Candidate Pre-Mainsequence F Stars with Circumstellar Dust Identified Using Combined 2MASS and uvby Data

We propose a method that uses near-infrared plus uvby photometry to identify potentially extensive circumstellar dusty environment about F and A stars. The method has been applied to a sample of ~900 metal rich reddened F stars with 2MASS and uvby data, suggesting the presence of circumstellar dust emitting in the near infrared for ~70 stars. The log T_e - M_V diagram suggests that most, if not all, of them are likely pre-mainsequence (PMS). They seem to be consistent with being a continuation of the class of Herbig Ae/Be PMS stars into the spectral type F. Their number drops sharply downward of log T_e ~ 3.84 (spectral types later than ~F5), which may provide new clues to the PMS evolution of stars with 1 to 2 solar mass. We present a list of 21 most conspicuous candidate stars with circumstellar dust. About half of them are associated with the extended star-forming region around rho Oph. The brightest of these 21 stars, with V < 7.5, turn out to be IRAS sources, suggesting the presence of heated dust emitting in the far infrared. Also in this list, HD 81270 is reported as a very unusual star moving away from the Galactic plane at a projected speed of 70 km/sec.Comment: 8 pages, 4 figures, 1 table. To appear in ApJ, part 2, v. 570, 2002 May

The Minimum Stellar Mass in Early Galaxies

The conditions for the fragmentation of the baryonic component during merging of dark matter halos in the early Universe are studied. We assume that the baryonic component undergoes a shock compression. The characteristic masses of protostellar molecular clouds and the minimum masses of protostars formed in these clouds decrease with increasing halo mass. This may indicate that the initial stellar mass function in more massive galaxies was shifted towards lower masses during the initial stages of their formation. This would result in an increase of the number of stars per unit halo mass, i.e., the efficiency of star formation.Comment: 18 pages, 7 figure

The NICMOS Snapshot Survey of nearby Galaxies

We present ``snapshot'' observations with the NearInfrared Camera and MultiObject Spectrometer (NICMOS) on board the Hubble Space Telescope (HST) of 94 nearby galaxies from the Revised Shapley Ames Catalog. Images with 0.2 as resolution were obtained in two filters, a broad-band continuum filter (F160W, roughly equivalent to the H-band) and a narrow band filter centered on the Paschen alpha line (F187N or F190N, depending on the galaxy redshift) with the 51x51 as field of view of the NICMOS camera 3. A first-order continuum subtraction is performed, and the resulting line maps and integrated Paschen alpha line fluxes are presented. A statistical analysis indicates that the average Paschen alpha surface brightness {\bf in the central regions} is highest in early-type (Sa-Sb) spirals.Comment: Original contained error in flux calibration. Table 1 now has correct Paschen Alpha fluxes. 14 pages LaTeX with JPEG and PS figures. Also available at http://icarus.stsci.edu/~boeker/publications.htm

The entropy and energy of intergalactic gas in galaxy clusters

Studies of the X-ray surface brightness profiles of clusters, coupled with theoretical considerations, suggest that the breaking of self-similarity in the hot gas results from an `entropy floor', established by some heating process, which affects the structure of the intracluster gas strongly in lower mass systems. Fitting analytical models for the radial variation in gas density and temperature to X-ray spectral images from the ROSAT PSPC and ASCA GIS, we derive gas entropy profiles for 20 galaxy clusters and groups. Scaling these profiles to coincide in the self-similar case, the lowest mass systems are found to have higher scaled entropy profiles than more massive systems. This appears to be due to a baseline entropy of 70-140 h50^-1/3 keV cm^2, depending on the extent to which shocks have been suppressed in low mass systems. The extra entropy may be present in all systems, but is detectable only in poor clusters, compared to the entropy generated by gravitational collapse. This excess entropy appears to be distributed uniformly with radius outside the central cooling regions. We determine the energy associated with this entropy floor, by studying the net reduction in binding energy of the gas in low mass systems, and find that it corresponds to a preheating temperature of ~0.3 keV. Since the relationship between entropy and energy injection depends upon gas density, we can combine the excesses of 70-140 keV cm^2 and 0.3 keV to derive the typical electron density of the gas into which the energy was injected. The resulting value of 1-3x10^-4 h50^1/2 cm-3, implies that the heating must have happened prior to cluster collapse but after a redshift z~7-10. The energy requirement is well matched to the energy from supernova explosions responsible for the metals which now pollute the intracluster gas.Comment: 15 pages, 10 figures, accepted for publication in MNRA

Dark Matter Search Perspectives with GAMMA-400

GAMMA-400 is a future high-energy gamma-ray telescope, designed to measure the fluxes of gamma-rays and cosmic-ray electrons + positrons, which can be produced by annihilation or decay of dark matter particles, and to survey the celestial sphere in order to study point and extended sources of gamma-rays, measure energy spectra of Galactic and extragalactic diffuse gamma-ray emission, gamma-ray bursts, and gamma-ray emission from the Sun. GAMMA-400 covers the energy range from 100 MeV to ~3000 GeV. Its angular resolution is ~0.01 deg(Eg > 100 GeV), and the energy resolution ~1% (Eg > 10 GeV). GAMMA-400 is planned to be launched on the Russian space platform Navigator in 2019. The GAMMA-400 perspectives in the search for dark matter in various scenarios are presented in this paperComment: 4 pages, 4 figures, submitted to the Proceedings of the International Cosmic-Ray Conference 2013, Brazil, Rio de Janeir

From 10 Kelvin to 10 TeraKelvin: Insights on the Interaction Between Cosmic Rays and Gas in Starbursts

