region, where the lack of 20 cm emission lead us to consider that there is no more ionized gas inside. It is likely that another energy source has triggered the formation of these YSOs, such as the explosion of an SN II.

Class I YSOs do not seem to be superimposed on the bubble N11 and many Class II YSOs can be found toward N11, as we can see in Figure 14. There is a remarkable concentration of transition disk sources surrounding the top frontier of N11. We consider, in this interpretation, that the concentration of Class II YSOs near the upper frontier of N11 is possibly the result of a past star formation activity related to that bubble.

The bubble MWP1G013134+000580, at coordinates l =13134 and b = 0058, has a size smaller than 2 pc. Interestingly, this small bubble should have about the same distance as N10, since its CO emission is contained in the same main velocity peak, clearly seen in channel maps with velocities between 51 and 53 km s⁻¹ in Figure 5.

We found three Class I YSOs in the region covered by 8.0 μm emission of MWP1G013134+000580. The age of the small bubble seems to have the same order of N10, as we can infer from the evolutionary stages of the YSOs.