Steven M. Crawford

Total Galaxy Magnitudes and Effective Radii from Petrosian Magnitudes and Radii

Petrosian magnitudes were designed to help with the difficult task of determining a galaxys total light. Although these magnitudes [taken here as the flux within 2RP, with the inverted Petrosian index 1/?(RP) = 0.2] can represent most of an objects flux, they do of course miss the light outside the Petrosian aperture (2RP). The size of this flux deficit varies monotonically with the shape of a galaxys light profile, i.e., its concentration. In the case of a de Vaucouleurs R1/4 profile, the deficit is 0.20 mag; for an R1/8 profile this figure rises to 0.50 mag. Here we provide a simple method for recovering total (S?rsic) magnitudes from Petrosian magnitudes using only the galaxy concentration (R90/R50 or R80/R20) within the Petrosian aperture. The corrections hold to the extent that S?rsics model provides a good description of a galaxys luminosity profile. We show how the concentration can also be used to convert Petrosian radii into effective half-light radii, enabling a robust measure of the mean effective surface brightness. Our technique is applied to the Sloan Digital Sky Survey Data Release 2 (SDSS DR2) Petrosian parameters, yielding good agreement with the total magnitudes, effective radii, and mean effective surface brightnesses obtained from the New York University Value-Added Galaxy Catalog S?rsic R1/n fits by Blanton and coworkers. Although the corrective procedure described here is specifically applicable to the SDSS DR2 and DR3, it is generally applicable to all imaging data where any Petrosian index and concentration can be constructed.

Possible detection of two giant extrasolar planets orbiting the eclipsing polar UZ Fornacis

We present new high-speed, multi-observatory, multi-instrument photometry of the eclipsing polar UZ For in order to measure precise mid-eclipse times with the aim of detecting any orbital period variations. When combined with published eclipse times and archival data spanning ∼27 years, we detect departures from a linear and quadratic trend of ∼60 s. The departures are strongly suggestive of two cyclic variations of 16(3) and 5.25(25) years. The two favoured mechanisms to drive the periodicities are either two giant extrasolar planets as companions to the binary [with minimum masses of 6.3(1.5) and 7.7(1.2)MJup) or a magnetic cycle mechanism (e.g. Applegate’s mechanism) of the secondary star. Applegate’s mechanism would require the entire radiant energy output of the secondary and would therefore seem to be the least likely of the two, barring any further refinements in the effect of magnetic fields (e.g. those of Lanza et al.). The two-planet model can provide realistic solutions but it does not quite capture all of the eclipse times measurements. A highly eccentric orbit for the outer planet would fit the data nicely, but we find that such a solution would be unstable. It is also possible that the periodicities are driven by some combination of both mechanisms. Further observations of this system are encouraged.

MULTI-WAVELENGTH OBSERVATIONS OF SUPERNOVA 2011ei: TIME-DEPENDENT CLASSIFICATION OF TYPE IIb AND Ib SUPERNOVAE AND IMPLICATIONS FOR THEIR PROGENITORS

We present X-ray, UV/optical, and radio observations of the stripped-envelope, core-collapse supernova (SN) 2011ei, one of the least luminous SNe IIb or Ib observed to date. Our observations begin with a discovery within � 1 day of explosion and span several months afterward. Early optical spectra exhibit broad, Type II-like hydrogen Balmer profiles that subside rapidly and are replaced by Type Ib-like He-rich features on the timescale of one week. High-cadence monitoring of this transition suggests that absorption attributable to a high velocity (& 12,000 km s −1 ) H-rich shell is not rare in Type Ib events. Radio observations imply a shock velocity of v � 0.13c and a progenitor star mass-loss rate of u M � 1.4 × 10 −5 M⊙ yr −1 (assuming wind velocity vw = 10 3 km s −1 ). This is consistent with independent constraints from deep X-ray observations with Swift-XRT and Chandra. Overall, the multi-wavelength properties of SN2011ei are consistent with the explosion of a lower-mass (3 4 M⊙), compact (R∗ . 1 × 10 11 cm), He core star. The star retained a thin hydrogen envelope at the time of explosion, and was embedded in an inhomogeneous circumstellar wind suggestive of modest episodic mass-loss. We conclude that SN2011ei’s rapid spectral metamorphosis is indicative of time-dependent classifications that bias estimates of explosion rates for Type IIb and Ib objects, and that important information about a progenitor star’s evolutionary state and mass-loss immediately prior to SN explosion can be inferred from timely multi-wavelength observations. Subject headings: supernovae: general — supernova: individual (SN2011ei)

