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Astronomy/Physics 410: Astrophysics
Fall Semester 2007

Astrophysics: Journal Articles

This page of links is assembled automatically via software algorithm. These are links to some current Astrophysics-related Journal Articles (which are peer-reviewed) and preprints (which are NOT peer-reviewed).

USAGE NOTE: The symbol next to each feed item can be clicked in order to bring up a more complete description.

Recent Peer-Reviewed Journal Articles

One of the things that sets science apart from many other fields of human-endeavor is that to prevent personal opinion from being represented as fact, articles that are submitted to scientific journals are "peer-reviewed", which means the paper is sent to a referee (possibly a scientific "competitor") who disects the article and makes sure its conclusions are valid based on the data presented. The process is not perfect, but it tends to drive a high level of scholarly quality in peer-reviewed journals.

Non Peer-Reviewed Preprints

Preprints are a time-honored way for scientists to pass on their cutting-edge work to their colleagues. Preprints have not generally been peer-reviewed, so any conclusions in the article may be considered tenttive. Furthermore, in the modern preprint process, authors upload their preprints to a publically accessible server, so in practice, a few completely non-scientific papers that would never survive the peer-review process do sneak onto the preprint servers. So Caveat Emptor.

Astro-Ph Preprint Server (20 Most Recent Items)

