Three new bricks in the wall: Berkeley 23, Berkeley 31, and King 8

A comprehensive census of Galactic open cluster properties places unique constraints on the Galactic disc structure and evolution. In this framework we investigate the evolutionary status of three poorly-studied open clusters, Berkeley 31, Berkeley 23 and King 8, all located in the Galactic anti-centre direction. To this aim, we make use of deep LBT observations, reaching more than 6 mag below the main sequence Turn- Off. To determine the cluster parameters, namely age, metallicity, distance, reddening and binary fraction, we compare the observational colour-magnitude diagrams (CMDs) with a library of synthetic CMDs generated with different evolutionary sets (Padova, FRANEC and FST) and metallicities. We find that Berkeley 31 is relatively old, with an age between 2.3 and 2.9 Gyr, and rather high above the Galactic plane, at about 700 pc. Berkeley 23 and King 8 are younger, with best fitting ages in the range 1.1-1.3 Gyr and 0.8-1.3 Gyr, respectively. The position above the Galactic plane is about 500- 600 pc for the former, and 200 pc for the latter. Although a spectroscopic confirmation is needed, our analysis suggests a sub-solar metallicity for all three clusters.Comment: 17 Pages, Accepted for publication in MNRA

The tidal tails of NGC 2298

We present an implementation of the matched-filter technique to detect tidal tails of globular clusters. The method was tested using SDSS data for the globular cluster Palomar 5 revealing its well known tidal tails. We also ran a simulation of a globular cluster with a tidal tail where we successfully recover the tails for a cluster at the same position and with the same characteristics of NGC 2298. Based on the simulation we estimate that the matched-filter increases the contrast of the tail relative to the background of stars by a factor of 2.5 for the case of NGC 2298. We also present the photometry of the globular cluster NGC 2298 using the MOSAIC2 camera installed on the CTIO 4m telescope. The photometry covers ~ 3deg2 reaching V ~ 23. A fit of a King profile to the radial density profile of NGC 2298 shows that this cluster has a tidal radius of 15.91' \pm 1.07' which is twice as in the literature. The application of the matched-filter to NGC 2298 reveals several extra-tidal structures, including a leading and trailing tail. We also find that NGC 2298 has extra-tidal structures stretching towards and against the Galactic disk, suggesting strong tidal interaction. Finally, we assess how the matched-filter performs when applied to a globular cluster with and without mass segregation taken into account. We find that disregarding the effects of mass segregation may significantly reduce the detection limit of the matched-filter.Comment: 11 pages, 9 figures, 1 table. Accepted for publication on MNRAS main journa

Searching for spiral features in the outer Galactic disk. The field towards WR38 and WR38a

The detailed spiral structure in the outer Galactic disk is still poorly known, and for several Galactic directions we rely on model extrapolations. One of these regions is the fourth Galactic quadrant, in the sector comprised between Vela and Carina (270 <l< 300) where no spiral arms have been detected so far in the optical beyond 270. By means of deep UBVI photometry, we search for spiral features in known low absorption windows.U photometry, although demanding, constitutes a powerful tool to detect and characterize distant aggregates, and allows to derive firmer distance estimates. We studied a direction close to the tangent (l=290) to the Carina arm, in an attempt to detect optical spiral tracers beyond the Carina branch, where radio observations and models predictions indicate the presence of the extension of the Perseus and Norma-Cygnus spiral arms in the fourth quadrant.Along this line of sight, we detect three distinct groups of young stars. Two of them, at 2.5 and 6.0 kpc, belong to the Carina spiral arm (which is crossed twice in this particular direction).The latter is here detected for the first time. The third group, at a distance of 12.7 kpc, is part of the Perseus arm which lies beyond the Carina arm, and constitutes the first optical detection of this arm in the fourth Galactic quadrant. The position of this feature is compatible with HI observations and model predictions. We furthermore present evidence that this extremely distant group, formerly thought to be a star cluster (Shorlin 1), is in fact a diffuse young population. In addition, our data-set does not support the possible presence of the Monoceros Ring toward this direction. This study highlights how multicolor optical studies can be effective to probe the spiral structure in the outer Galactic disk.Comment: 9 pages, 13 eps figure, in press in A&A, abstract rephrased and a few figures degraded in resolution to fit i

SMASH 1 : A VERY FAINT GLOBULAR CLUSTER DISRUPTING in the OUTER REACHES of the LMC?

