arXiv subjects

Alistair Walker

Publications and source records attributed to Alistair Walker.

30 records · Page 2Linked to original sources

arXiv2016

Variable stars in the field of the Hydra II ultra-faint dwarf galaxy

We report the discovery of one RR Lyrae star in the ultra--faint satellite galaxy Hydra II based on time series photometry in the g, r and i bands obtained with the Dark Energy Camera at Cerro Tololo Interamerican Observatory, Chile. The RR Lyrae star has a mean magnitude of $i = 21.30\pm 0.04$ which translates to a heliocentric distance of $151\pm 8$ kpc for Hydra II; this value is $\sim 13\%$ larger than the estimate from the discovery paper based on the average magnitude of several blue horizontal branch star candidates. The new distance implies a slightly larger half-light radius of $76^{+12}_{-10}$ pc and a brighter absolute magnitude of $M_V = -5.1 \pm 0.3$, which keeps this object within the realm of the dwarf galaxies. The pulsational properties of the RR Lyrae star ($P=0.645$ d, $Δg = 0.68$ mag) suggest Hydra II may be a member of the intermediate Oosterhoff or Oosterhoff II group. A comparison with other RR Lyrae stars in ultra--faint systems indicates similar pulsational properties among them, which are different to those found among halo field stars and those in the largest of the Milky Way satellites. We also report the discovery of 31 additional short period variables in the field of view (RR Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid). However, given their magnitudes and large angular separation from Hydra II, they must be field stars not related to Hydra II.

astro-ph.GA↗

arXiv2013

Towards a better understanding of the distance scale from RR Lyrae variable stars: A case study for the inner halo globular cluster NGC 6723

We present BV photometry for 54 variables in the metal-rich inner halo globular cluster NGC6723. With the discovery of new RR Lyrae variables (RRLs), we obtain = 0.541 +/- 0.066 and = 0.292 +/- 0.030 day, n(c)/n(ab+c) = 0.167, and _int = 15.459 +/- 0.055. We carry out the Fourier decomposition analysis and obtain [Fe/H]_ZW = -1.23 +/- 0.11 and E(B-V) = 0.063 +/- 0.015 for NGC 6723. By calibrating the zero-point from the recent absolute trigonometric parallax measurements for RR Lyr, we derive the revised M_V(RR)-[Fe/H] relation, providing M_V(RR)= 0.52 at [Fe/H] = -1.50 and (m-M)_0 = 18.54 for Large Magellanic Cloud (LMC), in excellent agreement with others. We obtain (m-M)_0 = 14.65 +/- 0.05, equivalent to the distance from the Sun of 8.47 +/- 0.17 kpc, for NGC 6723 from various distance measurement methods using RRLs. We find that RRLs in NGC 6723 do not have magnitude dependency on the radial distance, due to a not severe degree of the apparent crowdedness. Finally, we show that there exists a relation between the degree of photometric contamination and the apparent crowdedness of the central region of globular cluster systems. The use of this relation can play a significantly role on mitigating the discrepancy to establish a cosmic distance scale using RRLs in resolved stellar populations in the near-field cosmology.

astro-ph.GA↗

arXiv2013

Measuring Metallicities with HST/WFC3 photometry

We quantified and calibrated the metallicity and temperature sensitivities of colors derived from nine Wide Field Camera 3 (WFC3) filters aboard the Hubble Space Telescope using Dartmouth isochrones and Kurucz atmospheres models. The theoretical isochrone colors were tested and calibrated against observations of five well studied galactic clusters: M92, NGC 6752, NGC 104, NGC 5927, and NGC 6791, all of which have spectroscopically determined metallicities spanning -2.30 < [Fe/H] < +0.4. We found empirical corrections to the Dartmouth isochrone grid for each of the following color magnitude diagrams (CMD) (F555W--F814W, F814W), (F336W-F555W, F814W), (F390M-F555W, F814W) and (F390W-F555W, F814W). Using the empirical corrections we tested the accuracy and spread of the photometric metallicities assigned from CMDs and color-color diagrams (which are necessary to break the age-metallicity degeneracy). Testing three color-color diagrams [(F336W-F555W),(F390M-F555W),(F390W-F555W), vs (F555W-F814W)], we found the colors (F390M-F555W) and (F390W-F555W), to be the best suited to measure photometric metallicities. The color (F390W-F555W) requires much less integration time, but generally produces wider metallicity distributions, and, at very-low metallicity, the MDF from (F390W-F555W) is ~60% wider than that from (F390M-F555W). Using the calibrated isochrones we recovered the overall cluster metallicity to within ~0.1 dex in [Fe/H] when using CMDs (i.e. when the distance, reddening and ages are approximately known). The measured metallicity distribution function (MDF) from color-color diagrams show this method measures metallicities of stellar clusters of unknown age and metallicity with an accuracy of ~0.2 - 0.5 dex using F336W--F555W, ~0.15 - 0.25 dex using F390M-F555W, and ~0.2 - 0.4 dex with F390W-F555W, with the larger uncertainty pertaining to the lowest metallicity range.

