Searcharxiv⌕ Search

arXiv subjects

Ondrej Pejcha

Publications and source records attributed to Ondrej Pejcha.

26 records · Page 2Linked to original sources
arXiv2012

A Global Physical Model for Cepheids

We perform a global fit to ~5,000 radial velocity and ~177,000 magnitude measurements in 29 photometric bands covering 0.3 to 8.0 microns distributed among 287 Galactic, LMC, and SMC Cepheids with P > 10 days. We assume that the Cepheid light curves and radial velocities are fully characterized by distance, reddening, and time-dependent radius and temperature variations. We construct phase curves of radius and temperature for periods between 10 and 100 days, which yield light curve templates for all our photometric bands and can be easily generalized to any additional band. With only 4 to 6 parameters per Cepheid, depending on the existence of velocity data and the amount of freedom in the distance, the models have typical rms light and velocity curve residuals of 0.05 mag and 3.5 km/s. The model derives the mean Cepheid spectral energy distribution and its derivative with respect to temperature, which deviate from a black body in agreement with metal-line and molecular opacity effects. We determine a mean reddening law towards the Cepheids in our sample, which is not consistent with standard assumptions in either the optical or near-IR. Based on stellar atmosphere models we predict the biases in distance, reddening, and temperature determinations due to the metallicity and we quantify the metallicity signature expected for our fit residuals. The observed residuals as a function of wavelength show clear differences between the individual galaxies, which are compatible with these predictions. In particular, we find that metal-poor Cepheids appear hotter. Finally, we provide a framework for optimally selecting filters that yield the smallest overall errors in Cepheid parameter determination, or filter combinations for suppressing or enhancing the metallicity effects on distance determinations. We make our templates publicly available.

astro-ph.SR↗
arXiv2011

The Physics of the Neutrino Mechanism of Core-Collapse Supernovae

(Abridged) Neutrino heating may drive core-collapse supernova explosions. Although it is known that the stalled accretion shock turns into explosion when the neutrino luminosity from the collapsed core exceeds a critical value (L_crit) (the "neutrino mechanism"), the physics of L_crit, as well as its dependence on the properties of the proto-neutron star (PNS) and changes to the microphysics has never been systematically explored. We solve the one-dimensional steady-state accretion problem between the PNS surface and the accretion shock. We quantify the deep connection between the solution space of steady-state accretion flows with bounding shocks and the neutrino mechanism. We show that there is a maximum, critical sound speed above which it is impossible to maintain accretion with a standoff shock, because the shock jump conditions cannot be satisfied. The physics of this critical sound speed is general and does not depend on a specific heating mechanism. For the simple model of pressure-less free-fall onto a shock bounding an isothermal accretion flow with sound speed c_T, we show that if c_T^2/v_escape^2 > 3/16 explosion results. We generalize this result to the more complete supernova problem, showing explicitly that the same physics determines L_crit. We find that the critical condition for explosion can be written as c_S^2/v_escape^2 = 0.19, where c_S is the adiabatic sound speed. This "antesonic" condition describes L_crit over a broad range in accretion rate and microphysics. We show that the addition of the accretion luminosity (L_acc) reduces L_crit non-trivially. As in previous work, we find that L_crit is always significantly higher than the maximum possible value of L_acc. Finally, we provide evidence that the reduction in L_crit seen in recent multi-dimensional simulations results from a reduction in the efficiency of cooling, rather than an increase in the heating rate.

astro-ph.HE↗
arXiv2010

An R- and I-Band Photometric Variability Survey of the Cygnus OB2 Association

We present a catalog of photometrically variable stars discovered within two 21'.3 X 21'.3 fields centered on the Cygnus OB2 association. There have hitherto been no deep optical variability studies of Cyg OB2 despite it being replete with early-type massive stars, perhaps due to the high and variable extinction (up to A_V ~ 20) that permeates much of the region. Here we provide results of the first variability study with this combination of spatial coverage (~ 0.5 deg) and photometric depth (R ~ 21 mag). We find 121 stars to be variable in both R- and I-band, 116 of them newly discovered. Of the 121 variables, we identify 27 eclipsing binaries (EBs) and eclipsing binary candidates, 20 potential Herbig Ae/Be stars, and 52 pulsating variables. Confirming both the status and the cluster membership of the Herbig Ae/Be stars would address the uncertainty regarding the age and star formation history of Cyg OB2. We match our catalog to known variables and binaries in the region, 2MASS near-IR (NIR) data, and Chandra X-ray observations to find counterparts to new variables in other wavelengths.

astro-ph.SR↗
arXiv2009

The time-dependent rebrightenings in classical novae outbursts: a late-time episodic fuel burning?

A significant fraction of novae exhibit a series of rebrightenings on the decline branch of their light curves. We use visual observations to study this phase in several well-observed novae. We find that these rebrightenings are isolated flare-like events on otherwise smooth light curves and we show that in most novae in our sample the time intervals between consecutive flares gradually increase as a geometric series; rebrightenings are equally spaced in logarithmic time. We also find a correlation between the rate of increase in the time between rebrightenings with the speed class of the nova in the sense that slower novae tend to increase the time intervals faster. We attribute these rebrightenings to instabilities in the envelope hydrogen burning and we mention other cases of such timing pattern in astronomy and natural sciences in general.

astro-ph.SR↗
arXiv2008

Extended-Source Effect and Chromaticity in Two-Point-Mass Microlensing

We explore the sensitivity of two-point-mass gravitational microlensing to the extended nature of the source star, as well as the related sensitivity to its limb darkening. We demonstrate that the sensitive region, usually considered to be limited to a source-diameter-wide band along the caustic, is strongly expanded near cusps, most prominently along their outer axis. In the case of multi-component caustics, facing cusps may form a region with a non-negligible extended-source effect spanning the gap between them. We demonstrate that for smaller sources the size of the sensitive region extending from a cusp measured in units of source radii increases, scaling as the inverse cube root of the radius. We study the extent of different sensitivity contours and show that for a microlensed Galactic bulge giant the probability of encountering at least a 1% extended-source effect is higher than the probability of caustic crossing by 40-60% when averaged over a typical range of lens-component separations, with the actual value depending on the mass ratio of the components. We derive analytical expressions for the extended-source effect and chromaticity for a source positioned off the caustic. These formulae are more generally applicable to any gravitational lens with a sufficiently small source. Using exactly computed amplifications we test the often used linear-fold caustic approximation and show that it may lead to errors on the level of a few percent even in near-ideal caustic-crossing events. Finally, we discuss several interesting cases of observed binary and planetary microlensing events and point out the importance of our results for the measurement of stellar limb darkening from microlensing light curves.

astro-ph↗
arXiv2005

Light Variations of the Anomalous Central Star of Planetary Nebula Sh 2-71

We present an analysis of light variations in UBVRI of the anomalous object in the center of planetary nebula Sh 2-71. We refined the linear ephemeris of the light maxima to JD{max} = 2449862.0 + 68.101 (E-96), but also identified long-term, obviously non-periodic variations. The latter manifest themselves in large O-C shifts, a variable profile of light curves (hereafter LC) and changes in the mean brightness of the object. Our spectroscopic observations suggested the presence of a superdense nebula in the center of Sh 2-71.

astro-ph↗
arXiv2002

Observations of possible symbiotic star V335 Vul

We present visual and CCD VRI observations of a suspected symbiotic binary V335 Vul. Long term photometry suggests usual Mira or SRa pulsations. High speed V band CCD photometry didn't reveal anything unusual above 0.02 mag.

astro-ph↗