The discovery of Segue 2: a prototype of the population of satellites of satellites

We announce the discovery of a new Milky Way satellite Segue 2 found in the data of the Sloan Extension for Galactic Understanding and Exploration (SEGUE). We followed this up with deeper imaging and spectroscopy on the Multiple Mirror Telescope (MMT). From this, we derive a luminosity of   M _v =−2.5  , a half-light radius of 34 pc and a systemic velocity of  ∼−40 km s⁻¹  . Our data also provide evidence for a stream around Segue 2 at a similar heliocentric velocity, and the SEGUE data show that it is also present in neighbouring fields. We resolve the velocity dispersion of Segue 2 as 3.4 km s⁻¹ and the possible stream as  ∼7 km s⁻¹  . This object shows points of comparison with other recent discoveries, Segue 1, Boo II and Coma. We speculate that all four objects may be representatives of a population of satellites of satellites – survivors of accretion events that destroyed their larger but less dense parents. They are likely to have formed at redshifts   z > 10  and are good candidates for fossils of the reionization epoch.     

Metal-rich carbon stars in the Sagittarius dwarf spheroidal galaxy

We present spectroscopic observations from the Spitzer Space Telescope of six carbon-rich asymptotic giant branch (AGB) stars in the Sagittarius dwarf spheroidal galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C₂H₂ and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the Large Magellanic Cloud, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the dusty radiative transfer model and determine their dust mass-loss rates to be in the range  1.0–3.3 × 10⁻⁸ M_⊙ yr⁻¹  . The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars shows the strongest SiC feature in our present Local Group sample.     

Distant field blue horizontal branch stars – III. Identification of a probable outer halo stream at Galactocentric distance r= 70 kpc

We present FOcal Reducer/low dispersion Spectrograph-1 spectra (from the European Southern Observatory's Very Large Telescope) of a sample of 34 faint  20.0 < g * < 21.1  A-type stars selected from the Sloan Digital Sky Survey Early Data Release, with the goal of measuring the velocity dispersion of blue horizontal branch (BHB) stars in the remote Galactic halo,   R ∼ 80 kpc  . We show that colour selection with  1.08 < u *− g * < 1.40  and  −0.2 < g *− r * < −0.04  minimizes contamination of the sample by less luminous blue stragglers. In classifying the stars we confine our attention to the 20 stars with spectra of signal-to-noise ratio >15 Å⁻¹. Classification produces a sample of eight BHB stars at distances  65–102 kpc  from the Sun (mean 80 kpc), which represents the most distant sample of Galactic stars with measured radial velocities. The dispersion of the measured radial component of the velocity with respect to the centre of the Galaxy is  58 ± 15 km s⁻¹  . This value is anomalously low in comparison with measured values for stars at smaller distances, as well as for satellites at similar distances. Seeking an explanation for the low measured velocity dispersion, further analysis reveals that six of the eight remote BHB stars are plausibly associated with a single orbit. Three previously known outer halo carbon stars also appear to belong to this stream. The velocity dispersion of all nine stars relative to the orbit is only  15 ± 4 km s⁻¹  . Further observations along the orbit are required to trace the full extent of this structure on the sky.     

Late superhumps and the stream–disc impact in IY UMa

We present two simple dynamical models for Sagittarius based on N -body simulations of the progressive disruption of a satellite galaxy orbiting for 12.5 Gyr within a realistic Galactic potential. In both models the satellite initially has observable properties similar to those of current outlying dwarfs; in one case it is purely stellar while in the other it is embedded in an extended massive halo. The purely stellar progenitor is a King model with a total velocity dispersion of 18.9 km s⁻¹, a core radius of 0.44 kpc and a tidal radius of 3 kpc. The initial stellar distribution in the other case follows a King profile with the same core radius, a slightly larger total velocity dispersion and similar extent. Both these models are consistent with all published data on the current Sagittarius system, they match not only the observed properties of the main body of Sagittarius, but also those reported for unbound debris at larger distances.     

