NGC 7538: multiwavelength study of stellar cluster regions associated with IRS 1-3 and IRS 9 sources

Abstract

We present deep and high-resolution (FWHM ∼ 0.4 arcsec) near-infrared (NIR) imaging observations of the NGC 7538 IRS 1–3 region (in JHK bands), and IRS 9 region (in HK bands) using the 8.2 m Subaru telescope. The NIR analysis is complemented with Giant Metrewave Radio Telescope (GMRT) low-frequency observations at 325, 610, and 1280 MHz, molecular line observations of H¹³CO⁺ (J=1–0), and archival Chandra X-ray observations. Using the ‘J − H/H − K’ diagram, 144 Class II and 24 Class I young stellar object (YSO) candidates are identified in the IRS 1–3 region. Further analysis using ‘K/H − K’ diagram yields 145 and 96 red sources in the IRS 1–3 and IRS 9 regions, respectively. A total of 27 sources are found to have X-ray counterparts. The YSO mass function (MF), constructed using a theoretical mass–luminosity relation, shows peaks at substellar (∼0.08–0.18 M_⊙ ) and intermediate (∼1–1.78 M_⊙ ) mass ranges for the IRS 1–3 region. The MF can be fitted by a power law in the low-mass regime with a slope of Γ ∼ 0.54–0.75, which is much shallower than the Salpeter value of 1.35. An upper limit of 10.2 is obtained for the star to brown dwarf ratio in the IRS 1–3 region. GMRT maps show a compact H ii region associated with the IRS 1–3 sources, whose spectral index of 0.87 ± 0.11 suggests optical thickness. This compact region is resolved into three separate peaks in higher resolution 1280 MHz map, and the ‘east’ subpeak coincides with the IRS 2 source. H¹³CO⁺ (J=1–0) emission reveals peaks in both IRS 1–3 and IRS 9 regions, none of which are coincident with visible nebular emission, suggesting the presence of dense cloud nearby. The virial masses are approximately of the order of 1000 and 500 M_⊙ for the clumps in IRS 1–3 and IRS 9 regions, respectively.