1. High energy neutrinos from charm in astrophysical sources.
   Charm production gives rise to a flux of very high energy neutrinos from astrophysical sources with jets driven by central engines, such as gamma ray bursts or supernovae with jets. The neutrino flux from semi-leptonic decays of charmed mesons is subject to much less hadronic and radiative cooling than the conventional flux from pion and kaon decays and therefore has a dominant contribution at higher energies, of relevance to future ultrahigh energy neutrino experiments.
2. A search for rapid pulsations in the magnetic cool chemically peculiar star HD3980.
   The Ap star HD3980 appears to be a promising roAp candidate based on its fundamental parameters, leading us to search for rapid pulsations with the VLT UV-Visual Echelle Spectrograph (UVES). A precise Hipparcos parallax and estimated temperature of 8100K place HD3980 in the middle of the theoretical instability strip for rapidly oscillating Ap stars, about halfway through its main sequence evolution stage. The star has a strong, variable magnetic field, as is typical of the cool magnetic Ap stars. Dipole model parameters were determined from VLT observations using FORS1. From Doppler shift measurements for individual spectral lines of rare earth elements and the H-alpha line core, we find no pulsations above 20-30 m/s. This result is corroborated by inspection of lines of several other chemical elements, as well as with crosscorrelation for long spectral regions with the average spectrum as a template. Abundances of chemical elements were determined and show larger than solar abundances of rare earth elements....
3. AKARI/FIS Mapping of the ISM-Wind Bow Shock around Alpha Ori.
   We present 10' x 50' scan maps around an M supergiant Alpha Ori at 65, 90, 140 and 160 microns obtained with the AKARI Infrared Astronomy Satellite. Higher spatial resolution data with the exact analytic solution permit us to fit the de-projected shape of the stellar wind bow shock around Alpha Ori to have the stand-off distance of 4.8', position angle of 55 degrees and inclination angle of 56 degrees. The shape of the bow shock suggests that the peculiar velocity of Alpha Ori with respect to the local medium is v_* = 40 (n_H)^(-1/2), where n_H is the hydrogen nucleus density at Alpha Ori. We find that the local medium is of n_H = 1.5 to 1.9 cm^(-3) and the velocity of the local flow is at 11 km s^(-1) by using the most recent astrometric solutions for Alpha Ori under the assumption that the local medium is moving away from the Orion OB 1 association. AKARI images may also reveal a vortex ring due to instabilities on the surface of the bow shock as demonstrated by numerical models. This research...
4. On the dangers of using the growth equation on large scales.
   We examine the accuracy of the growth equation $\ddot{\delta} + 2H\dot{\delta} - 4\pi G\rho\delta = 0$, which is often used in the cosmological literature to study the growth of perturbations. By comparing the growth predicted by this equation to a numerical solution of the linearized Einstein equations in the $\Lambda$CDM scenario, we show that while this equation is a reliable approximation on small scales ($k\gtrsim $h Mpc$^{-1}$), it can be disastrously inaccurate ($\sim 10^4% $) on larger scales. We propose a modified version of the growth equation, which while preserving the simplicity of the original equation, provides considerably more accurate results for the growth of matter perturbations in a $\Lambda$CDM universe. Finally, we examine the implications of the failure of the growth equation on a few recent studies, aimed at discriminating general relativity from modified gravity, which use this equation as a starting point. We show that while the results of these studies are valid on small scales, they...
5. Statistical Tools for Analyzing the Cosmic Ray Energy Spectrum.
   In this paper un-binned statistical tools for analyzing the cosmic ray energy spectrum are developed and illustrated with a simulated data set. The methods are designed to extract accurate and precise model parameter estimators in the presence of statistical and systematic energy errors. Two robust methods are used to test for the presence of flux suppression at the highest energies: the Tail-Power statistic and a likelihood ratio test. Both tests give evidence of flux suppression in the simulated data. The tools presented can be generalized for use on any astrophysical data set where the power-law assumption is relevant and can be used to aid observational design.
6. Are There Meteors Originated from Near Earth Asteroid (25143) Itokawa?.
   As a result of a survey of Itokawid meteors (i.e., meteors originated from Near Earth Asteroid (25143) Itokawa = 1998SF36), from among the multi-station optical meteor orbit data of ~15000 orbits, and applying the D-criteria, we could find five Itokawid meteor candidates. We also analyzed corresponding mineral materials of the Itokawid candidates through their trajectory and atmospheric data. We conclude, on the basis of our investigation, that the fireball, MORP172, is the strongest Itokawid candidate.
7. The Acceleration of the Nebular Shells in Planetary Nebulae in the Milky Way Bulge.
   We present a systematic study of line widths in the [\ion{O}{3}]$\lambda$5007 and H$\alpha$ lines for a sample of 86 planetary nebulae in the Milky Way bulge based upon spectroscopy obtained at the \facility{Observatorio Astron\'omico Nacional in the Sierra San Pedro M\'artir (OAN-SPM)} using the Manchester Echelle Spectrograph. The planetary nebulae were selected with the intention of simulating samples of bright extragalactic planetary nebulae. We separate the planetary nebulae into two samples containing cooler and hotter central stars, defined by the absence or presence, respectively, of the \ion{He}{2} $\lambda$6560 line in the H$\alpha$ spectra. This division separates samples of younger and more evolved planetary nebulae. The sample of planetary nebulae with hotter central stars has systematically larger line widths, larger radii, lower electron densities, and lower H$\beta$ luminosities. The distributions of these parameters in the two samples all differ at significance levels exceeding 99%....
8. Combined Chandra, XMM-Newton and Hubble Space Telescope observations of the Galactic globular cluster NGC 2808.