Variable stars in the field of the old open cluster Melotte 66

We report the results of photometric monitoring of the Melotte 66 field in BVI filters. 10 variables were identified with nine being new discoveries. The sample includes eight eclipsing binaries of which four are W UMa type stars and one star is a candidate blue straggler. All four contact binaries are likely the members of the cluster based on their estimated distances. 10 blue stars with   U − B < −0.3  were detected inside a  14.8 × 22.8 arcmin²  field centred on the cluster. Time series photometry for seven of them showed no evidence for any variability. The brightest object in the sample of blue stars is a promising candidate for a hot subdwarf belonging to the cluster. We show that the anomalously wide main sequence of the cluster, reported in some earlier studies, results from a combination of two effects: variable reddening occurring across the cluster field and the presence of a rich population of binary stars in the cluster itself. The density profile of the cluster field is derived and the total number of the member stars with  16 < V < 21  or  2.8 < M _V < 7.8  is estimated conservatively at about 1100.     

Hyperfast pulsars as the remnants of massive stars ejected from young star clusters

Recent proper motion and parallax measurements for the pulsar PSR B1508+55 indicate a transverse velocity of  ∼1100 km s⁻¹  , which exceeds earlier measurements for any neutron star. The spin-down characteristics of PSR B1508+55 are typical for a non-recycled pulsar, which implies that the velocity of the pulsar cannot have originated from the second supernova disruption of a massive binary system. The high velocity of PSR B1508+55 can be accounted for by assuming that it received a kick at birth or that the neutron star was accelerated after its formation in the supernova explosion. We propose an explanation for the origin of hyperfast neutron stars based on the hypothesis that they could be the remnants of a symmetric supernova explosion of a high-velocity massive star which attained its peculiar velocity (similar to that of the pulsar) in the course of a strong dynamical three- or four-body encounter in the core of dense young star cluster. To check this hypothesis, we investigated three dynamical processes involving close encounters between: (i) two hard massive binaries, (ii) a hard binary and an intermediate-mass black hole (IMBH) and (iii) a single stars and a hard binary IMBH. We find that main-sequence O-type stars cannot be ejected from young massive star clusters with peculiar velocities high enough to explain the origin of hyperfast neutron stars, but lower mass main-sequence stars or the stripped helium cores of massive stars could be accelerated to hypervelocities. Our explanation for the origin of hyperfast pulsars requires a very dense stellar environment of the order of  10⁶– 10⁷ stars pc⁻³  . Although such high densities may exist during the core collapse of young massive star clusters, we caution that they have never been observed.     

An explanation for long flares from extragalactic globular cluster X-ray sources

We compare orbits in a thin axisymmetric disc potential in Modified Newtonian Dynamics (MOND) with those in a thin disc plus near-spherical dark matter halo predicted by a ΛCDM cosmology. Remarkably, the amount of orbital precession in MOND is nearly identical to that which occurs in a mildly oblate CDM Galactic halo (potential flattening   q = 0.9  ), consistent with recent constraints from the Sagittarius stream. Since very flattened mass distributions in MOND produce rounder potentials than in standard Newtonian mechanics, we show that it will be very difficult to use the tidal debris from streams to distinguish between a MOND galaxy and a standard CDM galaxy with a mildly oblate halo.
If a galaxy can be found with either a prolate halo or one that is more oblate than   q ∼ 0.9  this would rule out MOND as a viable theory. Improved data from the leading arm of the Sagittarius dwarf – which samples the Galactic potential at large radii – could rule out MOND if the orbital pole precession can be determined to an accuracy of the order of  ±1°  .     

