ALBERT

Description: We report the results of spectrophotometric observations of the massive star MN18 revealed via discovery of a bipolar nebula around it with the Spitzer Space Telescope . Using the optical spectrum obtained with the Southern African Large Telescope, we classify this star as B1 Ia. The evolved status of MN18 is supported by the detection of nitrogen overabundance in the nebula, which implies that it is composed of processed material ejected by the star. We analysed the spectrum of MN18 by using the code cmfgen , obtaining a stellar effective temperature of 21 kK. The star is highly reddened, E ( B – V )  2 mag. Adopting an absolute visual magnitude of M V  = –6.8 ± 0.5 (typical of B1 supergiants), MN18 has a luminosity of log L /L   5.42 ± 0.30, a mass-loss rate of (2.8-4.5) x 10 – 7 M yr – 1 , and resides at a distance of 5.6 $^{+1.5} _{-1.2}$  kpc. We discuss the origin of the nebula around MN18 and compare it with similar nebulae produced by other blue supergiants in the Galaxy (Sher 25, HD 168625, [SBW2007] 1) and the Large Magellanic Cloud (Sk–69 $\deg$ 202). The nitrogen abundances in these nebulae imply that blue supergiants can produce them from the main-sequence stage up to the pre-supernova stage. We also present a K -band spectrum of the candidate luminous blue variable MN56 (encircled by a ring-like nebula) and report the discovery of an OB star at 17 arcsec from MN18. The possible membership of MN18 and the OB star of the star cluster Lynga 3 is discussed.

Description: SDSS J0015+0104 is the lowest metallicity low surface brightness dwarf (LSBD) galaxy known. The oxygen abundance in its H ii region SDSS J001520.70+010436.9 (at ~1.5 kpc from the galaxy centre) is 12+log (O/H) = 7.07 (Guseva et al.). This galaxy, at the distance of 28.4 Mpc, appears to reside deeply in the volume devoid of luminous massive galaxies, known as the Eridanus void. SDSS J235437.29–000501.6 is another Eridanus void LSBD galaxy, with parameter 12+log (O/H) = 7.36 (also Guseva et al.). We present the results of their H i observations with the Nançay Radio Telescope revealing their high ratios of M (H i )/ L B  ~ 2.3. Based on the Sloan Digital Sky Survey images, we derived for both galaxies their radial surface brightness profiles and the main photometric parameters. Their colours and total magnitudes are used to estimate the galaxy stellar mass and ages. The related gas mass fractions, f g  ~ 0.98 and ~0.97, and the extremely low metallicities (much lower than for their more typical counterparts with the same luminosity) indicate their unevolved status. We compare these Eridanus void LSBDs with several extreme LSBD galaxies residing in the nearby Lynx–Cancer void. Based on the combination of all their unusual properties, the two discussed LSBD galaxies are similar to the unusual LSBDs residing in the closer void. This finding presents additional evidence for the existence in voids of a sizeable fraction of low-mass unevolved galaxies. Their dedicated search might result in the substantial increase of the number of such objects in the local Universe and in the advancement of understanding their nature.

Description: We report the results of optical spectroscopy of the candidate evolved massive star MN44 revealed via detection of a circular shell with the Spitzer Space Telescope . First spectra taken in 2009 May–June showed the Balmer lines in emission as well as numerous emission lines of iron, which is typical of luminous blue variables (LBVs) near the visual maximum. New observations carried out in 2015 May–September detected significant changes in the spectrum, indicating that the star became hotter. We found that these changes are accompanied by significant brightness variability of MN44. In particular, the I c -band brightness decreased by 1.6 mag during the last six years and after reaching its minimum in 2015 June has started to increase. Using archival data, we also found that the I c -band brightness increased by 3 mag in 30 yr preceding our observations. MN44 therefore represents the 17th known example of the Galactic bona fide LBVs. We detected a nitrogen-rich knot to the north-west of the star, which might represent an interstellar cloudlet interacting with the circumstellar shell. We discuss a possible association between MN44 and the INTEGRAL transient source of hard X-ray emission IGR J16327–4940, implying that MN44 might be either a colliding-wind binary or a high-mass X-ray binary.