We present the discovery of a very faint stellar system, SMASH 1, that is potentially a satellite of the Large Magellanic Cloud. Found within the Survey of the Magellanic Stellar History (SMASH), SMASH 1 is a compact (rh=9.1-3.4+5.9pc) and very low luminosity (Mv=-1.0±0.9,Lv=102.3±0.4L⊙ ) stellar system that is revealed by its sparsely populated main sequence and a handful of red giant branch candidate member stars. The photometric properties of these stars are compatible with a metal-poor ([Fe/H]=-2.2) and old (13 Gyr) isochrone located at a distance modulus of ∼18.8, i.e., a distance of . Situated at 11.°3 from the LMC in projection, its three-dimensional distance from the Cloud is 13 kpc, consistent with a connection to the LMC, whose tidal radius is at least . Although the nature of SMASH 1 remains uncertain, its compactness favors it being a stellar cluster and hence dark-matter free. If this is the case, its dynamical tidal radius is only ≲19 pc at this distance from the LMC, and smaller than the system's extent on the sky. Its low luminosity and apparent high ellipticity (ϵ=0.62-0.21+0.17) with its major axis pointing toward the LMC may well be the tell-tale sign of its imminent tidal demise.Peer reviewe

Draco, a flawless dwarf galaxy

The Draco dwarf spheroidal galaxy (dSph), with its apparent immense mass to light ratio and compact size, holds many clues to the nature of the enigmatic dark matter. Here we present deep photometric studies of this dwarf galaxy, undertaken with the MegaCam Camera at the Canada-France-hawaii Telescope, the Wide Field Camera at the Isaac Newton Telescope and the Wide-Field and Planetary Camera on board the Hubble Space Telescope. The new photometric data cover the entirety of the galaxy, and reach i=24.5 at 50% completeness, significantly deeper than previous panoramic studies, allowing searches for tidal disturbances of much lower surface brightness than has been possible before. With these improved statistics, we find no evidence for asymmetric disturbances or tidal tails that possess more than 3% of the stars found within the centre of the galaxy. We find that the central stellar density, as probed by the HST data, rises into the central 0.5'. Uncertainties in the position of the centroid of the galaxy do not allow us to determine whether the apparent flattening of the profile interior to 0.5' is reliable or not. Draco is therefore a flawless dwarf galaxy, featureless and apparently unaffected by Galactic tides.Comment: 13 pages, 16 figures, accepted for publication in MNRA

Employing a New BVIc Photometric Survey of IC 4665 to Investigate the Age of this Young Open Cluster

We present a new, BVIc photometric survey of the young open cluster IC4665, which improves on previous studies of this young cluster by incorporating a rigorous standardization procedure, thus providing high-fidelity colors and magnitudes for cluster members. We use this new photometric dataset to reevaluate the properties (age and distance) of IC4665. Namely, using a statistical approach incorporating Tau^2 CMD modeling, we measure a pre-main-sequence isochrone age and distance of 36+-9 Myr and 360+-12 pc, as well as a upper-main-sequence turn-off age and distance of 42+-12 Myr and 357+-12 pc. These ages and distances are highly dependent on the isochrone model and color used for the fitting procedure, with a possible range of ~10-20 Myr in age and ~20 pc in distance. This spread in calculated ages and distances seen between colors and models is likely due to limitations in the individual membership catalogs and/or systematic differences in the predicted stellar parameters from the different sets of models. Interestingly, when we compare the isochrone ages for IC4665 to the published lithium depletion boundary age, 28+-5 Myr, we observe that this cluster does not appear to follow the trend of isochrone ages being 1.5 times smaller than lithium depletion boundary ages. In addition, comparing the overall magnetic activity (X-ray and H-alpha emission) in IC4665 with other well studied open clusters, we find the observed activity distributions for this young cluster are best characterized by assuming an age of 30-40 Myr, thus in agreement with our pre-main-sequence and turn-off isochrone ages for IC4665. Overall, although some age discrepancies do exist, particularly in the ages measured from pre-main-sequence isochrones, the range of possible IC4665 ages derived from the various dating techniques employed here is relatively small compared to that found for other well studied open clusters.Comment: 17 pages, 13 figures, 6 tables; Accepted for publication in the Astronomical Journa

Prospects for multiwavelength polarization observations of GRB afterglows and the case GRB 030329

We explore the prospects for simultaneous, broad-band, multiwavelength polarimetric observations of GRB afterglows. We focus on the role of cosmic dust in GRB host galaxies on the observed percentage polarization of afterglows in the optical/near-infrared bands as a function of redshift. Our driving point is the afterglow of GRB 030329, for which we obtained polarimetric data in the R band and K band simultaneously about 1.5 days after the burst. We argue that polarimetric observations can be very sensitive to dust in a GRB host, because dust can render the polarization of an afterglow wavelength-dependent. We discuss the consequences for the interpretation of observational data and emphasize the important role of very early polarimetric follow-up observations in all bands, when afterglows are still bright, to study the physical properties of dust and magnetic fields in high-z galaxies.Comment: accepted for publication in Astronomy & Astrophysic