   Using new Chandra X-ray observations and existing XMM-Newton X-ray and Hubble far ultraviolet observations, we aim to detect and identify the faint X-ray sources belonging to the Galactic globular cluster NGC 2808 in order to understand their role in the evolution of globular clusters. We present a Chandra X-ray observation of the Galactic globular cluster NGC 2808. We classify the X-ray sources associated with the cluster by analysing their colours and variability. Previous observations with XMM-Newton and far ultraviolet observations with the Hubble Space Telescope are re-investigated to help identify the Chandra sources associated with the cluster. We compare our results to population synthesis models and observations of other Galactic globular clusters. We detect 113 sources, of which 16 fall inside the half-mass radius of NGC 2808 and are concentrated towards the cluster core. From statistical analysis, these 16 sources are very likely to be linked to the cluster. We detect short-term (1 day) variability...
9. Cosmology-Independent Distance Moduli of 42 Gamma-Ray Bursts between Redshift of 1.44 and 6.60.
   This report is an update and extension of our paper accepted for publication in ApJ (arXiv:0802.4262). Since objects at the same redshift should have the same luminosity distance and the distance moduli of type Ia supernovae (SNe Ia) obtained directly from observations are completely cosmology independent, we obtain the distance modulus of a gamma-ray burst (GRB) at a given redshift by interpolating or iterating from the Hubble diagram of SNe Ia. Then we calibrate five GRB relations without assuming a particular cosmological model, from different regression methods, and construct the GRB Hubble diagram to constrain cosmological parameters. Based upon these relations we list the cosmology-independent distance moduli of 42 GRBs between redshift of 1.44 and 6.60, with the 1-$\sigma$ uncertainties of 1-3%.
10. The Chemical and Ionization Conditions in Weak Mg II Absorbers.
    We present an analysis of the chemical and ionization conditions in a sample of 100 weak Mg II absorbers identified in the VLT/UVES archive of quasar spectra. Using a host of low ionization lines associated with each absorber in this sample, and on the basis of ionization models, we infer that the metallicity in a significant fraction of weak Mg II clouds is constrained to values of solar or higher, if they are sub-Lyman limit systems. Based on the observed constraints, we present a physical picture in which weak Mg II absorbers are predominantly tracing two different astrophysical processes/structures. A significant population of weak Mg II clouds, those in which N(Fe II) is much less than N(Mg II), identified at both low (z ~ 1) and high (z ~ 2) redshift, are potentially tracing gas in the extended halos of galaxies, analogous to the Galactic high velocity clouds. These absorbers might correspond to alpha-enhanced interstellar gas expelled from star-forming galaxies, in correlated supernova events. On the other...
11. The Extended Star Formation History of the Andromeda Spheroid at 35 Kpc on the Minor Axis.
    Using the HST ACS, we have obtained deep optical images reaching well below the oldest main sequence turnoff in fields on the southeast minor-axis of the Andromeda Galaxy, 35 kpc from the nucleus. These data probe the star formation history in the extended halo of Andromeda -- that region beyond 30 kpc that appears both chemically and morphologically distinct from the metal-rich, highly-disturbed inner spheroid. The present data, together with our previous data for fields at 11 and 21 kpc, do not show a simple trend toward older ages and lower metallicities, as one might expect for populations further removed from the obvious disturbances of the inner spheroid. Specifically, the mean ages and [Fe/H] values at 11 kpc, 21 kpc, and 35 kpc are 9.7 Gyr and -0.65, 11.0 Gyr and -0.87, and 10.5 Gyr and -0.98, respectively. In the best-fit model of the 35 kpc population, one third of the stars are younger than 10 Gyr, while only ~10% of the stars are truly ancient and metal-poor. The extended halo thus exhibits clear...
12. DLA kinematics and outflows from starburst galaxies.
    We present results from a numerical study of the multiphase interstellar medium in sub-Lyman-break galaxy protogalactic clumps. Such clumps are abundant at z=3 and are thought to be a major contributor to damped Ly-alpha absorption. We model the formation of winds from these clumps and show that during star formation episodes they feature outflows with neutral gas velocity widths up to several hundred km/s. Such outflows are consistent with the observed high-velocity dispersion in DLAs. In our models thermal energy feedback from winds and supernovae results in efficient outflows only when cold (~ 300 K), dense (> 100 msun/pc^3) clouds are resolved at grid resolution of 12 pc. At lower 24 pc resolution the first signs of the multiphase medium are spotted; however, at this low resolution thermal injection of feedback energy cannot yet create hot expanding bubbles around star-forming regions -- instead feedback tends to erase high-density peaks and suppress star formation. At 12 pc resolution feedback compresses...
13. A discontinuity in the low-mass IMF - the case of high multiplicity.
    The empirical binary properties of brown dwarfs (BDs) differ from those of normal stars suggesting BDs form a separate population. Recent work by Thies and Kroupa revealed a discontinuity of the initial mass function (IMF) in the very-low-mass star regime under the assumption of a low multiplicity of BDs of about 15 per cent. However, previous observations had suggested that the multiplicity of BDs may be significantly higher, up to 45 per cent. This contribution investigates the implication of a high BD multiplicity on the appearance of the IMF for the Orion Nebula Cluster, Taurus-Auriga, IC 348 and the Pleiades. We show that the discontinuity remains pronounced even if the observed MF appears to be continuous, even for a BD binary fraction as high as 60%. We find no evidence for a variation of the BD IMF with star-forming conditions. The BD IMF has a power-law index alpha = +0.3 and about 2 BDs form per 10 low-mass stars assuming equal-mass pairing of BDs.
14. Magnetic support of the optical emission line filaments in NGC 1275.
    The giant elliptical galaxy NGC 1275, at the centre of the Perseus cluster, is surrounded by a well-known giant nebulosity of emission-line filaments, which are plausibly about >10^8 yr old. The filaments are dragged out from the centre of the galaxy by the radio bubbles rising buoyantly in the hot intracluster gas before later falling back. They act as dramatic markers of the feedback process by which energy is transferred from the central massive black hole to the surrounding gas. The mechanism by which the filaments are stabilized against tidal shear and dissipation into the surrounding 4x10^7 K gas has been unclear. Here we report new observations that resolve thread-like structures in the filaments. Some threads extend over 6 kpc, yet are only 70 pc wide. We conclude that magnetic fields in the threads, in pressure balance with the surrounding gas, stabilize the filaments, so allowing a large mass of cold gas to accumulate and delay star formation.
