Vol 45, No 5 (2019)

For three stars in the ultra-faint dwarf (UFD) galaxy Boötes I we have determined the atmospheric parameters, performed a new reduction of high-resolution spectra from the Subaru archive, and derived the abundances of eight chemical elements without using the LTE assumption. As a result, among the galaxies of its class Boötes I now has the largest sample of stars (11) with a homogeneous set of atmospheric parameters and chemical abundances, and this makes it the most promising for studying the chemical evolution of UFD galaxies. We show that in the range −3 ≲ [Fe/H] ≲ −2 for each of the three α-process elements, magnesium, calcium, and titanium, a transition from their overabundance relative to iron with [α/Fe] ∼ 0.3 to the solar [α/Fe] ratio occurs. This most likely suggests the commenced production of iron in type Ia supernovae. The behavior of the carbon, sodium, nickel, and barium abundances does not differ from that in more massive galaxies, our Galaxy and classical dwarf spheroidal galaxies.

We calculate the X-ray signal that should arise due to reflection of the putative collimated X-ray emission of the Galactic supercritical accretor SS 433 on the molecular clouds in its vicinity. The molecular gas distribution in the region of interest has been constructed based on the data of the Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey in ¹³CO J= 1 → 0 emission line, while the collimated emission was assumed to be aligned with the direction of the relativistic jets, which are continuously launched by the system. We consider all the available Chandraobservations covering the regions possibly containing the reflection signal and put constraints on the apparent face-on luminosity of SS 433 above 4 keV. No signatures of the predicted signal have been found in the analysed regions down to a 4–8 keV surface brightness level of ∼10⁻¹¹ erg/s/cm²/deg². This translates into the limit on the apparent face-on 2–10 keV luminosity of SS 433 L_X2–10≲ 8 ×10³⁸ erg/s, provided that the considered clouds do fall inside the illumination cone of the collimated emission. This, however, might not be the case due to persisting uncertainty in the line-of-sight distances to SS 433 d_SS433 (4.5–5.5 kpc) and to the considered molecular clouds. For the half-opening angle of the collimation cone larger than or comparable to the amplitude of the jets’ precession (≈21°), the stringent upper limit quoted above is most relevant if d_SS433 < 5 kpc, provided that the kinematic distances to the considered molecular clouds are sufficiently accurate (within ∼1000 pc of the adopted values). Dropping the last assumption, a more conservative constraint is L_X2–10≲ 10⁴⁰ erg/s for d_SS433 = 4.65–4.85 kpc, and yet worse for d_SS433 outside this range. We conclude that SS 433 is not likely to belong to the brightest ultraluminous X-ray sources if it could be observed face-on, unless its X-ray emission is highly collimated. However, better X-ray coverage of the molecular clouds in the region of interest is needed to eliminate dependence of this conclusion on incidence of the individual clouds inside the putative X-ray illumination cone of SS 433.

Powerful flares in Galactic kilomasers are closely associated with regions of intense star formation. They contribute to the elucidation of physical processes occurring in these structures. We have recorded a superpowerful flare in the high-velocity −81 km s⁻¹ line in the Galactic maser source W49N. As a result of our monitoring at the RT-22 (Simeiz), RT-32 (Torun), RT-100 (Effelsberg), and RT-32 (Medicina) radio telescopes in the period from September 2017 to November 2018, we have obtained the shape of the spectral flux density variations in the source with time. At the peak the flux density reached P≈5 ×10⁴ Jy. The flare has a double pattern and different durations of its components. The pattern of spectral flux density variations for the first flare with a considerably shorter duration is apparently related to a sharp increase in the density of the medium and the photon flux and to a significant rise in the temperature to hundreds of kelvins. We propose a mechanism of primary energy release related to the existence of close massive multiple systems in star-forming regions. A powerful gravitational perturbation at the system’s periastron can lead to a partial ejection of the envelope of the central massive star in a direction close to the major axis of the ellipse of the companion’s orbit. This explains the significant asymmetry of high-velocity lines in W49N. The ejected envelope is an energy source more significant than the stellar wind and can explain the giant flares in the object. Further comprehensive studies in this direction, including monitoring VLBI studies, are needed to confirm this assumption.