Description: We present new or improved oxygen abundances (O/H) for the nearby Lynx-Cancer void updated galaxy sample. They are obtained via the SAO 6-m telescope spectroscopy (25 objects), or derived from the Sloan Digital Sky Survey spectra (14 galaxies, of which for seven objects O/H values were unknown). For eight galaxies with detected [O iii ] 4363 line, O/H values are derived via the direct ( T e ) method. For the remaining objects, O/H was estimated via semi-empirical and empirical methods. For all accumulated O/H data for 81 galaxies of this void (with 40 of them derived via T e method), their relation ‘O/H versus M B ’ is compared with that for similar late-type galaxies from denser environments (the Local Volume ‘reference sample’). We confirm our previous conclusion derived for a subsample of 48 objects: void galaxies show systematically reduced O/H for the same luminosity with respect to the reference sample, in average by 0.2 dex, or by a factor of ~1.6. Moreover, we confirm the fraction of ~20 per cent of strong outliers, with O/H of two to four times lower than the typical values for the ‘reference’ sample. The new data are consistent with the conclusion on the slower evolution of the main void galaxy population. We obtained Hα velocity for the faint optical counterpart of the most gas-rich ( M (H i )/ L B = 25) void object J0723+3624, confirming its connection with the respective H i blob. For similar extremely gas-rich dwarf J0706+3020, we give a tentative O/H ~(O/H) /45. In Appendix A , we present the results of calibration of semi-empirical method by Izotov & Thuan and of empirical calibrators by Pilyugin & Thuan and Yin et al. on the sample of ~150 galaxies from the literature with O/H measured by T e method.

Description: Low-luminosity galaxies may be the building blocks of more luminous systems. Southern African Large Telescope observations of the low-luminosity, early-type galaxy NGC 59 are obtained and analysed. These data are used to measure the stellar population parameters in the centre and off-centre regions of this galaxy, in order to uncover its likely star formation history. We find evidence of older stars, in addition to young stars in the emission-line regions. The metallicity of the stellar population is constrained to be [ Z /H] ~ –1.1 to –1.6, which is extremely low, even for this low-luminosity galaxy, since it is not classed as a dwarf spheroidal galaxy. The measured [α/Fe] ratio is subsolar, which indicates an extended star formation history in NGC 59. If such objects formed the building blocks of more massive, early-type galaxies, then they must have been gaseous mergers, rather than dry mergers, in order to increase the metals to observed levels in luminous, early-type galaxies.

Description: We report the results of spectrophotometric observations of the massive star MN18 revealed via discovery of a bipolar nebula around it with the Spitzer Space Telescope . Using the optical spectrum obtained with the Southern African Large Telescope, we classify this star as B1 Ia. The evolved status of MN18 is supported by the detection of nitrogen overabundance in the nebula, which implies that it is composed of processed material ejected by the star. We analysed the spectrum of MN18 by using the code cmfgen , obtaining a stellar effective temperature of 21 kK. The star is highly reddened, E ( B – V )  2 mag. Adopting an absolute visual magnitude of M V  = –6.8 ± 0.5 (typical of B1 supergiants), MN18 has a luminosity of log L /L   5.42 ± 0.30, a mass-loss rate of (2.8-4.5) x 10 – 7 M yr – 1 , and resides at a distance of 5.6 $^{+1.5} _{-1.2}$  kpc. We discuss the origin of the nebula around MN18 and compare it with similar nebulae produced by other blue supergiants in the Galaxy (Sher 25, HD 168625, [SBW2007] 1) and the Large Magellanic Cloud (Sk–69 $\deg$ 202). The nitrogen abundances in these nebulae imply that blue supergiants can produce them from the main-sequence stage up to the pre-supernova stage. We also present a K -band spectrum of the candidate luminous blue variable MN56 (encircled by a ring-like nebula) and report the discovery of an OB star at 17 arcsec from MN18. The possible membership of MN18 and the OB star of the star cluster Lynga 3 is discussed.

Description: We report early optical linear polarization observations of two gamma-ray bursts made with the MASTER robotic telescope network. We found the minimum polarization for GRB 150301B to be 8 per cent at the beginning of the initial stage, whereas we detected no polarization for GRB 150413A either at the rising branch or after the burst reached the power-law afterglow stage. This is the earliest measurement of the polarization (in cosmological rest frame) of gamma-ray bursts. The primary intent of the paper is to discover optical emission and publish extremely rare (unique) high-quality light curves of the prompt optical emission of gamma-ray bursts during the non-monotonic stage of their evolution. We report that our team has discovered the optical counterpart of one of the bursts, GRB 150413A.

