Infrared study of the star-forming region associated with the UC HII regions G45.07+0.13 and G45.12+0.13

Abstract

Ultra-compact H ii (UC HII) regions are an important phase in the formation and early evolution of massive stars and a key component of the interstellar medium (ISM). The main objectives of this work are to study the young stellar population associated with the G45.07+0.13 and G45.12+0.13 UC HII regions, as well as the ISM in which they are embedded. We determined the distribution of the hydrogen column density (N( $\mathrm{H}_2$ )) and dust temperature ( $T_d$ ) in the molecular cloud using Modified blackbody fitting on Herschel images obtained in four bands: 160, 250, 350, and $500\,\unicode{x03BC}\mathrm{m}$ . We used near-, mid-, and far-infrared photometric data to identify and classify the young stellar objects (YSOs). Their main parameters were determined by the radiation transfer models. We also constructed a colour-magnitude diagram and K luminosity functions (KLFs) to compare the parameters of stellar objects with the results of the radiative transfer models. We found that N( $\mathrm{H}_2$ ) varies from ${\sim}3.0 \times 10^{23}$ to $5.5 \times 10^{23}\,\mathrm{cm}^{-2}$ within the G45.07+0.13 and G45.12+0.13 regions, respectively. The maximum $T_d$ value is 35 K in G45.12+0.13 and 42 K in G45.07+0.13. $T_d$ then drops significantly from the centre to the periphery, reaching about 18–20 K at distances of ${\sim}2.6$ and ${\sim}3.7\,\mathrm{pc}$ from InfraRed Astronomical Satellite (IRAS) 19110+1045 (G45.07+0.13) and IRAS 19111+1048 (G45.12+0.13), respectively. The gas plus dust mass value included in G45.12+0.13 is ${\sim}3.4 \times 10^5\,\mathrm{M}_\odot$ and ${\sim}1.7 \times 10^5\,\mathrm{M}_\odot$ in G45.07+0.13. The UC HII regions are connected through a cold ( $T_d = 19\,\mathrm{K}$ ) bridge. The radial surface density distribution of the identified 518 YSOs exhibits dense clusters in the vicinity of both IRAS sources. The parameters of YSOs in the IRAS clusters (124 objects) and 394 non-cluster objects surrounding them show some differences. About 75% of the YSOs belonging to the IRAS clusters have an evolutionary age greater than $10^6$ yr. Their slope $\alpha$ of the KLF agrees well with a Salpeter-type initial mass function (IMF) ( $\gamma = 1.35$ ) for a high mass range (O–F stars, $\beta \sim 2$ ) at 1 Myr. The non-cluster objects are uniformly distributed in the molecular cloud, 80% of which are located to the right of the 0.1 Myr isochrone. The slope $\alpha$ of the KLF of non-cluster objects is $0.55\,\pm\,0.09$ , corresponding better to a Salpeter-type IMF for low-mass objects (G–M stars, $\beta \sim 1$ ). Our results show that two dense stellar clusters are embedded in these two physically connected UC HII regions. The clusters include several high- and intermediate-mass zero-age main sequence stellar objects. Based on the small age spread of the stellar objects, we suggest that the clusters originate from a single triggering shock. The extended emission observed in both UC HII regions is likely due to the stellar clusters.

Properties of interstellar matter and stellar population in two star-forming regions

“…The c-m diagram shows that the cluster members are well above the photometric limit. Indeed, no objects with a mass equal to or less than 1.4 M ⊙ were found in Azatyan et al (2022). For comparison, it should be noted that no such phenomenon is observed in the IRAS 5168+3634 region.…”

“…Using the Modified blackbody fitting on the Herschel images it was found that N(H 2 ) varies from 3.0 x 10 23 to 5.5 x 10 23 cm −2 within the G45.07+0.13 and G45.12+0.13 regions (Azatyan et al, 2022). The maximum T d values are 35 K in G45.12+0.13 and 42 K in G45.07+0.13.…”

“…The second one is a physically connected pair of ultra compact H II (UCHII) regions, G45.07+0.13 and G45.12+0.13, associated with IRAS 19110+1045 and IRAS 19111+1048 sources, respectively. For both regions, detailed studies of both the ISM and the stars were carried out in Azatyan (2019), Azatyan et al (2022), Nikoghosyan et al (2021).…”

“…That also reflects the distribution of the objects on the (J-K) 0 histogram. In Azatyan et al (2022) it was shown that about 75% of the YSOs belonging to the IRAS clusters have an evolutionary age greater than 10 6 yr. Therefore, in general, the evolutionary age spread of the vast majority of stellar objects in the both clusters is small.…”

“…The arrow is parallel to the excess vector and passes through the ZAMS point with coordinates (0.34, 2.05). The Y-coordinate corresponds to the photometric limit of UKIDSS in the K band (18.02 mag) corrected for distance (7.8 kpc) and interstellar extinction (A v = 13 mag, (Azatyan et al, 2022)). In turn, photometric limit K = 2.05 mag corresponds to the ZAMS stars with ∼ 1.4 M ⊙ (Siess et al, 2000).…”