Goldman S.R.,Keele University |
van Loon J.T.,Keele University |
Zijlstra A.A.,University of Manchester |
Green J.A.,Jodrell Bank Observatory |
And 8 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2017
We present the results of our survey of 1612-MHz circumstellar OH maser emission from asymptotic giant branch (AGB) stars and red supergiants (RSGs) in the Large Magellanic Cloud (LMC). We have discovered four new circumstellar maser sources in the LMC, and increased the number of reliable wind speeds from infrared (IR) stars in the LMC from 5 to 13. Using our new wind speeds, as well as those from Galactic sources, we have derived an updated relation for dust-driven winds: vexp ∝ ZL0.4. We compare the subsolar metallicityLMC OH/IR stars with carefully selected samples of more metal-rich OH/IR stars, also at known distances, in the Galactic Centre and Galactic bulge. We derive pulsation periods for eight of the bulge stars for the first time by using near-IR photometry from the VistaVariables in the Via Lactea survey.We have modelled our LMC OH/IR stars and developed an empirical method of deriving gas-to-dust ratios and mass-loss rates by scaling the models to the results from maser profiles.We have done this also for samples in the Galactic Centre and bulge and derived a new mass-loss prescription which includes luminosity, pulsation period, and gas-to-dust ratio M˙ = 1.06-0.8 +3.5 × 10-5(L/104 L⊙)0.9±0.1(P/500 d)0.75±0.3(rgd/200)-0.03±0.07 M⊙ yr-1. The tightest correlation is found between mass-loss rate and luminosity. We find that the gas-to-dust ratio has little effect on the mass-loss of oxygen-rich AGB stars and RSGs within the Galaxy and the LMC. This suggests that the mass-loss of oxygen-rich AGB stars and RSGs is (nearly) independent of metallicity between a half and twice solar. © 2016 The Authors.
Ramatsoku M.,University of Groningen |
Ramatsoku M.,University of Cape Town |
Ramatsoku M.,Netherlands Institute for Radio Astronomy |
Verheijen M.A.W.,University of Groningen |
And 11 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2016
We present results of a blind 21cm HI-line imaging survey of a galaxy overdensity located behind theMilkyWay at ℓ, b ≈ 160°, 0. ° 5. The overdensity corresponds to a zone-of-avoidance crossing of the Perseus-Pisces Supercluster filament. Although it is known that this filament contains an X-ray galaxy cluster (3C 129) hosting two strong radio galaxies, little is known about galaxies associated with this potentially rich cluster because of the high Galactic dust extinction. We mapped a sky area of~9.6 deg2 using theWesterbork Synthesis RadioTelescope in a hexagonal mosaic of 35 pointings observed for 12 h each, in the radial velocity range cz = 2400-16 600 km s-1. The survey has a sensitivity of 0.36 mJy beam-1 rms at a velocity resolution of 16.5 km s-1. We detected 211 galaxies, 62 per cent of which have a near-infrared counterpart in the UKIDSS Galactic Plane Survey. We present a catalogue of the HI properties and an HI atlas containing total intensity maps, position-velocity diagrams, global HI profiles and UKIDSS counterpart images. For the resolved galaxies we also present HI velocity fields and radial HI surface density profiles. A brief analysis of the structures outlined by these galaxies finds that 87 of them lie at the distance of the Perseus-Pisces Supercluster (cz ~ 4000-8000 km s-1) and seem to form part of the 3C 129 cluster. Further 72 detections trace an overdensity at a velocity of cz ≈ 10 000 km s-1 and seem to coincide with a structure predicted from mass density reconstructions in the first 2MASS Redshift Survey. © 2016 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society.
Childress M.J.,Australian National University |
Scalzo R.A.,Australian National University |
Sim S.A.,Australian National University |
Sim S.A.,Queen's University of Belfast |
And 58 more authors.
Astrophysical Journal | Year: 2013
We present 65 optical spectra of the Type Ia SN 2012fr, 33 of which were obtained before maximum light. At early times, SN 2012fr shows clear evidence of a high-velocity feature (HVF) in the Si II λ6355 line that can be cleanly decoupled from the lower velocity "photospheric" component. This Si II λ6355 HVF fades by phase -5; subsequently, the photospheric component exhibits a very narrow velocity width and remains at a nearly constant velocity of 12,000 km s-1 until at least five weeks after maximum brightness. The Ca II infrared triplet exhibits similar evidence for both a photospheric component at v 12,000 km s-1 with narrow line width and long velocity plateau, as well as an HVF beginning at v 31,000 km s-1 two weeks before maximum. SN 2012fr resides on the border between the "shallow silicon" and "core-normal" subclasses in the Branch et al. classification scheme, and on the border between normal and high-velocity Type Ia supernovae (SNe Ia) in the Wang et al. system. Though it is a clear member of the "low velocity gradient" group of SNe Ia and exhibits a very slow light-curve decline, it shows key dissimilarities with the overluminous SN 1991T or SN 1999aa subclasses of SNe Ia. SN 2012fr represents a well-observed SN Ia at the luminous end of the normal SN Ia distribution and a key transitional event between nominal spectroscopic subclasses of SNe Ia. © 2013. The American Astronomical Society. All rights reserved..