astro-ph.GA↗

arXiv2013

Theoretical Modeling of the RR Lyrae Variables in NGC 1851

The RR Lyrae instability strip (IS) in NGC 1851 is investigated, and a model is presented which reproduces the pulsational properties of the RR Lyrae population. In our model, a stellar component within the IS that displays minor helium variations (Y~0.248-0.270) is able to reproduce the observed periods and amplitudes of the RR Lyrae variables, as well as the frequency of fundamental and first-overtone RR Lyrae variables. The RR Lyrae variables therefore may belong to an He-enriched second generation of stars. The RR Lyrae variables with a slightly enhanced helium (Y~0.270-0.280) have longer periods at a given amplitude, as is seen with Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with Y~0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI) variables.

astro-ph.SR↗

arXiv2012

The Horizontal Branch of NGC 1851: Constraints from its RR Lyrae Variables

We use the pulsational properties of the RR Lyrae variables in the globular cluster NGC 1851 to obtain detailed constraints of the various sub-stellar populations present along its horizontal branch. On the basis of detailed synthetic horizontal branch modeling, we find that minor helium variations (Y~0.248-0.280) are able to reproduce the observed periods and amplitudes of the RR Lyrae variables, as well as the frequency of fundamental and first-overtone RR Lyrae stars. Comparison of number ratios amongst the blue and red horizontal branch components and the two observed subgiant branches also suggest that the RR Lyrae variables originated from the progeny of the bright subgiant branch. The RR Lyrae variables with a slightly enhanced helium (Y~0.270-0.280) have longer periods at a given amplitude, as is seen with Oosterhoff II (OoII) RR Lyrae variables, whereas the RR Lyrae variables with Y~0.248-0.270 have shorter periods, exhibiting properties of Oosterhoff I (OoI) variables. This correlation does suggest that the pulsational properties of RR Lyrae stars can be very useful for tracing the various sub-populations and can provide suitable constraints on the multiple population phenomenon. It appears of great interest to explore whether this conclusion can be generalized to other globular clusters hosting multiple populations.

astro-ph.SR↗

arXiv2010

Period Change Similarities among the RR Lyrae Variables in Oosterhoff I and Oosterhoff II Globular Systems

We present period change rates (dP/dt) for 42 RR Lyrae variables in the globular cluster IC$\,$4499. Despite clear evidence of these period increases or decreases, the observed period change rates are an order of magnitude larger than predicted from theoretical models of this cluster. We find there is a preference for increasing periods, a phenomenon observed in most RR Lyrae stars in Milky Way globular clusters. The period-change rates as a function of position in the period-amplitude plane are used to examine possible evolutionary effects in OoI clusters, OoII clusters, field RR Lyrae stars and the mixed-population cluster $ω$~ Centauri. It is found that there is no correlation between the period change rate and the typical definition of Oosterhoff groups. If the RR Lyrae period changes correspond with evolutionary effects, this would be in contrast to the hypothesis that RR Lyrae variables in OoII systems are evolved HB stars that spent their ZAHB phase on the blue side of the instability strip. This may suggest that age may not be the primary explanation for the Oosterhoff types.

astro-ph.SR↗

arXiv2008

The Araucaria Project. The Distance of the Large Magellanic Cloud from Near-Infrared Photometry of RR Lyrae Variables

We have obtained deep infrared $J$ and $K$ band observations of five fields located in the Large Magellanic Cloud (LMC) bar with the ESO New Technology Telescope equipped with the SOFI infrared camera. In our fields, 65 RR Lyrae stars catalogued by the OGLE collaboration were identified. Using different theoretical and empirical calibrations of the period-luminosity-metallicity relation, we find consistent LMC distance moduli values. Since the observed fields are situated very close to the center of the LMC, the correction for the tilt of the LMC bar with respect to the line of sight is negligible. Our adopted best true distance modulus to the LMC of $18.58 \pm 0.03$ (statistical) $\pm$ 0.11 (systematic) mag agrees very well with most independent determinations to this galaxy.

astro-ph↗

arXiv2002

B,V Photometry of Variable Stars in the Northeast Arm of the Small Magellanic Cloud