Rotation and activity in the solar-type stars of NGC 2547

We present high-resolution spectroscopy of a sample of 24 solar-type stars in the young (15–40 Myr), open cluster NGC 2547. We use our spectra to confirm cluster membership in 23 of these stars, to determine projected equatorial velocities and chromospheric activity, and to search for the presence of accretion discs. We find examples of both fast ( v _e sin  i >50 km s⁻¹) and slow ( v _e sin  i <10 km s⁻¹) rotators, but no evidence for active accretion in any of the sample. The distribution of projected rotation velocities is indistinguishable from the slightly older IC 2391 and IC 2602 clusters, implying similar initial angular momentum distributions and circumstellar disc lifetimes. The presence of very slow rotators indicates either that long (10–40 Myr) disc lifetimes or internal differential rotation are needed, or that NGC 2547 (and IC 2391/2602) were born with more slowly rotating stars than are presently seen in even younger clusters and associations. The solar-type stars in NGC 2547 follow a similar rotation–activity relationship to that seen in older clusters. X-ray activity increases until a saturation level is reached for v _e sin  i >15–20 km s⁻¹. We are unable to explain why this saturation level, of log( L _x L _bol)≃−3.3, is a factor of 2 lower than in other clusters, but rule out anomalously slow rotation rates or uncertainties in X-ray flux calculations.     

On the origin of high-velocity runaway stars

We explore the hypothesis that some high-velocity runaway stars attain their peculiar velocities in the course of exchange encounters between hard massive binaries and a very massive star (either an ordinary  50–100 M_⊙  star or a more massive one, formed through runaway mergers of ordinary stars in the core of a young massive star cluster). In this process, one of the binary components becomes gravitationally bound to the very massive star, while the second one is ejected, sometimes with a high speed. We performed three-body scattering experiments and found that early B-type stars (the progenitors of the majority of neutron stars) can be ejected with velocities of  ≳200–400 km s⁻¹  (typical of pulsars), while  3–4 M_⊙  stars can attain velocities of  ≳300–400 km s⁻¹  (typical of the bound population of halo late B-type stars). We also found that the ejected stars can occasionally attain velocities exceeding the Milky Ways's escape velocity.     

A stochastic Monte Carlo approach to model real star cluster evolution – II. Self-consistent models and primordial binaries

The new approach outlined in Paper I to follow the individual formation and evolution of binaries in an evolving, equal point-mass star cluster is extended for the self-consistent treatment of relaxation and close three- and four-body encounters for many binaries (typically a few per cent of the initial number of stars in the cluster mass). The distribution of single stars is treated as a conducting gas sphere with a standard anisotropic gaseous model. A Monte Carlo technique is used to model the motion of binaries, their formation and subsequent hardening by close encounters, and their relaxation (dynamical friction) with single stars and other binaries. The results are a further approach towards a realistic model of globular clusters with primordial binaries without using special hardware. We present, as our main result, the self-consistent evolution of a cluster consisting of 300 000 equal point-mass stars, plus 30 000 equal-mass binaries over several hundred half-mass relaxation times, well into the phase where most of the binaries have been dissolved and evacuated from the core. The cluster evolution is about three times slower than found by Gao et al. Other features are rather comparable. At every moment we are able to show the individual distribution of binaries in the cluster.     

Optical spectroscopy of X-Mega targets – I. CPD −59° 2635: a new double-lined O-type binary in the Carina Nebula

Optical spectroscopy of CPD −59° 2635, one of the O-type stars in the open cluster Trumpler 16 in the Carina Nebula, reveals this star to be a double-lined binary system. We have obtained the first radial velocity orbit for this system, consisting of a circular solution with a period of 2.2999 d and semi-amplitudes of 208 and 273 km s⁻¹. This results in minimum masses of 15 and 11 M_⊙ for the binary components of CPD −59° 2635, which we classified as O8V and O9.5V, although spectral type variations of the order of 1 subclass, which we identify as the Struve–Sahade effect , seem to be present in both components. From ROSAT HRI observations of CPD −59° 2635 we determine a luminosity ratio log( L _x/ L _bol)≈−7 , which is similar to that observed for other O-type stars in the Carina Nebula region. No evidence of light variations is present in the available optical or X-ray data sets.     

