arXiv subjects

D. M. Bramich

Publications and source records attributed to D. M. Bramich.

161 records · Page 9Linked to original sources

arXiv2006

An Orphan in the "Field of Streams"

We use Sloan Digital Sky Survey Data Release 5 photometry and spectroscopy to study a tidal stream that extends over 50 degrees in the North Galactic Cap. From the analysis of the path of the stream and the colors and magnitudes of its stars, the stream is about 20 kpc away at its nearest detection (the celestial equator). We detect a distance gradient -- the stream is farther away from us at higher declination. The contents of the stream are made up from a predominantly old and metal-poor population that is similar to the globular clusters M13 and M92. The integrated absolute magnitude of the stream stars is estimated to be M_r = -7.5. There istentative evidence for a velocity signature, with the stream moving at -40 km/s at low declinations and +100 km/s at high declinations. The stream lies on the same great circle as Complex A, a roughly linear association of HI high velocity clouds stretching over 30 degrees on the sky, and as Ursa Major II, a recently discovered dwarf spheroidal galaxy. Lying close to the same great circle are a number of anomalous, young and metal-poor globular clusters, including Palomar 1 and Ruprecht 106.

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arXiv2006

Cats and Dogs, Hair and A Hero: A Quintet of New Milky Way Companions

We present five new satellites of the Milky Way discovered in Sloan Digital Sky Survey (SDSS) imaging data, four of which were followed-up with either the Subaru or the Isaac Newton Telescopes. They include four probable new dwarf galaxies -- one each in the constellations of Coma Berenices, Canes Venatici, Leo and Hercules -- together with one unusually extended globular cluster, Segue 1. We provide distances, absolute magnitudes, half-light radii and color-magnitude diagrams for all five satellites. The morphological features of the color-magnitude diagrams are generally well described by the ridge line of the old, metal-poor globular cluster M92. In the last two years, a total of ten new Milky Way satellites with effective surface brightness mu_v >~ 28 mag/sq. arcsec have been discovered in SDSS data. They are less luminous, more irregular and appear to be more metal-poor than the previously-known nine Milky Way dwarf spheroidals. The relationship between these objects and other populations is discussed. We note that there is a paucity of objects with half-light radii between ~40 pc and ~ 100 pc. We conjecture that this may represent the division between star clusters and dwarf galaxies.

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arXiv2006

A Curious New Milky Way Satellite in Ursa Major

In this Letter, we study a localized stellar overdensity in the constellation of Ursa Major, first identified in Sloan Digital Sky Survey (SDSS) data and subsequently followed up with Subaru imaging. Its color-magnitude diagram (CMD) shows a well-defined sub-giant branch, main sequence and turn-off, from which we estimate a distance of ~30 kpc and a projected size of ~250 x 125 pc. The CMD suggests a composite population with some range in metallicity and/or age. Based on its extent and stellar population, we argue that this is a previously unknown satellite galaxy of the Milky Way, hereby named Ursa Major II (UMa II) after its constellation. Using SDSS data, we find an absolute magnitude of M_V \~ -3.8, which would make it the faintest known satellite galaxy. UMa II's isophotes are irregular and distorted with evidence for multiple concentrations; this suggests that the satellite is in the process of disruption.

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arXiv2006

A Faint New Milky Way Satellite in Bootes

In this Letter, we announce the discovery of a new satellite of the Milky Way in the constellation of Bootes at a distance of 60 kpc. It was found in a systematic search for stellar overdensities in the North Galactic Cap using Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The color-magnitude diagram shows a well-defined turn-off, red giant branch, and extended horizontal branch. Its absolute magnitude is -5.8, which makes it one of the faintest galaxies known. The half-light radius is 220 pc. The isodensity contours are elongated and have an irregular shape, suggesting that Boo may be a disrupted dwarf spheroidal galaxy.

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arXiv2006

CCD Photometry of the globular cluster M2. RR Lyrae physical parameters and new variables

We report the results of CCD V and R photometry of the RR Lyrae stars in M2. The periodicities of most variables are revised and new ephemerides are calculated. Light curve decomposition of the RR Lyrae stars was carried out and the corresponding mean physical parameters [Fe/H] = -1.47, Teff = 6276 K, log L = 1.63 Lsun and Mv = 0.71 from nine RRab and [Fe/H] = -1.61, M = 0.54 Msun, Teff = 7215 K, log L = 1.74 Lsun and Mv = 0.71 from two RRc stars were calculated. A comparison of the radii obtained from the above luminosity and temperature with predicted radii from nonlinear convective models is discussed. The estimated mean distance to the cluster is 10.49 +- 0.15 kpc. These results place M2 correctly in the general globular cluster sequences Oosterhoff type, mass, luminosity and temperature, all as a function of the metallicity. Mean relationships for M, log L/Lsun, Teff and Mv as a function of [Fe/H] for a family of globular clusters are offered. These trends are consistent with evolutionary and structural notions on the horizontal branch. Eight new variables are reported.