15. Effects of Supernova Feedback on the Formation of Galaxies.
    We study the effects of Supernova (SN) feedback on the formation of galaxies using hydrodynamical simulations in a Lambda-CDM cosmology. We use an extended version of the code GADGET-2 which includes chemical enrichment and energy feedback by Type II and Type Ia SN, metal-dependent cooling and a multiphase model for the gas component. We focus on the effects of SN feedback on the star formation process, galaxy morphology, evolution of the specific angular momentum and chemical properties. We find that SN feedback plays a fundamental role in galaxy evolution, producing a self-regulated cycle for star formation, preventing the early consumption of gas and allowing disks to form at late times. The SN feedback model is able to reproduce the expected dependence on virial mass, with less massive systems being more strongly affected.
16. Another forbidden solar oxygen abundance: the [O I] 5577 A line.
    Context: Recent works with improved model atmospheres, line formation, atomic and molecular data, and detailed treatment of blends, have resulted in a significant downward revision of the solar oxygen abundance. Aims: Considering the importance of the Sun as an astrophysical standard and the current conflict of standard solar models using the new solar abundances with helioseismological observations we have performed a new study of the solar oxygen abundance based on the forbidden [OI] line at 5577.34 A, not previously considered. Methods: High-resolution (R > 500 000), high signal-to-noise (S/N > 1000) solar spectra of the [O I] 5577.34 A line have been analyzed employing both three-dimensional (3D) and a variety of 1D (spatially and temporally averaged 3D, Holweger & Muller, MARCS and Kurucz models with and without convective overshooting) model atmospheres. Results: The oxygen abundance obtained from the [OI] 5577.3 A forbidden line is almost insensitive to the input model atmosphere and has a...
17. On the formation and abundance of CO in AGB envelopes.
    It is generally considered, as a rule of thumb, that carbon monoxide forms very early in envelopes of AGB stars, and that it consumes most of the carbon, or most of the oxygen, depending on whether the photosphere is oxygen-rich or carbon-rich, respectively. \rm This work focuses on the latter case, with the purpose of quantifying the remaining fraction of gaseous carbon which is then available for forming carbonaceous grains. Since AGB stars are (probably) the main providers of cosmic carbon grains, this residual fraction is essential in establishing the validity of current grain models. Here, we use a kinetic treatment to follow the chemical evolution of circumstellar shells towards steady state. It is shown that the residual fraction depends essentially on the atomic ratio of pristine gaseous carbon and oxygen, and on the cross-section for CH formation by collision of C and H atoms. It lies between 55 and 144 C atoms per million H atoms, depending on the values adopted for unknown reaction rates and cosmic...
18. A mid-infrared study of HII regions in the Magellanic Clouds: N88A and N160A.
    To show the importance of high-spatial resolution observations of HII regions when compared with observations obtained with larger apertures such as ISO, we present mid-infrared spectra of two Magellanic Cloud HII regions, N88A and N160A. We obtained mid-infrared (8-13 um), long-slit spectra with TIMMI2 on the ESO 3.6m telescope. These are combined with archival spectra obtained with the Infrared Spectrograph (IRS) onboard the Spitzer Space Telescope, and are compared with the low-spatial resolution ISO-SWS data. An inventory of the spectra in terms of atomic fine-structure lines and molecular bands is presented. Concerning N88A, an isolated HII region with no adjacent infrared sources, the observations indicate that the line fluxes observed by ISO-SWS and Spitzer-IRS come exclusively from the compact HII region of about 3 arcsec in diameter. This is not the case for N160A, which has a more complex morphology. We have spectroscopically isolated for the first time the individual contributions of the three components...
19. Core Formation in Giant Gaseous Protoplanets.
    Sedimentation rates of silicate grains in gas giant protoplanets formed by disk instability are calculated for protoplanetary masses between 1 M_Saturn to 10 M_Jupiter. Giant protoplanets with masses of 5 M_Jupiter or larger are found to be too hot for grain sedimentation to form a silicate core. Smaller protoplanets are cold enough to allow grain settling and core formation. Grain sedimentation and core formation occur in the low mass protoplanets because of their slow contraction rate and low internal temperature. It is predicted that massive giant planets will not have cores, while smaller planets will have small rocky cores whose masses depend on the planetary mass, the amount of solids within the body, and the disk environment. The protoplanets are found to be too hot to allow the existence of icy grains, and therefore the cores are predicted not to contain any ices. It is suggested that the atmospheres of low mass giant planets are depleted in refractory elements compared with the atmospheres of more massive...
20. Tidal Heating of Terrestrial Extra-Solar Planets and Implications for their Habitability.
    The tidal heating of hypothetical rocky (or terrestrial) extra-solar planets spans a wide range of values depending on stellar masses and initial orbits. Tidal heating may be sufficiently large (in many cases, in excess of radiogenic heating) and long-lived to drive plate tectonics, similar to the Earth's, which may enhance the planet's habitability. In other cases, excessive tidal heating may result in Io-like planets with violent volcanism, probably rendering them unsuitable for life. On water-rich planets, tidal heating may generate sub-surface oceans analogous to Europa's with similar prospects for habitability. Tidal heating may enhance the outgassing of volatiles, contributing to the formation and replenishment of a planet's atmosphere. To address these issues, we model the tidal heating and evolution of hypothetical extra-solar terrestrial planets. The results presented here constrain the orbital and physical properties required for planets to be habitable.