arXiv subjects

Behrooz Karamiqucham

Publications and source records attributed to Behrooz Karamiqucham.

5 recordsLinked to original sources

arXiv2025

Secondary eclipses of two brown dwarfs in the K2 fields: detection by multiple dataset merging

By using various data sources for the stellar fluxes in overlapping campaign fields and employing full time series modeling, we report the detection of the secondary eclipses of two brown dwarfs (CWW 89Ab = EPIC 219388192b and HSHJ 430b = EPIC 211946007b). The detections yielded timings in agreement with the orbital elements derived from the earlier radial velocity measurements and eclipse depths of 70+/-12 ppm (CWW 89Ab) and 852+/-123 ppm (HSHJ 430b). While the high depth in the Kepler waveband for HSHJ 430b is in agreement with the assumption that the emitted flux comes mostly from the internal heat source and the absorbed stellar irradiation, the case of CWW 89Ab suggests very high albedo, because of the lack of sufficient thermal radiation in the Kepler waveband. Assuming completely reflective dayside hemisphere, without circulation, the maximum value of the eclipse depth due to the reflection of the stellar light is 56 ppm. By making the extreme assumption that the true eclipse depth is 3 sigma less than the observed depth, the minimum geometric albedo becomes ~0.6.

astro-ph.EP↗

arXiv2025

Constraining Brown Dwarf Desert Formation Mechanisms Through Bayesian Statistical Comparison of Observed and Simulated Populations

We present a comprehensive Bayesian statistical analysis of brown dwarf companions to investigate the physical mechanisms responsible for the observed ``brown dwarf desert'' -- the notable paucity of brown dwarf companions at orbital separations $<$5~AU. Using a carefully vetted sample of 88 confirmed brown dwarf companions from the \texttt{exoplanet.eu} catalog with masses 13--80~$\mjup$ and semi-major axes 0.1--5.0~AU, we employ Markov Chain Monte Carlo (MCMC) optimization and two-dimensional Kolmogorov-Smirnov tests to compare observed orbital and mass distributions with three theoretical formation scenarios: (A) Type II disk-driven migration, (B) core accretion with mass-dependent survival, and (C) dynamical scattering from wide orbits. Our analysis spans 4-parameter models for each scenario, with proper posterior distributions quantifying parameter uncertainties and correlations. The disk migration model provides statistically superior fits (2D KS $p = 0.18$), with optimal parameters $\log_{10}\nu = -6.47^{+0.42}_{-0.31}$, $\sigma_\nu = 0.34^{+0.23}_{-0.17}$, $t_{\rm disk} = 1.66^{+1.24}_{-0.84}$~Myr, and $M_{\rm gap} = 12.0^{+4.7}_{-8.3}~\mjup$, consistent with Type II migration theory. The dynamical scattering model achieves intermediate performance ($p = 0.08$), while core accretion scenarios show poor agreement ($p < 0.001$) despite theoretical sophistication. Occurrence rate analysis reveals the desert region (0.1--5~AU) is depleted by a factor of $\approx$1.6 relative to wide separations ($>$5~AU), a constraint successfully reproduced only by the migration model. Our results provide quantitative evidence that brown dwarfs form at wide separations (10--30~AU) through disk fragmentation and undergo limited Type II migration to reach observed close-in locations, with migration naturally halting near 1~AU through gap-opening processes.

astro-ph.EP↗

arXiv2021

Bright single-mode RR Lyrae stars: matching Gaia EDR3 with pulsation and evolutionary models

We combine observed metallicity, optical and infrared magnitudes with evolutionary and pulsation models to derive average luminosities for 156 single-mode RR Lyrae stars. These luminosities are compared with those obtained from the Gaia EDR3 parallaxes, and found in excellent agreement with the high accuracy subsample (62 stars, with relative parallax errors less than 2%). With the temperature and metallicity scale used, no parallax shift seems to be necessary when alpha-enhanced evolutionary models are employed. Some 10% of the sample shows curious `distance keeping' between the evolutionary and pulsation models. The cause of this behavior is not clear at this moment but can be cured by an excessive increase of the reddening.

astro-ph.SR↗

arXiv2021

Near-Infrared atmospheric modelling of Jupiter's South Equatorial Belt (SEB) observed with AAT/IRIS2

Near-Infrared spectra of Jupiter's South Equatorial Belt (SEB) with AAT/IRIS2 in H and K bands at a resolving power of R~2400 have been obtained. By creating line-by-line radiative transfer models with the latest improved spectral line data for ammonia and methane (HITRAN2016), we derive best models of cloud/haze parameters in Jupiter's South Equatorial Belt. The modelled spectra fit the observations well except for small, isolated discrepancies in the trough region of H2-H2 collision-induced-absorption around 2.08 μm and the methane absorption level between 2.16 and 2.19 μm in K band and at the high pressure methane window between 1.596 to 1.618 μm in H band.

astro-ph.EP↗

arXiv2021

Probing galactic double-mode RR Lyrae stars against Gaia EDR3

Classical double-mode pulsators (RR Lyrae stars and delta Cepheids) are important for their simultaneous pulsation in low-order radial modes. This enables us to put stringent constraints on their physical parameters. We use 30 bright galactic double-mode RR~Lyrae (RRd) stars to estimate their luminosities and compare them with those derived from the parallaxes of the recent data release (EDR3) of the Gaia survey. We employ pulsation and evolutionary models, together with observationally determined effective temperatures to derive the basic stellar parameters. Excluding 6 outlying stars (e.g., with blending issues) the RRd and Gaia luminosities correlate well. With the adopted temperature zero point from one of the works based on the infrared flux method, we find it necessary to increase the Gaia parallaxes by 0.02 mas to bring the RRd and Gaia luminosities into agreement. This value is consonant with those derived from studies on binary stars in the context of Gaia. We examine also the resulting period-luminosity-metallicity (PLZ) relation in the 2MASS K band as follows from the RRd parameters. This leads to the verification of two independently derived other PLZs. No significant zero point differences are found. Furthermore, the predicted K absolute magnitudes agree within sigma=0.005-0.01mag.

astro-ph.SR↗