OB association members in the ACT and TRC catalogues

The Hipparcos Catalogue contains members of nearby OB associations brighter than 12th magnitude in V . However, membership lists are complete only to magnitude V =7.3. In this paper we discuss whether proper motions listed in the 'Astrographic Catalogue+Tycho' reference catalogue (ACT) and the Tycho Reference Catalogue (TRC), which are complete to V ∼10.5 mag, can be used to find additional association members. Proper motions in the ACT/TRC have an average accuracy of ∼3 mas yr⁻¹. We search for ACT/TRC stars which have proper motions consistent with the spatial velocity of the Hipparcos members of the nearby OB associations already identified by de Zeeuw et al. These stars are first selected using a convergent-point method, and then subjected to further constraints on the proper-motion distribution, magnitude and colour to narrow down the final number of candidate members. Monte Carlo simulations show that the proper-motion distribution, magnitude, and colour constraints remove ∼97 per cent of the field stars, while at the same time retain more than 90 per cent of the cluster stars.
The procedure has been applied to five nearby associations: the three subgroups of Sco OB2, plus Per OB3 and Cep OB6. In all cases except Cep OB6, we find evidence for new association members fainter than the completeness limit of the Hipparcos Catalogue. However, narrow-band photometry and/or radial velocities are needed to pinpoint the cluster members, and to study their physical characteristics.     

Supernova kicks and misaligned Be star binaries

Be stars are rapidly spinning B stars surrounded by an outflowing disc of gas in Keplerian rotation. Be star/X-ray binary systems contain a Be star and a neutron star. They are found to have non-zero eccentricities and there is evidence that some systems have a misalignment between the spin axis of the star and the spin axis of the binary orbit. The eccentricities in these systems are caused by a kick to the neutron star during the supernova that formed it. Such kicks would also give rise to misalignments. In this paper, we investigate the extent to which the same kick distribution can give rise to both the observed eccentricity distribution and the observed misalignments. We find that a Maxwellian distribution of velocity kicks with a low velocity dispersion,  σ_k≈ 15 km s⁻¹  , is consistent with the observed eccentricity distribution but is hard to reconcile with the observed misalignments, typically   i ≥ 25°  . Alternatively, a higher velocity kick distribution,  σ_k= 265 km s⁻¹  , is consistent with the observed misalignments but not with the observed eccentricities, unless post-supernova circularization of the binary orbits has taken place. We discuss briefly how this might be achieved.     

A search for the progenitors of two Type Ia Supernovae in NGC 1316

Recent evidence of a young progenitor population for many Type Ia supernovae (SNe Ia) raises the possibility that evolved intermediate-mass progenitor stars may be detected in pre-explosion images. NGC 1316, a radio galaxy in the Fornax cluster, is a prolific producer of SNe Ia, with four detected since 1980. We analyse Hubble Space Telescope ( HST ) pre-explosion images of the sites of two of the SNe Ia that exploded in this galaxy, SN2006dd (a normal Type Ia) and SN2006mr (likely a subluminous, 1991bg-like, SN Ia). Astrometric positions are obtained from optical and near-infrared ground-based images of the events. We find no candidate point sources at either location, and set upper limits on the flux in B, V and I from any such progenitors. We also estimate the amount of extinction that could be present, based on analysis of the surface-brightness inhomogeneities in the HST images themselves. At the distance of NGC 1316, the limits correspond to absolute magnitudes of  ∼−5.5, −5.4  and −6.0 mag in   M _B , M _V   and   M _I   , respectively. Comparison to stellar evolution models argues against the presence at the supernova sites, 3 yr prior to the explosion, of normal stars with initial masses  ≳6 M_⊙  at the tip of their asymptotic-giant branch (AGB) evolution, young post-AGB stars that had initial masses  ≳4 M_⊙  and post-red giant stars of initial masses  ≳9 M_⊙  .     

