Increase In Stellar Mass Research Articles

Dynamical processes in star forming regions: feedback and turbulence generation

AbstractYoung stellar objects (YSOs) inject large amounts of momentum and kinetic energy into their surroundings. Feedback from low mass YSOs is dominated by their outflows. However, as stellar mass increases, UV photo-heating and ionization play increasingly important roles. Massive stars produce powerful stellar winds and explode as supernovae within 3 – 40 Myr after birth. While low-mass protostellar feedback can drive turbulence in cloud cores and even disrupt the star forming environment, feedback from massive stars plays important roles in the generation of cloud structure and motions in the entire ISM.

Gravitational Collapse and Neutrino Emission of Population III Massive Stars

Pop III stars are the first stars in the universe. They do not contain metals and their formation and evolution may be different from that of stars of later generations. In fact, according to the theory of star formation, Pop III stars might have very massive components ($\sim 100 - 10000M_\odot$). In this paper, we compute the spherically symmetric gravitational collapse of these Pop III massive stars. We solve the general relativistic hydrodynamics and neutrino transfer equations simultaneously, treating neutrino reactions in detail. Unlike supermassive stars ($\gtrsim 10^5 M_\odot$), the stars of concern in this paper become opaque to neutrinos. The collapse is simulated until after an apparent horizon is formed. We confirm that the neutrino transfer plays a crucial role in the dynamics of gravitational collapse, and find also that the $\beta$-equilibration leads to a somewhat unfamiliar evolution of electron fraction. Contrary to the naive expectation, the neutrino spectrum does not become harder for more massive stars. This is mainly because the neutrino cooling is more efficient and the outer core is more massive as the stellar mass increases. Here the outer core is the outer part of the iron core falling supersonically. We also evaluate the flux of relic neutrino from Pop III massive stars. As expected, the detection of these neutrinos is difficult for the currently operating detectors. However, if ever observed, the spectrum will enable us to obtain the information on the formation history of Pop III stars. We investigate 18 models covering the mass range of $300 - 10^4 M_\odot$, making this study the most detailed numerical exploration of spherical gravitational collapse of Pop III massive stars. This will also serve as an important foundation for multi-dimensional investigations.

Nearly 5000 Distant Early‐Type Galaxies in COMBO‐17: A Red Sequence and Its Evolution sincez ∼ 1

We present the rest-frame colors and luminosities of ~25,000 mR 24 galaxies in the redshift range 0.2 < z ≤ 1.1 drawn from 0.78 deg2 of the COMBO-17 survey (Classifying Objects by Medium-Band Observations in 17 Filters). We find that the rest-frame color distribution of these galaxies is bimodal at all redshifts out to z ~ 1. This bimodality permits a model-independent definition of red early-type galaxies and blue late-type galaxies at any given redshift. The colors of the blue peak become redder toward the present day, and the number density of blue luminous galaxies has dropped strongly since z ~ 1. Focusing on the red galaxies, we find that they populate a color-magnitude relation. Such red sequences have been identified in galaxy cluster environments, but our data show that such a sequence exists over this redshift range even when averaging over all environments. The mean color of the red galaxy sequence evolves with redshift in a way that is consistent with the aging of an ancient stellar population. The rest-frame B-band luminosity density in red galaxies evolves only mildly with redshift in a Λ-dominated cold dark matter universe. When we account for the change in stellar mass-to-light ratio implied by the redshift evolution in red galaxy colors, the COMBO-17 data indicate an increase in stellar mass on the red sequence by a factor of 2 since z ~ 1. The largest source of uncertainty is large-scale structure, implying that considerably larger surveys are necessary to further refine this result. We explore mechanisms that may drive this evolution in the red galaxy population, finding that both galaxy merging and truncation of star formation in some fraction of the blue star-forming population are required to fully explain the properties of these galaxies.