The Red Sequence Luminosity Function in Massive Intermediate Redshift Galaxy Clusters

We measure the rest-frame B-band luminosity function of red-sequence galaxies of five intermediate-redshift (0.5 950 km s–1) clusters. Cluster galaxies are identified through photometric redshifts based on imaging in seven bands (five broad and two narrow) using the WIYN 3.5 m telescope. The luminosity functions (LFs) are well-fit down to M* B + 3 for all of the clusters out to a radius of R 200. For comparison, the LFs for a sample of 59 low-redshift clusters selected from the Sloan Digital Sky Survey are measured as well. There is a brightening trend (M* B increases by 0.7 mag at z = 0.75) with redshift comparable to what is seen in the field for similarly defined galaxies, although there is a hint that the cluster red-sequence brightening is more rapid in the past (z > 0.5), and relatively shallow at more recent times. Contrary to other claims, we find little evidence for evolution of the faint-end slope. Previous indications of evolution may be due to limitations in measurement technique, bias in the sample selection, and cluster-to-cluster variation. As seen in both the low- and high-redshift samples, a significant amount of variation in LF parameters α and M* exists between individual clusters.

PySALT: the SALT science pipeline

PySALT is the python/PyRAF-based data reduction and analysis pipeline for the Southern African Large Telescope (SALT), a modern 10m class telescope with a large user community consisting of 13 partner institutions. The two first generation instruments on SALT are SALTICAM, a wide-field imager, and the Robert Stobie Spectrograph (RSS). Along with traditional imaging and spectroscopy modes, these instruments provide a wide range of observing modes, including Fabry-Perot imaging, polarimetric observations, and high-speed observations. Due to the large user community, resources available, and unique observational modes of SALT, the development of reduction and analysis software is key to maximizing the scientific return of the telescope. PySALT is developed in the Python/PyRAF environment and takes advantage of a large library of open-source astronomical software. The goals in the development of PySALT are: (1) Provide science quality reductions for the major operational modes of SALT, (2) Create analysis tools for the unique modes of SALT, and (3) Create a framework for the archiving and distribution of SALT data. The data reduction software currently provides support for the reduction and analysis of regular imaging, high-speed imaging, and long slit spectroscopy with planned support for multi-object spectroscopy, high-speed spectroscopy, Fabry-Perot imaging, and polarimetric data sets. We will describe the development and current status of PySALT and highlight its benefits through early scientific results from SALT.

Characterization of the nearby L/T Binary Brown Dwarf WISE J104915.57–531906.1 at 2 Pc from the Sun

WISE J104915.57–531906.1 is a L/T brown dwarf binary located 2 pc from the Sun. The pair contains the closest known brown dwarfs and is the third closest known system, stellar or sub-stellar. We report comprehensive follow-up observations of this newly uncovered system. We have determined the spectral types of both components (L8 ± 1, for the primary, agreeing with the discovery paper; T1.5 ± 2 for the secondary, which was lacking spectroscopic type determination in the discovery paper) and, for the first time, their radial velocities (V_(rad) ~ 23.1, 19.5 km s^(–1)) using optical spectra obtained at the Southern African Large Telescope and other facilities located at the South African Astronomical Observatory (SAAO). The relative radial velocity of the two components is smaller than the range of orbital velocities for theoretically predicted masses, implying that they form a gravitationally bound system. We report resolved near-infrared JHK_S photometry from the Infrared Survey Facility telescope at the SAAO which yields colors consistent with the spectroscopically derived spectral types. The available kinematic and photometric information excludes the possibility that the object belongs to any of the known nearby young moving groups or associations. Simultaneous optical polarimetry observations taken at the SAAO 1.9 m give a non-detection with an upper limit of 0.07%. For the given spectral types and absolute magnitudes, 1 Gyr theoretical models predict masses of 0.04-0.05 M _☉ for the primary, and 0.03-0.05 M _☉ for the secondary.