Recent work has both illuminated and mystified our attempts to understand cosmic rays (CRs) in starburst galaxies. I discuss my new research exploring how CRs interact with the ISM in starbursts. Molecular clouds provide targets for CR protons to produce pionic gamma rays and ionization, but those same losses may shield the cloud interiors. In the densest molecular clouds, gamma rays and Al-26 decay can provide ionization, at rates up to those in Milky Way molecular clouds. I then consider the free-free absorption of low frequency radio emission from starbursts, which I argue arises from many small, discrete H II regions rather than from a "uniform slab" of ionized gas, whereas synchrotron emission arises outside them. Finally, noting that the hot superwind gas phase fills most of the volume of starbursts, I suggest that it has turbulent-driven magnetic fields powered by supernovae, and that this phase is where most synchrotron emission arises. I show how such a scenario could explain the far-infrared radio correlation, in context of my previous work. A big issue is that radio and gamma-ray observations imply CRs also must interact with dense gas. Understanding how this happens requires a more advanced understanding of turbulence and CR propagation.Comment: Conference proceedings for "Cosmic-ray induced phenomenology in star-forming environments: Proceedings of the 2nd Session of the Sant Cugat Forum of Astrophysics" (April 16-19, 2012). 16 pages, 5 figure

The Stellar IMF from Turbulent Fragmentation

The morphology and kinematics of molecular clouds (MCs) are best explained as the consequence of super--sonic turbulence. Super--sonic turbulence fragments MCs into dense sheets, filaments and cores and large low density ``voids'', via the action of highly radiative shocks. We refer to this process as "turbulent fragmentation". In this work we derive the mass distribution of gravitationally unstable cores generated by the process of turbulent fragmentation. The mass distribution above one solar mass depends primarily on the power spectrum of the turbulent flow and on the jump conditions for isothermal shocks in a magnetized gas. For a power spectrum index \beta=-1.74, consistent with Larson's velocity dispersion--size relation as well as with new numerical and analytic results on super--sonic turbulence, we obtain a power law mass distribution of dense cores with a slope equal to 3/(4-\beta) = 1.33, consistent with the slope of the stellar IMF. Below one solar mass, the mass distribution flattens and turns around at a fraction of a solar mass, as observed for the stellar IMF in a number of stellar clusters, because only the densest cores are gravitationally unstable. The mass distribution at low masses is determined by the probability distribution of the gas density, which is known to be approximately Log--Normal for an isothermal turbulent gas. The intermittent nature of the turbulent density distribution is thus responsible for the existence of a significant number of small collapsing cores, even of sub--stellar mass. Since turbulent fragmentation is unavoidable in super--sonically turbulent molecular clouds, and given the success of the present model in predicting the observed shape of the stellar IMF, we conclude that turbulent fragmentation is essential to the origin of the stellar IMF.Comment: 15 pages, 3 figures included, submitted to Ap

A New Model for the Spiral Structure of the Galaxy. Superposition of 2+4-armed patterns

We investigate the possibility of describing the spiral pattern of the Milky Way in terms of a model of superposition 2- and 4-armed wave harmonics (the simplest description, besides pure modes). Two complementary methods are used: a study of stellar kinematics, and direct tracing of positions of spiral arms. In the first method, the parameters of the galactic rotation curve and the free parameters of the spiral density waves were obtained from Cepheid kinematics, under different assumptions. To turn visible the structure corresponding to these models, we computed the evolution of an ensemble of N-particles, simulating the ISM clouds, in the perturbed galactic gravitational field. In the second method, we present a new analysis of the longitude-velocity (l-v) diagram of the sample of galactic HII regions, converting positions of spiral arms in the galactic plane into locii of these arms in the l-v diagram. Both methods indicate that the ``self-sustained'' model, in which the 2-armed and 4-armed mode have different pitch angles (6 arcdeg and 12 arcdeg, respectively) is a good description of the disk structure. An important conclusion is that the Sun happens to be practically at the corotation circle. As an additional result of our study, we propose an independent test for localization of the corotation circle in a spiral galaxy: a gap in the radial distribution of interstellar gas has to be observed in the corotation region.Comment: 17 pages, 9 figures, Latex, uses aas2pp4.st

A separation of electrons and protons in the GAMMA-400 gamma-ray telescope

The GAMMA-400 gamma-ray telescope is intended to measure the fluxes of gamma rays and cosmic-ray electrons and positrons in the energy range from 100 MeV to several TeV. Such measurements concern with the following scientific goals: search for signatures of dark matter, investigation of gamma-ray point and extended sources, studies of the energy spectra of Galactic and extragalactic diffuse emission, studies of gamma-ray bursts and gamma-ray emission from the active Sun, as well as high-precision measurements of spectra of high-energy electrons and positrons, protons, and nuclei up to the knee. The main components of cosmic rays are protons and helium nuclei, whereas the part of lepton component in the total flux is ~10E-3 for high energies. In present paper, the capability of the GAMMA-400 gamma-ray telescope to distinguish electrons and positrons from protons in cosmic rays is investigated. The individual contribution to the proton rejection is studied for each detector system of the GAMMA-400 gamma-ray telescope. Using combined information from all detector systems allow us to provide the proton rejection from electrons with a factor of ~4x10E5 for vertical incident particles and ~3x10E5 for particles with initial inclination of 30 degrees. The calculations were performed for the electron energy range from 50 GeV to 1 TeV.Comment: 19 pages, 10 figures, submitted to Advances and Space Researc