We present the results of deep imaging obtained at the Canada-France-Hawaii Telescope with MegaCam in the anticenter direction at two different heights above the Galactic disk. We detect the presence ...of the Monoceros ring in both fields as a conspicuous and narrow main-sequence feature which dominates star counts over a large portion of the color-magnitude diagram down to g' ~ 24. The comparison of the morphology and density of this feature with a large variety of Galactic models excludes the possibility that it can be due to a flare of the Galactic disk, supporting an extra-Galactic origin for this ring-like structure.
We examine the star-forming history of the M31 disk during the past few hundred Myr. The luminosity functions (LFs) of main-sequence stars at distances R sub(GC) > 21 kpc (i.e., >4 disk scale ...lengths) are matched by models that assume a constant star formation rate (SFR). However, at smaller R sub(GC) the LFs suggest that during the past ~10 Myr the SFR was 2-3 times higher than during the preceding ~100 Myr. The rings of cool gas that harbor a significant fraction of the current star-forming activity are traced by stars with ages ~100 Myr, indicating that (1) these structures have ages of at least 100 Myr and (2) stars in these structures do not follow the same relation between age and random velocity as their counterparts throughout the disks of other spiral galaxies, probably due to the inherently narrow orbital angular momentum distribution of the giant molecular clouds in these structures. The distribution of evolved red stars is not azimuthally symmetric, in the sense that the projected density along the northeast segment of the major axis is roughly twice that on the opposite side of the galaxy. The northeast arm of the major axis thus appears to be a fossil star-forming area that dates to intermediate epochs. Such a structure may be the consequence of interactions with a companion galaxy.
Two 5 deg2 regions around the NGC 7332/9 galaxy pair and the isolated galaxy NGC 1156 have been mapped in the 21 cm line of neutral hydrogen (H I) with the Arecibo L-band Feed Array out to a redshift ...of ~0.065 (~20,000 km s--1) as part of the Arecibo Galaxy Environment Survey. One of the aims of this survey is to investigate the environment of galaxies by identifying dwarf companions and interaction remnants; both of these areas provide the potential for such discoveries. The neutral hydrogen observations were complemented by optical and radio follow-up observations with a number of telescopes. A total of 87 galaxies were found, of which 39 (45%) were previously cataloged and 15 (17%) have prior redshifts. Two dwarf galaxies have been discovered in the NGC 7332 group and a single dwarf galaxy in the vicinity of NGC 1156. A parallel optical search of the area revealed one further possible dwarf galaxy near NGC 7332.
Stellar ages have traditionally been used to constrain cosmological models by setting lower limits to the age of the universe. We quickly review the basic cosmological concepts that underpin the ...derivation of a finite age, and the constraints given by the measure of cosmological parameters at a redshift z ∼ 1000. The most direct way of measuring stellar ages through the abundance of radioactive elements is shown to suffer from systematics in the evaluation of the production ratios, to the extent that the constrains are less stringent that previously thought. The discrepancy between the ages of the oldest stars and the cosmologically inferred age of the universe may well be dominated by systematc effects in the predicted evolution of low-mass stars.
We introduce a new method for solving maximum likelihood problems through variational calculus, and apply it to the case of recovering an unknown star formation history, SFR(t), from the resulting ...Hertzsprung—Russell (HR) diagram. This approach allows a totally non-parametric solution, which has the advantage of requiring no initial assumptions about SFR(t). As a full maximum likelihood statistical model is used, and we take advantage of all the information available in the HR diagram, rather than concentrating on particular features such as turn-off points or luminosity functions. We test the method using a series of synthetic HR diagrams produced from known SFR(t), and find it to be quite successful under noise conditions comparable to those present in current observations. At this point we restrict the analysis to situations in which the metallicity of the system is known, as is the case with the resolved population of dwarf spheroidal companions to the Milky Way or the solar neighbourhood Hipparcos data. We also include tests to quantify the way uncertainties in the assumed metallicity, binary fraction and initial mass function (IMF) affect our inferences.