Mguda Z.,University of Cape Town |
Mguda Z.,South African Astronomical Observatory SAAO |
Faltenbacher A.,University of Witwatersrand |
Van der Heyden K.,University of Cape Town |
And 5 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014
In this paper,we use the MareNostrum Universe Simulation, a large-scale, hydrodynamic, nonradiative simulation in combination with a simple abundance matching approach to determine the ram pressure statistics for bent radio sources (BRSs). The abundance matching approach allows us to determine the locations of all galaxies with stellar masses ≥1011 h-1M⊙ in the simulation volume. Assuming that ram pressure exceeding a critical value causes bent morphology, we compute the ratio of all galaxies exceeding the ram pressure limit (RPEX galaxies) relative to all galaxies in our sample. According to our model 50 per cent of the RPEX galaxies at z = 0 are found in clusters with masses larger than 1014.5 h-1M⊙ the other half resides in lower mass clusters. Therefore, the appearance of bent tail morphology alone does not put tight constraints on the host cluster mass. In low-mass clusters, M ≤ 1014 h-1M⊙, RPEX galaxies are confined to the central 500 h-1 kpc whereas in clusters of ≥1015 h-1M⊙ they can be found at distances up to 1.5 h-1 Mpc. Only clusters with masses ≥1015 h-1M⊙ are likely to host more than one BRS. Both criteria may prove useful in the search for distant, high-mass clusters. © 2014 The Authors.
Hakala P.J.,University of Turku |
Charles P.A.,South African Astronomical Observatory SAAO |
Muhli P.,National Land Survey of Finland
Monthly Notices of the Royal Astronomical Society | Year: 2011
We present results from several years of fast optical photometry of 4U0614+091 (V1055 Orionis), a candidate ultracompact X-ray binary most likely consisting of a neutron star and a degenerate secondary. We find evidence for strong accretion-driven variability at all epochs, which manifests itself as red noise. This flickering produces transient peaks in the observed power spectrum in the 15-65min period range. Only in one of our 12 optical data sets can we see evidence for a period that cannot be reproduced using the red noise model. This period of 51min coincides with the strongest period detected by Shahbaz et al. and can thus be taken as the prime candidate for the orbital period of the system. Furthermore, we find some tentative evidence for the X-ray versus optical flux anticorrelation discovered by Machin et al. using our data together with the all-sky X-ray monitoring data from RXTE/All Sky Monitor. We propose that the complex time series behaviour of 4U0614+09 is a result of drastic changes in the accretion disc geometry/structure on time-scales from hours to days. Finally, we want to draw attention to the interpretation of moderately strong peaks in the power spectra of especially accreting sources. Many of such 'periods' can probably be attributed to the presence of red noise (i.e. correlated events) in the data. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.
Wittkowski M.,ESO |
Boboltz D.A.,Us Naval Observatory |
Ireland M.,Macquarie University |
Ireland M.,Australian Astronomical Observatory |
And 9 more authors.
Astronomy and Astrophysics | Year: 2011
Aims. We investigate the structure and shape of the photospheric and molecular layers of the atmospheres of four Mira variables. Methods. We obtained near-infrared K-band spectro-interferometric observations of the Mira variables R Cnc, X Hya, W Vel, and RW Vel with a spectral resolution of about 1500 using the AMBER instrument at the VLTI. We obtained concurrent JHKL photometry using the the Mk II instrument at the SAAO. Results. The Mira stars in our sample are found to have wavelength-dependent visibility values that are consistent with earlier low-resolution AMBER observations of S Ori and with the predictions of dynamic model atmosphere series based on self-excited pulsation models. The corresponding wavelength-dependent uniform disk (UD) diameters show a minimum near the near-continuum bandpass at 2.25â€‰ μm. They then increase by up to 30% toward the H2O band at 2.0â€‰ μm and by up to 70% at the CO bandheads between 2.29â€‰ μm and 2.48â€‰ μm. The dynamic model atmosphere series show a consistent wavelength-dependence, and their parameters such as the visual phase, effective temperature, and distances are consistent with independent estimates. The closure phases have significantly wavelength-dependent and non-zero values at all wavelengths indicating deviations from point symmetry. For example, the R Cnc closure phase is 110°â€‰ ± â€‰ 4° in the 2.0â€‰ μm H 2O band, corresponding for instance to an additional unresolved spot contributing 3% of the total flux at a separation of ~4â€‰ mas. Conclusions. Our observations are consistent with the predictions of the latest dynamic model atmosphere series based on self-excited pulsation models. The wavelength-dependent radius variations are interpreted as the effect of molecular layers lying above the photosphere. The wavelength-dependent closure phase values are indicative of deviations from point symmetry at all wavelengths, thus a complex non-spherical stratification of the extended atmosphere. In particular, the significant deviation from point symmetry in the H2O band is interpreted as a signature on large scales (there being a few across the stellar disk) of inhomogeneities or clumps in the water vapor layer. The observed inhomogeneities might possibly be caused by pulsation-and shock-induced chaotic motion in the extended atmosphere. © 2011 ESO.