A Clue to the Extent of Convective Mixing Inside Massive Stars: The Surface Hydrogen Abundances of Luminous Blue Variables and Hydrogen‐poor Wolf‐Rayet Stars

Interior layers of stars that have been exposed by surface mass loss reveal aspects of their chemical and convective histories that are otherwise inaccessible to observation. It must be significant that the surface hydrogen abundances of luminous blue variables (LBVs) show a remarkable uniformity, specifically X(sub surf) = 0.3 - 0.4, while those of hydrogen-poor Wolf-Rayet (WN) stars fall, almost without exception, below these values, ranging down to X(sub surf) = 0. According to our stellar model calculations, most LBVs are post-red-supergiant objects in a late blue phase of dynamical instability, and most hydrogen-poor WN stars are their immediate descendants. If this is so, stellar models constructed with the Schwarzschild (temperature-gradient) criterion for convection account well for the observed hydrogen abundances, whereas models built with the Ledoux (density-gradient) criterion fail. At the brightest luminosities, the observed hydrogen abundances of LBVs are too large to be explained by any of our highly evolved stellar models, but these LBVs may occupy transient blue loops that exist during an earlier phase of dynamical instability when the star first becomes a yellow supergiant. Independent evidence concerning the criterion for convection, which is based mostly on traditional color distributions of less massive supergiants on the Hertzsprung-Russell diagram, tends to favor the Ledoux criterion. It is quite possible that the true criterion for convection changes over from something like the Ledoux criterion to something like the Schwarzschild criterion as the stellar mass increases.

The Nobeyama Millimeter Array Survey for protoplanetary disks around protostar candidates and T Tauri stars in Taurus

The Nobeyama Millimeter Array Survey for protoplanetary disks has been made for 19 protostellar IRAS sources in Taurus; 13 of them were optically invisible protostars and 6 were young T Tauri stars. We observed 98-GHz continuum and CS(J = 2 − 1) line emissions simultaneously with spatial resolutions of 2 . ″ 8-8 . ″ 8 (360-1,200 AU). The continuum emission was detected from 5 out of 6 T Tauri stars and 2 out of 13 protostar candidates: the emission was not spatially resolved and was consistent with being originated from compact circumstellar disks. Extended CS emission was detected around 2 T Tauri stars and 11 protostar candidates. There is a remarkable tendency for the detectability of the 98-GHz continuum emission to be small for protostar candidates. This tendency is explained if the mass of protoplanetary disks around protostars is not as large as that around T Tauri stars; the disk mass may increase with the increase of central stellar mass by dynamical accretion in the course of evolution from protostars to T Tauri stars.

The Nobeyama Millimeter Array Survey for Protoplanetary Disks Around Protostar Candidates and T Tauri Stars in Taurus

AbstractThe Nobeyama Millimeter Array Survey for protoplanetary disks has been made for 19 protostellar IRAS sources in Taurus; 13 were invisible protostars and 6 were youngest T Tauri stars. We observed the 98 GHz continuum and CS(J=2-1) line emissions simultaneously with spatial resolutions of 2.8”- 8.8” (360 AU-1,200 AU). Unresolved continuum emission was detected from 5 of 6 T Tauri stars and 2 of 13 protostar candidates. The continuum emission arose from compact circumstellar disks. Extended CS emission was detected around 2 T Tauri stars and 11 protostar candidates. There is a remarkable tendency for the detectability for the 98 GHz continuum emission to be small for protostar candidates. This tendency is explained if the mass of protoplanetary disks around protostars is not as large as that around T Tauri stars; the disk mass may increase with the increase of central stellar mass by dynamical accretion in the course of evolution from protostars to T Tauri stars.

The instability strip of the anomalous Cepheids in the Draco dwarf spheroidal galaxy

view Abstract Citations (22) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The instability strip of the anomalous Cepheids in the Draco dwarf spheroidal galaxy. Deupree, R. G. ; Hodson, S. W. Abstract The instability strip of the anomalous Cepheids is located, and the location verifies that the variables must be more massive than the RR Lyrae variables. It is found that the red edge of the Population II instability strip becomes bluer as the stellar mass increases, as does the blue edge. This indicates that our previous RR Lyrae color-width results are effectively independent of mass. The changes in the anomalous Cepheid instability strip indicated by a new opacity table, whose prime improvement is the inclusion of a number of molecules, and by the hypothesis that the helium abundance in the Draco system is slightly less than in the Galaxy, are examined. Subject headings: clusters: globular - galaxies: individual - stars: horizontal branch - stars: interiors - stars: pulsation - stars: RR Lyrae Publication: The Astrophysical Journal Pub Date: December 1977 DOI: 10.1086/155721 Bibcode: 1977ApJ...218..654D Keywords: Cepheid Variables; Galactic Clusters; Globular Clusters; Star Distribution; Stellar Luminosity; Stellar Mass; Stellar Models; Stellar Structure; Astrophysics full text sources ADS | data products NED (1)