Photometry of 2006 RH120: an asteroid temporary captured into a geocentric orbit

Aims. From July 2006 to July 2007 a very small asteroid orbited the Earth within its Hill sphere. We used this opportunity to study its rotation and estimate its diameter and shape. Methods. Due to its faintness, 2006 RH120 was observed photometrically with the new 10-m SALT telescope at the SAAO (South Africa). We obtained data on four nights: 11, 15, 16, and 17 March 2007 when the solar phase angle remained almost constant at 74 ◦ . The observations lasted about an hour each night and the object was exposed for 7−10 s through the “clear” filter. Results. From the lightcurves obtained on three nights we derived two solutions for a synodical period of rotation: P1 = 1.375 ± 0.001 min and P2 = 2.750 ± 0.002 min. The available data are not sufficient to choose between them. The absolute magnitude of the object was found to be H = 29.9 ± 0.3 mag (with the assumed slope parameter G = 0.25) and its effective diameter D = 2− 7m , depending on the geometric albedo pV (with the most typical near-Earth asteroids albedo pV = 0.18 its diameter would be D = 3.3 ± 0.4 m). The body has an elongated shape with the a/b ratio greater than 1.4. It probably originates in low-eccentricity Amor or Apollo orbits. There is still a possibility, which needs further investigation, that it is a typical near-Earth asteroid that survived the aerobraking in the Earth’s atmosphere and returned to a heliocentric orbit similar to that of the Earth.

The Herschel-ATLAS: a sample of 500 μm-selected lensed galaxies over 600 deg2

We present a sample of 80 candidate strongly lensed galaxies with flux density above 100 mJy at 500 μm extracted from the Herschel Astrophysical Terahertz Large Area Survey, over an area of 600 deg2. Available imaging and spectroscopic data allow us to confirm the strong lensing in 20 cases and to reject it in one case. For other eight objects, the lensing scenario is strongly supported by the presence of two sources along the same line of sight with distinct photometric redshifts. The remaining objects await more follow-up observations to confirm their nature. The lenses and the background sources have median redshifts zL = 0.6 and zS = 2.5, respectively, and are observed out to zL = 1.2 and zS = 4.2. We measure the number counts of candidate lensed galaxies at 500 μm and compare them with theoretical predictions, finding a good agreement for a maximum magnification of the background sources in the range 10–20. These values are consistent with the magnification factors derived from the lens modelling of individual systems. The catalogue presented here provides sub-mm bright targets for follow-up observations aimed at exploiting gravitational lensing, to study with unprecedented details the morphological and dynamical properties of dusty star-forming regions in z ≳ 1.5 galaxies.

Luminous red galaxies in simulations: cosmic chronometers?

There have been a number of attempts to measure the expansion rate of the Universe at high redshift using luminous red galaxies (LRGs) as ‘chronometers’. The method generally assumes that stars in LRGs are all formed at the same time. In this paper, we quantify the uncertainties on the measurement of H(z) which arise when one considers more realistic, extended star formation histories. In selecting galaxies from the Millennium simulation for this study, we show that using rest-frame criteria significantly improves the homogeneity of the sample and that H(z) can be recovered to within 3 per cent at z ∼ 0.42 even when extended star formation histories are considered. We demonstrate explicitly that using single stellar populations (SSPs) to age-date galaxies from the semi-analytical simulations provides insufficient accuracy for this experiment but accurate ages are obtainable if the complex star formation histories extracted from the simulation are used. We note, however, that problems with SSP fitting might be overestimated since the semi-analytical models tend to overpredict the late-time star formation in LRGs. Finally, we optimize an observational programme to carry out this experiment.

Abell 48 – a rare WN-type central star of a planetary nebula

A considerable fraction of the central stars of planetary nebulae (CSPNe) are hydrogendeficient. Almost all of these H-deficient central stars (CSs) display spectra with strong carbon and helium lines. Most of them exhibit emission-line spectra resembling those of massive WC stars. Therefore these stars are classed as CSPNe of spectral type [WC]. Recently, quantitative spectral analysis of two emission-line CSs, PB 8 and IC 4663, revealed that these stars do not belong to the [WC] class. Instead PB 8 has been classified as [WN/WC] type and IC 4663 as [WN] type. In this work we report the spectroscopic identification of another rare [WN] star, the CS of Abell 48. We performed a spectral analysis of Abell 48 with the Potsdam Wolf– Rayet (PoWR) models for expanding atmospheres. We find that the expanding atmosphere of Abell 48 is mainly composed of helium (85 per cent by mass), hydrogen (10 per cent) and nitrogen (5 per cent). The residual hydrogen and the enhanced nitrogen abundance make this object different from the other [WN] star IC 4663. We discuss the possible origin of this atmospheric composition.