Hernandez A.G.,University of Porto |
Lignieres F.,CNRS Institute for research in astrophysics and planetology |
Balona L.,South African Astronomical Observatory SAAO |
Reese D.R.,University of Birmingham |
And 3 more authors.
EPJ Web of Conferences | Year: 2015
High precision photometric observations from space has led to the detection of hundreds of frequencies in the light curves of δ Scuti pulsators. In this work, we analyzed a sample of Kepler δ Sct stars to search for frequency patterns in the p-mode regime. To avoid g-modes, we looked at the mode density histogram (MDH). We then used the Fourier transform technique (FT), histograms of frequency differences (HFD) and Echelle diagrams (ED) to find periodicities in the frequency content. We compared the results with those expected for SCF rotating models  with the aim of identifying large separations and rotational splittings. © Owned by the authors, published by EDP Sciences, 2015.
Whitelock P.A.,South African Astronomical Observatory SAAO |
Whitelock P.A.,University of Cape Town |
Menzies J.W.,University of Cape Town |
Feast M.W.,South African Astronomical Observatory SAAO |
And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013
JHKS photometry is presented from a 3.5 yr survey of the central regions of the irregular galaxy NGC 6822. The morphology of the colour-magnitude and colour-colour diagrams is discussed with particular reference to M-, S- and C-type asymptotic giant branch (AGB) stars and to M supergiants. Mean JHKS magnitudes and periods are given for 11 O-rich and 50 presumed C-rich Miras. Data are also listed for 27 large-amplitude AGB stars without periods and for 69 small-amplitude AGB variables. The slope of the bolometric period-luminosity relation for the C-rich Miras is in good agreement with that in the Large Magellanic Cloud. Distance moduli derived from the C- and O-rich Miras are in agreement with other estimates. The period distribution of C-rich Miras in NGC 6822 is similar to that in the Magellanic Clouds, but differs from that in the dwarf spheroidals in the Local Group. In the latter there is a significant proportion of large-amplitude, short-period variables indicating a population producing old carbon-rich AGB stars. © 2012 The Authors.
Whitelock P.A.,South African Astronomical Observatory SAAO |
Whitelock P.A.,University of Cape Town |
Feast M.W.,South African Astronomical Observatory SAAO |
Feast M.W.,University of Cape Town
EAS Publications Series | Year: 2014
Some recent observations of Classical Cepheids, RR Lyrae and Mira variables are used to illustrate their continued importance as standard candles. Gaia will provide a calibration of these stars as distance indicators which should lead to a greatly improved understanding of the formation and evolution of our Galaxy as well as a vital calibration of the extra-galactic distance scale. The importance of infrared observations, as well as a deeper understanding of interstellar reddening and photometric systems, will play a part if the Gaia calibration is to be put to good use. © EAS, EDP Sciences, 2015.
Johnston R.,University of the Western Cape |
Johnston R.,South African Astronomical Observatory SAAO |
Johnston R.,University of Glasgow
Astronomy and Astrophysics Review | Year: 2011
From as early as the 1930s, astronomers have tried to quantify the statistical nature of the evolution and large-scale structure of galaxies by studying their luminosity distribution as a function of redshift - known as the galaxy luminosity function (LF). Accurately constructing the LF remains a popular and yet tricky pursuit in modern observational cosmology where the presence of observational selection effects due to e.g. detection thresholds in apparent magnitude, colour, surface brightness or some combination thereof can render any given galaxy survey incomplete and thus introduce bias into the LF. Over the last 70 years there have been numerous sophisticated statistical approaches devised to tackle these issues; all have advantages - but not one is perfect. This review takes a broad historical look at the key statistical tools that have been developed over this period, discussing their relative merits and highlighting any significant extensions and modifications. In addition, the more generalised methods that have emerged within the last few years are examined. These methods propose a more rigorous statistical framework within which to determine the LF compared to some of the more traditional methods. I also look at how photometric redshift estimations are being incorporated into the LF methodology as well as considering the construction of bivariate LFs. Finally, I review the ongoing development of completeness estimators which test some of the fundamental assumptions going into LF estimators and can be powerful probes of any residual systematic effects inherent magnitude-redshift data. © Springer-Verlag 2011.