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D. Zurek

Publications and source records attributed to D. Zurek.

10 recordsLinked to original sources
arXiv2021

The outburst of a 60 min AM CVn reveals peculiar color evolution: implications for outbursts in long period double white dwarfs

We report on multi-wavelength observations during quiescence and of the first detected outburst of the ~60 min orbital period AM CVn SDSS J113732+405458. Using X-ray and UV observations we determined an upper limit duration of the event of about one year. The amplitude of the outburst was remarkably small, of around one magnitude in r and 0.5 magnitudes in g. We have also investigated the color variations of SDSS J113732+405458 and other long period AM CVns in outbursts and identified a track on the color-magnitude diagram that is not compatible with the predictions of the disk instability model, suggesting that some outbursts in long period AM CVns are caused by enhanced mass-transfer. To our knowledge, these are the first studies of the color evolution in AM CVns. During quiescence we measured an X-ray luminosity for SDSS J113732+405458 of ~3E29 erg/s in the 0.5-10 keV band. This indicates a very low accretion rate, in agreement with the disk instability model for long period systems. However, such a model predicts stable disks at somewhat long periods. The discovery of this system outburst, along with similarities to the long period system SDSS J080710+485259 with a comparably long, weak outburst, indicates that these enhanced mass-transfer events may be more common in long period AM CVns. A larger sample would be needed to determine empirically at what period, if any, the disk instability stops functioning entirely. Finally, we identified an infrared excess in the quiescence spectrum attributable to the donor. This makes SDSS J113732+405458 the second AM CVn to have a directly detected donor.

astro-ph.SR↗
arXiv2018

Constraints on blue straggler formation mechanisms in Galactic globular clusters from proper motion velocity distributions

For a sample of 38 Galactic globular clusters (GCs), we confront the observed distributions of blue straggler (BS) proper motions and masses (derived from isochrone fitting) from the BS catalog of Simunovic & Puzia with theoretical predictions for each of the two main competing BS formation mechanisms. These are mass transfer from an evolved donor on to a main-sequence (MS) star in a close binary system, and direct collisions involving MS stars during binary encounters. We use the \texttt{FEWBODY} code to perform simulations of single-binary and binary-binary interactions. This provides collisional velocity and mass distributions for comparison to the observed distributions. Most clusters are consistent with BSs derived from a dynamically relaxed population, supportive of the binary mass-transfer scenario. In a few clusters, including all the post-core collapse clusters in our sample, the collisional velocities provide the best fit.

astro-ph.SR↗
arXiv2017

The First Optical Spectra of Wolf Rayet Stars in M101 Revealed with Gemini/GMOS

Deep narrow-band HST imaging of the iconic spiral galaxy M101 has revealed over a thousand new Wolf Rayet (WR) candidates. We report spectrographic confirmation of 10 HeII emission line sources hosting 15 WR stars. We find WR stars present at both sub- and super-solar metalicities with WC stars favouring more metal-rich regions compared to WN stars. We investigate the association of WR stars with HII regions using archival HST imaging and conclude that the majority of WR stars are in or associated with HII regions. Of the 10 emission lines sources, only one appears to be unassociated with a star-forming region. Our spectroscopic survey provides confidence that our narrow-band photometric candidates are in fact bonafide WR stars, which will allow us to characterise the progenitors of any core-collapse supernovae that erupt in the future in M101.

astro-ph.GA↗
arXiv2015

Deep near-IR observations of the Globular Cluster M4: Hunting for Brown Dwarfs

We present an analysis of deep HST/WFC3 near-IR (NIR) imaging data of the globular cluster M4. The best-photometry NIR colour-magnitude diagram (CMD) clearly shows the main sequence extending towards the expected end of the Hydrogen-burning limit and going beyond this point towards fainter sources. The white dwarf sequence can be identified. As such, this is the deepest NIR CMD of a globular cluster to date. Archival HST optical data were used for proper-motion cleaning of the CMD and for distinguishing the white dwarfs (WDs) from brown dwarf (BD) candidates. Detection limits in the NIR are around F110W approx 26.5 mag and F160W approx27 mag, and in the optical around F775W approx 28 mag. Comparing our observed CMDs with theoretical models, we conclude that we have reached beyond the H-burning limit in our NIR CMD and are probably just above or around this limit in our optical-NIR CMDs. Thus, any faint NIR sources that have no optical counterpart are potential BD candidates, since the optical data are not deep enough to detect them. We visually inspected the positions of NIR sources which are fainter than the H-burning limit in F110W and for which the optical photometry did not return a counterpart. We found in total five sources for which we did not get an optical measurement. For four of these five sources, a faint optical counterpart could be visually identified, and an upper optical magnitude was estimated. Based on these upper optical magnitude limits, we conclude that one source is likely a WD, one source could either be a WD or BD candidate, and the remaining two sources agree with being BD candidates. For only one source no optical counterpart could be detected, which makes this source a good BD candidate. We conclude that we found in total four good BD candidates.