Description: Most runaway OB stars, like the majority of massive stars residing in their parent clusters, go through the red supergiant (RSG) phase during their lifetimes. Nonetheless, although many dozens of massive runaways were found to be associated with bow shocks, only two RSG bow-shock-producing stars, Betelgeuse and μ Cep, are known to date. In this paper, we report the discovery of an arc-like nebula around the late M-type star IRC –10414 using the SuperCOSMOS H-alpha Survey. Our spectroscopic follow-up of IRC –10414 with the Southern African Large Telescope (SALT) showed that it is a M7 supergiant, which supports previous claims on the RSG nature of this star based on observations of its maser emission. This was reinforced by our new radio- and (sub)millimetre-wavelength molecular line observations made with the Atacama Pathfinder Experiment 12-m telescope and the Effelsberg 100-m radio telescope, respectively. The SALT spectrum of the nebula indicates that its emission is the result of shock excitation. This finding along with the arc-like shape of the nebula and an estimate of the space velocity of IRC –10414 (70 ± 20 km s –1 ) imply the bow shock interpretation for the nebula. Thus, IRC –10414 represents the third case of a bow-shock-producing RSG and the first one with a bow shock visible at optical wavelengths. We discuss the smooth appearance of the bow shocks around IRC –10414 and Betelgeuse and propose that one of the necessary conditions for stability of bow shocks generated by RSGs is the ionization of the stellar wind. Possible ionization sources of the wind of IRC –10414 are proposed and discussed.

Description: We use US Naval Observatory CCD Astrograph Catalogue (UCAC4) proper motions and Wide-Field Infrared Survey Explorer (WISE) W 1-band apparent magnitudes and intensity means for almost 400 field RR Lyrae variables to determine the parameters of the velocity distribution of the Galactic RR Lyrae population and constrain the zero-points of the metallicity–〈 M V 〉, period–metallicity–〈 M K s 〉-band and period–metallicity–〈 M W 1 〉-band luminosity relations via statistical parallax. We find the mean velocities of the halo- and thick-disc RR Lyrae populations in the solar neighbourhood to be [ U 0 (Halo), V 0 (Halo), W 0 (Halo)] = (–7 ± 9, –214 ± 10, –10 ± 6) km s –1 and [ U 0 (Disc), V 0 (Disc), W 0 (Disc)] = (–13 ± 7, –37 ± 6, –17 ± 4) km s –1 , respectively, while the corresponding components of the velocity-dispersion ellipsoids are [ V R (Halo), V (Halo), V (Halo)] = (153 ± 9, 101 ± 6, 96 ± 5) km s –1 and [ V R (Disc), V (Disc), V (Disc)] = (46 ± 7, 37 ± 5, 27 ± 4) km s –1 , respectively. The fraction of thick-disc stars is estimated at 0.22 ± 0.03. The corrected infrared period–metallicity–luminosity relations are $\langle M_{K_s}\rangle = -0.769 +0.088 {[Fe/H]}- 2.33 \mathop {\rm log} P_{\rm F}$ and $\langle M_{W1}\rangle = -0.825 + 0.088 {[Fe/H]} -2.33 \mathop {\rm log} P_{\rm F}$ and the optical metallicity–luminosity relation [Fe/H]–〈 M V 〉 is 〈 M V 〉 = +1.094 + 0.232[Fe/H] with a standard error of ±0.089, implying a Large Magellanic Cloud distance modulus of 18.32 ± 0.09, a solar Galactocentric distance of 7.73 ± 0.36 kpc and M31 and M33 distance moduli of DM M31  = 24.24 ± 0.09 ( D  = 705 ± 30 kpc) and DM M33  = 24.36 ± 0.09 ( D  = 745 ± 31 kpc), respectively. Extragalactic distances calibrated with our RR Lyrae star luminosity scale imply a Hubble constant of ~80 km s –1 Mpc –1 . Our results suggest marginal prograde rotation for the population of halo RR Lyraes in the Milky Way.

Description: Recent observations reveal that the central star of the planetary nebula Abell 48 exhibits spectral features similar to massive nitrogen-sequence Wolf–Rayet stars. This raises a pertinent question, whether it is still a planetary nebula or rather a ring nebula of a massive star. In this study, we have constructed a three-dimensional photoionization model of Abell 48, constrained by our new optical integral field spectroscopy. An analysis of the spatially resolved velocity distributions allowed us to constrain the geometry of Abell 48. We used the collisionally excited lines to obtain the nebular physical conditions and ionic abundances of nitrogen, oxygen, neon, sulphur and argon, relative to hydrogen. We also determined helium temperatures and ionic abundances of helium and carbon from the optical recombination lines. We obtained a good fit to the observations for most of the emission-line fluxes in our photoionization model. The ionic abundances deduced from our model are in decent agreement with those derived by the empirical analysis. However, we notice obvious discrepancies between helium temperatures derived from the model and the empirical analysis, as overestimated by our model. This could be due to the presence of a small fraction of cold metal-rich structures, which were not included in our model. It is found that the observed nebular line fluxes were best reproduced by using a hydrogen-deficient expanding model atmosphere as the ionizing source with an effective temperature of T eff  = 70 kK and a stellar luminosity of L *  = 5500 L , which corresponds to a relatively low-mass progenitor star (~3 M ) rather than a massive Pop I star.