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arXiv2006

The Origin of the Bifurcation in the Sagittarius Stream

The latest Sloan Digital Sky Survey data reveal a prominent bifurcation in the distribution of debris of the Sagittarius dwarf spheroidal (Sgr) beginning at a right ascension of roughly 190 degrees. Two branches of the stream (A and B) persist at roughly the same heliocentric distance over at least 50 degrees of arc. There is also evidence for a more distant structure (C) well behind the A branch. This paper provides the first explanation for the bifurcation. It is caused by the projection of the young leading (A) and old trailing (B) tidal arms of the Sgr, whilst the old leading arm (C) lies well behind A. This explanation is only possible if the halo is close to spherical, as the angular difference between the branches is a measure of the precession of the orbital plane.

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arXiv2006

The Field of Streams: Sagittarius and its Siblings

We use Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) u,g,r,i,z photometry to study Milky Way halo substructure in the area around the North Galactic Cap. A simple color cut (g-r < 0.4) reveals the tidal stream of the Sagittarius dwarf spheroidal, as well as a number of other stellar structures in the field. Two branches (A and B) of the Sagittarius stream are clearly visible in an RGB-composite image created from 3 magnitude slices, and there is also evidence for a still more distant wrap behind the A branch. A comparison of these data with numerical models suggests that the shape of the Galactic dark halo is close to spherical.

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arXiv2006

A New Milky Way Dwarf Satellite in Canes Venatici

In this Letter, we announce the discovery of a new dwarf satellite of the Milky Way, located in the constellation Canes Venatici. It was found as a stellar overdensity in the North Galactic Cap using Sloan Digital Sky Survey Data Release 5 (SDSS DR5). The satellite's color-magnitude diagram shows a well-defined red giant branch, as well as a horizontal branch. As judged from the tip of the red giant branch, it lies at a distance of ~220 kpc. Based on the SDSS data, we estimate an absolute magnitude of Mv ~ -7.9, a central surface brightness of mu_0,V ~ 28 mag arcsecond^-2, and a half-light radius of \~ 8.5' (~ 550 pc at the measured distance). The outer regions of Canes Venatici appear extended and distorted. The discovery of such a faint galaxy in proximity to the Milky Way strongly suggests that more such objects remain to be found.

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arXiv2006

Microlens OGLE-2005-BLG-169 Implies Cool Neptune-Like Planets are Common

We detect a Neptune mass-ratio (q~8e-5) planetary companion to the lens star in the extremely high-magnification (A~800) microlensing event OGLE-2005-BLG-169. If the parent is a main-sequence star, it has mass M~0.5 M_sun implying a planet mass of ~13 M_earth and projected separation of ~2.7 AU. When intensely monitored over their peak, high-magnification events similar to OGLE-2005-BLG-169 have nearly complete sensitivity to Neptune mass-ratio planets with projected separations of 0.6 to 1.6 Einstein radii, corresponding to 1.6--4.3 AU in the present case. Only two other such events were monitored well enough to detect Neptunes, and so this detection by itself suggests that Neptune mass-ratio planets are common. Moreover, another Neptune was recently discovered at a similar distance from its parent star in a low-magnification event, which are more common but are individually much less sensitive to planets. Combining the two detections yields 90% upper and lower frequency limits f=0.37^{+0.30}_{-0.21} over just 0.4 decades of planet-star separation. In particular, f>16% at 90% confidence. The parent star hosts no Jupiter-mass companions with projected separations within a factor 5 of that of the detected planet. The lens-source relative proper motion is μ~7--10 mas/yr, implying that if the lens is sufficiently bright, I<23.8, it will be detectable by HST by 3 years after peak. This would permit a more precise estimate of the lens mass and distance, and so the mass and projected separation of the planet. Analogs of OGLE-2005-BLG-169Lb orbiting nearby stars would be difficult to detect by other methods of planet detection, including radial velocities, transits, or astrometry.

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arXiv2005

Upper limits on the hot Jupiter fraction in the field of NGC 7789

We describe a method of estimating the abundance of short-period extrasolar planets based on the results of a photometric survey for planetary transits. We apply the method to a 21-night survey with the 2.5m Isaac Newton Telescope of \~32000 stars in a ~0.5 deg by 0.5 deg square field including the open cluster NGC 7789. From the colour-magnitude diagram we estimate the mass and radius of each star by comparison with the cluster main sequence. We search for injected synthetic transits throughout the lightcurve of each star in order to determine their recovery rate, and thus calculate the expected number of transit detections and false alarms in the survey. We take proper account of the photometric accuracy, time sampling of the observations and criteria (signal-to-noise and number of transits) adopted for transit detection. Assuming that none of the transit candidates found in the survey will be confirmed as real planets, we place conservative upper limits on the abundance of planets as a function of planet radius, orbital period and spectral type.