An Ancient Metal-Poor Population in M32, and Halo Satellite Accretion in M31, Identified by RR Lyrae Stars

We present time-series photometry of two fields near M32 using archival observations from ACS/WFC onboard HST. One field is centered about 2 arcmin from M32 while the other is located 15 arcmin to the southeast of M31. We identify a total of 1139 RR Lyrae variables of which 821 are ab-type and 318 are c-type. In the field near M32, we find a radial gradient in the density of RR Lyraes relative to the center of M32. This gradient is consistent with the surface brightness profile of M32 suggesting that a significant number of the RR Lyraes in this region belong to M32. This provides further confirmation that M32 contains an ancient stellar population formed around the same time as the oldest population in M31 and the Milky Way. The RR Lyrae stars in M32 exhibit a mean metal abundance of [Fe/H] ~ -1.42 +/- 0.02, which is ~15 times lower than the metal abundance of the overall M32 stellar population. Moreover, the abundance of RR Lyrae stars normalized to the luminosity of M32 in the field analyzed further indicates that the ancient metal-poor population in M32 represents only a very minor component of this galaxy, consistent with the 1% to 4.5% in mass inferred from the CMD analysis of Monachesi et al. In the other field, we find unprecedented evidence for two populations of RR Lyraes in M31 as shown by two distinct sequences among the ab-type variables in the Bailey Diagram. When interpreted in terms of metal abundance, one population exhibits a peak at [Fe/H] ~ -1.3 and the other is at [Fe/H] ~ -1.9. One possible interpretation of this result is that the more metal-rich population represents the dominant M31 halo, while the metal-poorer group could be a disrupted dwarf satellite galaxy orbiting M31. If true, this represents a further indication that the formation of the M31 spheroid has been significantly influenced by the merger and accretion of dwarf galaxy satellites. [abridged]Comment: 22 pages, 17 figures, accepted for publication in the MNRA

J08069+1527: A newly discovered high amplitude, hybrid subdwarf B pulsator

We present our discovery of a new hybrid pulsating subdwarf B star, J08069+1527. The effective temperature and surface gravity of 28,500$\pm$400\,K and 5.37$\pm$0.04\,dex, respectively, place this object inside the instability strip and also among other pulsating hot subdwarfs of a hybrid nature, right next to another fascinating star: Balloon\,090100001. From this proximity, we anticipated this star could pulsate in both high and low frequency modes. Indeed, our analysis of photometric data confirmed our prediction. We detected two peaks in the high frequency region and two other peaks at low frequencies. In addition, the amplitude of the dominant mode is very high and comparable to the dominant peaks in other hybrid subdwarf B stars. Since this star is bright, we performed time-series low resolution spectroscopy. Despite a low signal-to-noise (S/N) ratio, we were able to detect the main peak from these data. All our results strongly indicate that J08069+1527 is a high amplitude pulsating hot subdwarf B star of hybrid nature. By analogy to the other pulsating sdB star, we judge that the dominant mode we detected here has radial nature. Future stellar modeling should provide us with quite good constrains as p- and g-modes presented in this star are driven in different parts of its interior.Comment: 7 pages, 10 figures, accepted for publication in MNRA

Long-term photometric monitoring of RR Lyr stars in M3

The period-change behaviour of 134 RR Lyrae stars in the globular cluster Messier 3 (M3) is investigated on the ~120-year time base of the photometric observations. The mean period-change rates (\beta \approx 0.01 d Myr^-1) of the subsamples of variables exhibiting the most regular behaviour are in good agreement with theoretical expectations based on Horizontal-Branch stellar evolution models. However, a large fraction of variables show period changes that contradict the evolutionary expectations. Among the 134 stars studied, the period-change behaviour of only 54 variables is regular (constant or linearly changing), slight irregularities are superimposed on the regular variations in 23 cases and the remaining 57 stars display irregular period variations. The light curve of ~50 per cent of the RRab stars is not stable, i.e., these variables exhibit Blazhko modulation. The large fraction of variables with peculiar behaviour (showing light-curve modulation and/or irregular O-C variation) indicate that, probably, variables with regular period changes incompatible with their evolutionary stages also could display some kind of instability of the pulsation light curve and/or period, but the available observations have not disclosed it yet. The temporal appearence of the Blazhko effect in some stars, and the 70-90 years long regular changes preceded or followed by irregular, rapid changes of the pulsation period in some cases support this hypothesis. [...] Abstract truncated due to the limitations of astroph. See full abstract in the paper.Comment: 22 pages, 14 figures, accepted for publication in MNRA