The detection of transits is an efficient technique to uncover faint companions around stars. The full characterisation of the companions (M-type stars, brown dwarfs or exoplanets) requires ...high-resolution spectroscopy to measure properly masses and radii. With the advent of massive variability surveys over wide fields, the large number of possible candidates makes such a full characterisation for all of them impractical. We explore here a fast technique to pre-select the most promising candidates using either near-IR photometry or low resolution spectroscopy. We develop a new method based on the well-calibrated surface brightness relation along with the correlation between mass and luminosity for main sequence stars, so that not only can giant stars be excluded but also accurate effective temperatures and radii measured. The main source of uncertainty arises from the unknown dispersion of extinction at a given distance. We apply this technique to our observations of a sample of 34 stars extracted from the 62 low-depth transits identified by OGLE during their survey of some 105 stars in the Carina fields of the Galactic disc. We infer that at least 78% of the companions of the stars which are well characterised in this sample are not exoplanets. Stars OGLE-TR-105, OGLE-TR-109 and OGLE-TR-111 are the likeliest to host exoplanets and deserve high-resolution follow-up studies. Most recently, OGLE-TR-111 was confirmed as an exoplanet with $M_{\rm planet} \cong 0.53 \pm 0.11 ~M_{\rm Jup}$ (Pont et al. CITE), confirming the efficiency of our method in pre-selecting reliable planetary transit candidates.
► We propose theoretical fragile binary properties: separation,
q-ratio, and period. ► We generate theoretical distributions of fragile binary populations. ► Theoretical distributions are evolved and ...compared to observed distributions. ► Resulting comparisons of evolved pairs show similarities. ► Resulting distributions of theoretical fragile binary properties provide insight to observed distributions.
The secular stellar mass loss causes an amplification of the orbital separation in fragile, common proper motion, binary systems with separations of the order of 1000 A.U. In these systems, companions evolve as two independent coeval stars as they experience negligible mutual tidal interactions or mass transfer. We present models for how post-main sequence mass-loss statistically distorts the frequency distribution of separations in fragile binaries. These models demonstrate the expected increase in orbital separation resulting from stellar mass-loss, as well as a perturbation of associated orbital parameters. Comparisons between our models and observations resulting from the Luyten survey of wide visual binaries, specifically those containing MS and white-dwarf pairs, demonstrate a good agreement between the calculated and the observed angular separation distribution functions.
POINT—AGAPE is an Anglo-French collaboration which is employing the Isaac Newton Telescope (INT) to conduct a pixel-lensing survey towards M31. Pixel lensing is a technique which permits the ...detection of microlensing against unresolved stellar fields. The survey aims to constrain the stellar population in M31, and also the distribution and nature of massive compact halo objects (MACHOs) in both M31 and the Galaxy. In this paper we investigate what we can learn from pixel-lensing observables about the MACHO mass and fractional contribution in M31 and the Galaxy for the case of spherically symmetric, near-isothermal haloes. We employ detailed pixel-lensing simulations which include many of the factors that affect the observables, such as non-uniform sampling and signal-to-noise ratio degradation owing to changing observing conditions. For a maximum MACHO halo we predict an event rate in V of up to 100 per observing season for M31 and 40 per season for the Galaxy. However, the Einstein radius crossing time is measurable for less than 10 per cent of the events, and the observed full-width at half-maximum duration provides only a weak tracer of lens mass. None the less, we find that the near—far asymmetry in the spatial distribution of M31 MACHOs provides significant information on their mass and density contribution. We present a likelihood estimator for measuring the fractional contribution and mass of both M31 and Galaxy MACHOs, which permits an unbiased determination to be made of MACHO parameters, even from data sets strongly contaminated by variable stars. If M31 does not have a significant population of MACHOs in the mass range 0.001−1 M⊙, strong limits will result from the first season of INT observations. Simulations based on currently favoured density and mass values indicate that, after three seasons, the M31 MACHO parameters should be constrained to within a factor of 4 uncertainty in halo fraction and an order of magnitude uncertainty in mass (90 per cent confidence). Interesting constraints on Galaxy MACHOs may also be possible. For a campaign lasting 10 years, comparable to the lifetime of current LMC surveys, reliable estimates of MACHO parameters in both galaxies should be possible.
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