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arXiv2005

A Survey of Eclipsing Binary Stars in the Eastern Spiral Arm of M31

Results of an archival survey are presented using B-band imaging of the eastern spiral arm of M31. Focusing on the eclipsing binary star population, a matched-filter technique has been used to identify 280 binary systems. Of these, 127 systems (98 of which are newly discovered) have sufficient phase coverage to allow accurate orbital periods to be determined. At least nine of these binaries are detached systems which could, in principle, be used for distance determination. The light curves of the detached and other selected systems are presented along with a discussion of some of the more interesting binaries. The impact of unresolved stellar blends on these lightcurves is considered.

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arXiv2005

A Jovian-mass Planet in Microlensing Event OGLE-2005-BLG-071

We report the discovery of a several-Jupiter mass planetary companion to the primary lens star in microlensing event OGLE-2005-BLG-071. Precise (<1%) photometry at the peak of the event yields an extremely high signal-to-noise ratio detection of a deviation from the light curve expected from an isolated lens. The planetary character of this deviation is easily and unambiguously discernible from the gross features of the light curve. Detailed modeling yields a tightly-constrained planet-star mass ratio of q=m_p/M=0.0071+/-0.0003. This is the second robust detection of a planet with microlensing, demonstrating that the technique itself is viable and that planets are not rare in the systems probed by microlensing, which typically lie several kpc toward the Galactic center.

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arXiv2005

A dearth of planetary transits in the direction of NGC 6940

We present results of our survey for planetary transits in the field of NGC 6940. We think nearly all of our observed stars are field stars. We have obtained high precision (3-10 millimags at the bright end) photometric observations of 50,000 stars spanning 18 nights in an attempt to identify low amplitude and short period transit events. We have used a matched filter analysis to identify 14 stars that show multiple events, and four stars that show single transits. Of these 18 candidates, we have identified two that should be further researched. However, none of the candidates are convincing hot Jupiters.

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arXiv2005

A Survey for Planetary Transits in the Field of NGC 7789

We present results from 30 nights of observations of the open cluster NGC 7789 with the WFC camera on the INT telescope in La Palma. From ~900 epochs, we obtained lightcurves and Sloan r-i colours for ~33000 stars, with ~2400 stars with better than 1% precision. We expected to detect ~2 transiting hot Jupiter planets if 1% of stars host such a companion and that a typical hot Jupiter radius is ~1.2RJ. We find 24 transit candidates, 14 of which we can assign a period. We rule out the transiting planet model for 21 of these candidates using various robust arguments. For 2 candidates we are unable to decide on their nature, although it seems most likely that they are eclipsing binaries as well. We have one candidate exhibiting a single eclipse for which we derive a radius of 1.81+0.09-0.00RJ. Three candidates remain that require follow-up observations in order to determine their nature.

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arXiv2004

Structure and star formation in disk galaxies II. Optical imaging

We present optical observations of a sample of 57 spiral galaxies and describe the procedures followed to reduce the data. We have obtained images in the optical B and I broad bands, as well as in H alpha, with moderate spatial resolution and across wide enough fields to image the complete disks of the galaxies. In addition, we observed 55 of our sample galaxies in the R and eight in the V band, and imaged a subset through a dedicated narrow continuum filter for the H alpha line. We describe the data reduction procedures we developed in the course of this work to register, combine and photometrically calibrate each set of images for an individual galaxy. We describe in some detail the procedure used to subtract the continuum emission from our H alpha images. In companion papers, we describe near-infrared imaging of the galaxy sample, and present analyses of disk scale lengths, and of properties of bars, rings, and HII regions in the sample galaxies. The images described here will be made available for use by other researchers through the CDS.

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arXiv2003

Searching For Planetary Transits In The Open Cluster NGC 7789

Open clusters are ideal targets for searching for transiting Hot Jupiters. They provide a relatively large concentration of stars on the sky and cluster members have similar metallicities, ages and distances. Fainter cluster members are likely to show deeper transit signatures, helping to offset sky noise contributions. A survey of open clusters will then help to characterise the Hot Jupiter fraction of main sequence stars, and how this may depend on primordial metallicity and stellar age. We present results from 11 nights of observations of the open cluster NGC 7789 with the WFC camera on the INT telescope in La Palma. From 684 epochs, we obtained lightcurves and B-V colours for ~25600 stars, with ~2400 stars with better than 1% precision. We expect to detect ~1 transiting Hot Jupiter in our sample assuming that 1% of stars host a Hot Jupiter companion.

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arXiv2003

Structure and star formation in disk galaxies I. Sample selection and near infrared imaging

We present near-infrared imaging of a sample of 57 relatively large, Northern spiral galaxies with low inclination. After describing the selection criteria and some of the basic properties of the sample, we give a detailed description of the data collection and reduction procedures. The K_s lambda=2.2 micron images cover most of the disk for all galaxies, with a field of view of at least 4.2 arcmin. The spatial resolution is better than an arcsec for most images. We fit bulge and exponential disk components to radial profiles of the light distribution. We then derive the basic parameters of these components, as well as the bulge/disk ratio, and explore correlations of these parameters with several galaxy parameters.

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