Velocity dispersions of dwarf spheroidal galaxies: dark matter versus MOND

We present predictions for the line-of-sight velocity dispersion profiles of dwarf spheroidal galaxies and compare them to observations in the case of the Fornax dwarf. The predictions are made in the framework of standard dynamical theory of spherical systems with different velocity distributions. The stars are assumed to be distributed according to Sérsic laws with parameters fitted to observations. We compare predictions obtained assuming the presence of dark matter haloes (with density profiles adopted from N -body simulations) with those resulting from Modified Newtonian Dynamics (MOND). If the anisotropy of velocity distribution is treated as a free parameter, observational data for Fornax are reproduced equally well by models with dark matter and with MOND. If stellar mass-to-light ratio of 1 M_⊙/L_⊙ is assumed, the required mass of the dark halo is     , two orders of magnitude larger than the mass in stars. The derived MOND acceleration scale is     . In both cases a certain amount of tangential anisotropy in the velocity distribution is needed to reproduce the shape of the velocity dispersion profile in Fornax.     

Accretion-driven core collapse and the collisional formation of massive stars

We consider the conditions required for a cluster core to shrink, by adiabatic accretion of gas from the surrounding cluster, to densities such that stellar collisions are a likely outcome. We show that the maximum densities attained, and hence the viability of collisions, depend on the balance between core shrinkage (driven by accretion) and core puffing up (driven by relaxation effects). The expected number of collisions scales as     , where N _core is the number of stars in the cluster core and     is the free-fall velocity of the parent cluster (gas reservoir). Thus, whereas collisions are very unlikely in a relatively low-mass, low-internal-velocity system such as the Orion Nebula Cluster, they become considerably more important at the mass and velocity scales characteristic of globular clusters. Thus, stellar collisions in response to accretion-induced core shrinkage remain a viable prospect in more massive clusters, and may contribute to the production of intermediate-mass black holes in these systems.     

Single star scattering on an open cluster

We examine the scattering of single stars from an open star cluster. The probability of the capture of a star by a star cluster is dependent on the velocity and mass of the star, and the stars that are not captured experience a velocity change. For low-velocity stars there is an exponential decrease of the capture probability with the initial velocity, and the velocity change decreases almost linearly. For high-velocity stars there is a v ⁻⁶ dependence for the capture probability, and a v ⁻¹ dependence for the velocity change. Analytical estimations, Monte Carlo and full N -body simulations are all in good agreement.     

Optical spectroscopy of XMEGA targets in the Carina Nebula – III. The multiple system Tr 16-104 (≡CPD −59° 2603)

We discuss the orbital elements of the multiple system Tr 16-104 which is usually believed to be a member of the open cluster Trumpler 16 in the Carina complex. We show that Tr 16-104 could be a hierarchical triple system consisting of a short-period (2.15 d) eclipsing O7 V+O9.5 V binary bound to a B0.2 IV star. Our preliminary orbital solution of the third body indicates that the B star most probably describes an eccentric orbit with a period of ∼285 or ∼1341 d around the close binary. Folding photometric data from the literature with our new ephemerides, we find that the light curve of the close binary exhibits rather narrow eclipses indicating that the two O stars must be well inside their Roche lobes. Our analysis of the photometric data yields a lower limit on the inclination of the orbit of the close binary of i ≥77° . The stellar radii and luminosities of the O7 V and O9.5 V stars are significantly smaller than expected for stars of this spectral type. Our results suggest that Tr 16-104 lies at a distance of the order of 2.5 kpc and support a fainter absolute magnitude for zero-age main-sequence O stars than usually adopted. We find that the dynamical configuration of Tr 16-104 corresponds to a hierarchical system that should remain stable provided that it suffers no strong perturbation. Finally, we also report long-term temporal variations of high-velocity interstellar Ca  ii absorptions in the line of sight towards Tr 16-104.     

The globular cluster system of the Sagittarius dwarf spheroidal galaxy: the age of Terzan 8

We present deep V - and I -band CCD photometry of the globular cluster Terzan 8, recently found to be a member of the globular cluster system of the Sagittarius dwarf spheroidal galaxy. We accurately estimate the metallicity of Terzan 8, and provide the first direct determination of the colour excess toward this cluster. Our robust age estimate confirms that this cluster is indeed coeval with typical Galactic globulars of comparable metal content, and thus it is probably significantly older than at least two other Sagittarius clusters, Terzan 7 and Arp 2. The implications of this result on the star formation history of the Sagittarius galaxy are briefly discussed.