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arXiv2012

A multi-wavelength survey of NGC\,6752: X-ray counterparts, two new dwarf novae, and a core-collapsed radial profile

We present the results of a multi-wavelength (FUV to I-band) survey of the stellar populations of the globular cluster NGC 6752, using STIS, ACS and WFC3 on board the Hubble Space Telescope. We have confirmed that two previously identified CV candidates are, in fact, dwarf novae which underwent outbursts during our observations. We have also identified previously unknown optical counterparts to two X-ray sources. We estimate the position of the centre of the cluster, and show that the stellar density profile is not well described by a single King model, indicating that this cluster is in a core-collapsed or post-core collapse phase. The colour-magnitude diagram shows a well-populated horizontal branch, numerous blue stragglers and white dwarfs (WDs), as well as 87 sources in the gap region where we expect to find WD - main sequence binaries, including cataclysmic variables (CVs). The X-ray sources and WD binary systems are the most centrally concentrated populations, with dynamically estimated characteristic masses >1.1Msun and >0.8Msun, respectively.

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arXiv2012

Searching for Wolf-Rayet Stars in M101

Wolf-Rayet (WR) stars are the evolved descendants of massive O-type stars and are considered to be progenitor candidates for Type Ib/c core-collapse supernovae (SNe). Recent results of our HST/WFC3 survey of Wolf-Rayet stars in M101 are summarised based on the detection efficiency of narrow-band optical imaging compared to broad-band methods. Weshow that on average of 42% WR stars, increasing to ~85% in central regions, are only detected in the narrow-band imaging. Hence, the non-detection of a WR star at the location of ~10 Type Ib/c SNe in broad-band imaging is no longer strong evidence for a non-WR progenitor channel.

astro-ph.SR↗
arXiv2009

Blue Hook Stars in Globular Clusters

Blue hook (BHk) stars are a rare class of horizontal branch stars that so far have been found in only very few Galactic globular clusters (GCs). The dominant mechanism for producing these objects is currently still unclear. In order to test if the presence of BHk populations in a given GC is linked to specific physical or structural cluster properties, we have constructed a parent sample of GCs for which existing data is sufficient to establish the presence or absence of BHk populations with confidence. We then compare the properties of those clusters in our parent sample that do contain a BHk population to those that do not. We find that there is only one compelling difference between BHk and non-BHk clusters: all known BHk clusters are unusually massive. However, we also find that the BHk clusters are consistent with being uniformly distributed within the cumulative mass distribution of the parent sample. Thus, while it is attractive to suggest there is is a lower mass cut-off for clusters capable of forming BHk stars, the data do not require this. Instead, the apparent preference for massive clusters could still be a purely statistical effect: intrinsically rare objects can only be found by searching a sufficiently large number of stars.

astro-ph.GA↗
arXiv2005

A Spectroscopic Analysis of Blue Stragglers, Horizontal Branch and Turn-Off Stars in Four Globular Clusters

We present a spectroscopic analysis of HST/STIS and FOS low- and intermediate-resolution spectroscopy of 55 stars (turn-off stars, horizontal branch stars and blue stragglers) in four globular clusters (47 Tucanae, M3, NGC6752, and NGC6397). Stars were analyzed with non-Local Thermodynamic Equilibrium model atmospheres, and values for their effective temperatures and gravities and some rotation rates were obtained. Using photometric fluxes, we also obtained radii, luminosities and spectroscopic masses.

astro-ph↗
arXiv1999

Nuclear Brightness Profiles of Merger Remnants: Constraints on the Formation of Ellipticals by Mergers

We present preliminary results of an HST/NICMOS program to image merger remnants in the J, H and K bands. The nuclear brightness profiles for most sample galaxies are similar to those typical for elliptical galaxies, but some (including the well-studied NGC 3921 and 7252) have an unusually high luminosity density at small radii. This is consistent with the prediction of N-body simulations that gas flows to the center during a merger and forms new stars.

astro-ph↗