New Massive Contact Twin Binary in a Radio-quiet H ii Region Associated with the M17 Complex

Early-B stars, much less energetic than O stars, may create an H ii region that appears as radio-quiet. We report the identification of new early-B stars associated with the radio-quiet H ii region G014.645-00.606 in the M17 complex. The radio-quiet H ii region G014.645-00.606 is adjacent to three radio-quiet WISE H ii region candidates. The ionizing sources of the radio-quiet H ii regions are expected to be later than B1V, given the sensitivity about 1-2 mJy of the MAGPIS 20 cm survey. The stars were first selected if their parallaxes of Gaia EDR3 match that of the 22 GHz H2O maser source within the same region. We used the color-magnitude diagram made from the Zwicky Transient Facility photometric catalog to select the candidates for massive stars because the intrinsic g - r colors of massive stars change little from B-type to O-type stars. Five stars lie in the areas of the color-magnitude diagram where either reddened massive stars or evolved post-main sequence stars of lower masses are commonly found. Three of the five stars, sources 1, 2, and 3, are located at the cavities of the three IR bubbles, and extended H alpha emission is detected around the three IR bubbles. We suggest that sources 1, 2, and 3 are candidates for early-B stars associated with the radio-quiet region G014.645-00.606. Particularly, source 1 is an EW type eclipsing binary with a short period of 0.825 day, while source 2 is an EA type eclipsing binary with a short period of 0.919 day. The physical parameters of the two binary systems have been derived through the PHOEBE model. Source 1 is a twin binary of two stars with T (eff) approximate to 23,500 K, and source 2 contains a hotter component (T (eff) approximate to 20,100 K) and a cooler one (T (eff) approximate to 15,500 K). The O - C values of source 1 show a trend of decline, implying that the period of the source is deceasing. Source 1 is likely a contact early-B twin binary, for which mass transfer might cause its orbit to shrink.

Automated summary

This study reports the identification of new early-B stars associated with the radio-quiet H ii region G014.645-00.606 in the M17 complex. The candidates for massive stars were selected using Gaia EDR3 parallaxes and the color-magnitude diagram, and the physical parameters of the binary systems were derived through modeling. Source 1 is an EW type eclipsing binary with a short period, while source 2 contains a hotter and a cooler component.