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We present the preliminary results of an image-subtraction analysis of the Galactic globular cluster M79 (NGC 1904), as well as a new investigation of the variable star population in NGC 1851. Both M79 and NGC 1851 have been previously associated with the Canis Major overdensity, which has been suggested to have an extragalactic origin. We found 6 new RR Lyrae in M79, and also recovered 3 previously known RR Lyrae. The average period of the 5 ab-type RR Lyrae is 0.68 ~ d, corresponding to an Oosterhoff II classification -- which is unusual, though not unprecedented, for systems of extragalactic origin. We also report on the discovery of at least 4 previously unknown variables in NGC 1851.
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We present a new search for variable stars in the Galactic globular cluster M28 (NGC 6626). The search is based on a series of BVI images obtained with the SMARTS Consortiums 1.3m telescope at Cerro Tololo Inter-American Observatory, Chile. The search was carried out using the ISIS v2.2 image subtraction package. We find a total of 25 variable stars in the field of the cluster, 9 being new discoveries. Of the newly found variables, 1 is an ab-type RR Lyrae star, 6 are c-type RR Lyrae, and 2 are long-period/semi-regular variables. V22, previously classified as a type II Cepheid, appears as a bona-fide RRc in our data. In turn, V20, previously classified as an ab-type RR Lyrae, could not be properly phased with any reasonable period. The properties of the ab-type RR Lyrae stars in M28 appear most consistent with an Oosterhoff-intermediate classification, which is unusual for bona-fide Galactic globulars clusters. However, the clusters c-type variables do not clearly support such an Oosterhoff type, and a hybrid Oosterhoff I/II system is accordingly another possibility, thus raising the intriguing possibility of multiple populations being present in M28. Coordinates, periods, and light curves in differential fluxes are provided for all the detected variables.
We present the results of a search for variable stars in the globular cluster NGC 5286, which has recently been suggested to be associated with the Canis Major dwarf spheroidal galaxy. 57 variable stars were detected, only 19 of which had previously been known. Among our detections one finds 52 RR Lyrae (22 RRc and 30 RRab), 4 LPVs, and 1 type II Cepheid of the BL Herculis type. Periods are derived for all of the RR Lyrae as well as the Cepheid, and BV light curves are provided for all the variables. The mean period of the RRab variables is <Pab> = 0.656 days, and the number fraction of RRc stars is N(c)/N(RR) = 0.42, both consistent with an Oosterhoff II (OoII) type -- thus making NGC 5286 one of the most metal-rich ([Fe/H] = -1.67; Harris 1996) OoII globulars known to date. The minimum period of the RRabs, namely Pab,min = 0.513 d, while still consistent with an OoII classification, falls towards the short end of the observed Pab,min distribution for OoII globular clusters. As was recently found in the case of the prototypical OoII globular cluster M15 (NGC 7078), the distribution of stars in the Bailey diagram does not strictly conform to the previously reported locus for OoII stars. We provide Fourier decomposition parameters for all of the RR Lyrae stars detected in our survey, and discuss the physical parameters derived therefrom. The values derived for the RRcs are not consistent with those typically found for OoII clusters, which may be due to the clusters relatively high metallicity -- the latter being confirmed by our Fourier analysis of the ab-type RR Lyrae light curves. We derive for the cluster a revised distance modulus of (m-M)V = 16.04 mag. (ABRIDGED)
We have found three new LBV candidates in the star-forming galaxy NGC 4736. They show typical well-known LBV spectra, broad and strong hydrogen lines, He I lines, many Fe II lines, and forbidden [Fe II] and [Fe III]. Using archival Hubble Space Telescope and ground-based telescope data, we have estimated the bolometric magnitudes of these objects from -8.4 to -11.5, temperatures, and reddening. Source NGC 4736_1 (Mv = -10.2 +/- 0.1 mag) demonstrated variability between 2005 and 2018 as Delta V = 1.1 mag and Delta B = 0.82 mag, the object belongs to LBV stars. NGC 4736_2 (Mv < -8.6 mag) shows P Cyg profiles and its spectrum has changed from 2015 to 2018. The brightness variability of NGC 4736_2 is Delta V = 0.5 mag and Delta B = 0.4 mag. In NGC 4736_3 (Mv = -8.2 +/- 0.2 mag), we found strong nebular lines, broad wings of hydrogen; the brightness variation is only 0.2 mag. Therefore, the last two objects may reside to LBV candidates.
We use deep images taken with the Advanced Camera for Surveys on board the Hubble Space Telescope of the disk galaxy NGC 891, to search for globular cluster candidates. This galaxy has long been considered to be a close analog in size and structure to the Milky Way and is nearly edge-on, facilitating studies of its halo population. These extraplanar ACS images, originally intended to study the halo field-star populations, reach deep enough to reveal even the faintest globular clusters that would be similar to those in the Milky Way. From the three pointings we have identified a total of 43 candidates after culling by object morphology, magnitude, and colour. We present (V,I) photometry for all of these, along with measurements of their effective radius and ellipticity. The 16 highest-rank candidates within the whole sample are found to fall in very much the same regions of parameter space occupied by the classic Milky Way globular clusters. Our provisional conclusion from this survey is that the total globular cluster population in NGC 891 as a whole may be almost as large as that of the Milky Way.
We search for LBV stars in galaxies outside the Local Group. Here we present a study of two bright $Halpha$ sources in the NGC 247 galaxy. Object j004703.27-204708.4 ($M_V=-9.08 pm 0.15^m$) shows the spectral lines typical for well-studied LBV stars: broad and bright emission lines of hydrogen and helium He I with P Cyg profiles, emission lines of iron Fe II, silicon Si II, nitrogen N II and carbon C II, forbidden iron [Fe II] and nitrogen [N II] lines. The variability of the object is $Delta B = 0.74pm0.09^m$ and $Delta V = 0.88pm0.09^m$, which makes it reliable LBV candidate. The star j004702.18-204739.93 ($M_V=-9.66 pm 0.23^m$) shows many emission lines of iron Fe II, forbidden iron lines [Fe II], bright hydrogen lines with broad wings, and also forbidden lines of oxygen [O I] and calcium [Ca II] formed in the circumstellar matter. The study of the light curve of this star also did not reveal significant variations in brightness ($Delta V = 0.29pm0.09^m$). We obtained estimates of interstellar absorption, the photosphere temperature, as well as bolometric magnitudes $M_text{bol}=-10.5^{+0.5}_{-0.4}$ and $M_text{bol}=-10.8^{+0.5}_{-0.6}$, which corresponds to bolometric luminosities $log(L_text{bol}/L_{odot})=6.11^{+0.20}_{-0.16}$ and $6.24^{+0.20}_{-0.25}$ for j004703.27-204708.4 and j004702.18-204739.93 respectively. Thus, the object j004703.27-204708.4 remains a reliable LBV candidate, while the object j004702.18-204739.93 can be classified as B[e]-supergiant.
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