B and V photometry has been obtained for variable stars in the northeast arm of the Small Magellanic Cloud (SMC). Periods and lightcurves have been determined for 237 periodic variables, including 201 Cepheids, 68 of which are newly discovered. Fundamental mode Cepheids and first overtone mode Cepheids are generally well separated in the B,V color-magnitude diagram, with the latter having bluer mean colors than the former. The Cepheid period-color relationship for this outlying SMC field is indistinguishable from that seen in more centrally located SMC fields, and is bluer than theoretical predictions. The red edge to the populated portion of the instability strip shifts to bluer colors for fainter Cepheids. There is support from our sample for a previously reported steepening in the slope of the period-luminosity relation for fundamental mode Cepheids near a period of 2 days. The Cepheids of the northeast arm may be closer to us than are those of the main body of the SMC, but the difference is smaller than or equal to about 4 kpc, comparable to the tidal radius of the SMC.

astro-ph↗

arXiv2001

Wide-field High-precision CCD Photometry of omega Centauri and Its RR Lyrae Stars

We present wide-field and high-precision BV and Ca & Stromgren by photometry of omega Centauri and its RR Lyrae stars, which represents one of the most extensive photometric surveys to date for this cluster. The member stars of omega Cen are well discriminated from foreground Galactic disk stars from the different distributions in the hk [=(Ca-b)-(b-y)] vs. b-y diagram. The color-magnitude diagrams show the presence of several distinct red-giant branches with a red, metal-rich, sequence clearly separated from other bluer metal-poor ones. Comparison with our population models suggests the most metal-rich population is few billion years younger than the most metal-poor population. From our new BV photometry and hk metallicity measurements for RR Lyrae stars in omega Cen, we also confirm that the luminosity of RR Lyrae stars depends on evolutionary status as well as metallicity. From the presence of several distinct populations and the internal age-metallicity relation, we conclude omega Cen was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing now.

astro-ph↗

arXiv2001

Omega Centauri as a Disrupted Dwarf Galaxy: Evidence from Multiple Stellar Populations

Our recent CCD photometry (Lee et al. 1999) has shown, for the first time, that omega Cen has several distinct stellar populations, which is reminiscent of the Sagittarius dwarf galaxy. Here we present more detailed analysis of the data along with the population models. We confirm the presence of several distinct red-giant-branches (RGBs) with a red metal-rich sequence well separated from other bluer metal-poor ones. Our population models suggest the red clump associated with the most metal-rich RGB is about 4 Gyr younger than the dominant metal-poor component, indicating that omega Cen was enriched over this timescale. These features, taken together with this cluster's other unusual characteristics, provide good evidence that omega Cen was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were much more frequent in the early history of the Galaxy and omega Cen appears to be a relict of this era.

astro-ph↗

arXiv2000

CCD Photometry of the Globular Cluster Omega Centauri. I. Metallicity of RR Lyrae Stars from Caby Photometry

We present new measurements of the metallicity of 131 RR Lyrae stars in the globular cluster Omega Centauri, using the hk index of the Caby photometric system. The hk method has distinct advantages over Delta-S and other techniques in determining the metallicity of RR Lyrae stars, and has allowed us to obtain the most complete and homogeneous metallicity data to date for the RR Lyrae stars in this cluster. For RR Lyrae stars in common with the Delta-S observations of Butler et al. (1978) and Gratton et al. (1986), we have found that our metallicities, [Fe/H](hk), deviate systematically from their Delta-S metallicity, while our [Fe/H](hk) for well observed field RRab stars are consistent with previous spectroscopic measurements. We conclude that this is due to the larger errors associated with the previous Delta-S observations for this cluster. The Mv(RR) - [Fe/H] and period-shift - [Fe/H] relations obtained from our new data are consistent with the evolutionary models predicted by Lee (1991), confirming that the luminosity of RR Lyrae stars depends on evolutionary status as well as metallicity. Using the period - amplitude diagram, we have also identified highly evolved RRab stars in the range of -1.9 < [Fe/H] < -1.5, as predicted from the synthetic horizontal-branch models.

astro-ph↗

arXiv1999

Multiple Stellar Populations in the Globular Cluster omega Centauri as Tracers of a Merger Event

The discovery of the Sagittarius dwarf galaxy, which is being tidally disrupted by and merging with the Milky Way, supports the view that the halo of the Galaxy has been built up at least partially by the accretion of similar dwarf systems. The Sagittarius dwarf contains several distinct populations of stars, and includes M54 as its nucleus, which is the second most massive globular cluster associated with the Milky Way. The most massive globular cluster is omega Centauri, and here we report that omega Centauri also has several distinct stellar populations, as traced by red-giant-branch stars. The most metal-rich red-giant-branch stars are about 2 Gyr younger than the dominant metal-poor component, indicating that omega Centauri was enriched over this timescale. The presence of more than one epoch of star formation in a globular cluster is quite surprising, and suggests that omega Centauri was once part of a more massive system that merged with the Milky Way, as the Sagittarius dwarf galaxy is in the process of doing now. Mergers probably were much more frequent in the early history of the Galaxy and omega Centauri appears to be a relict of this era.

astro-ph↗