Carbon-Enhanced Metal-Poor Stars, the Cosmic Microwave Background, and the Stellar IMF in the Early Universe

The characteristic mass of stars at early times may have been higher than today owing to the cosmic microwave background (CMB). This study proposes that (1) the testable predictions of this "CMB-IMF" hypothesis are an increase in the fraction of carbon-enhanced metal-poor (CEMP) stars with declining metallicity and an increase from younger to older populations at a single metallicity (e.g. disk to halo), and (2) these signatures are already seen in recent samples of CEMP stars and can be better tested with anticipated data. The expected spatial variation may explain discrepancies of CEMP frequency among published surveys. The ubiquity and time dependence of the CMB will substantially alter the reconstruction of star formation histories in the Local Group and early Universe.Comment: 7 pages emulateapj format, three figures, accepted for ApJ Letter

The Multiphase Intergalactic Medium towards PKS 2155-304

We study the cluster of H I and O VI absorption systems and the claimed detection of O VIII absorption from the intergalactic medium at z ~ 0.0567, associated with a group of galaxies toward the BL Lac object PKS 2155-304. As measured by spectrographs on the Hubble Space Telescope, Far Ultraviolet Spectroscopic Explorer, and Chandra, this system appears to contain gas at a variety of temperatures. We analyze this multi-phase gas in a clumpy-infall model. From the absence of C IV and Si III absorption in the Ly-alpha clouds, we infer metallicities less than 2.5-10% of solar values. The only metals are detected in two O VI absorption components, offset by +/- 400 km/s from the group barycenter (cz ~ 16,600 km/s). The O VI components may signify "nearside" and "backside" infall into the group potential well, which coincides with the claimed O VIII absorption. If the claimed O VIII detection is real, our analysis suggests that clusters of strong Ly-alpha and O VI absorbers, associated with groups of galaxies, may be the "signposts" of shock-heated, metal-enriched baryons. Through combined UV and X-ray spectra of H I and O VI, O VII, and O VIII, one may be able to clarify the heating mechanism of this multiphase gas.Comment: Accepted for ApJL (2003), 11 pages, 1 color figur

Evolving Spectra of Pop III Stars: Consequences for Cosmological Reionization

We examine the significance of the first metal-free stars (Pop III) for the cosmological reionization of HI and HeII. These stars have unusually hard spectra, with the integrated ionizing photon rates from a Pop III stellar cluster for HI and HeII being 1.6 and $10^5$ times stronger respectively than those from a Pop II cluster. For the currently favored cosmology, we find that Pop III stars alone can reionize HI and HeII at redshifts, $z$ of about 9 (4.7) and 5.1 (0.7) for continuous (instantaneous) modes of star formation. More realistic scenarios involving combinations of Pop III and Pop II stellar spectra yield similar results for hydrogen. Helium never reionizes completely in these cases; the ionization fraction of HeIII reaches a maximum of about 60 percent at $z$ of about 5.6 if Pop III star formation lasts for $10^9$ yr. Future data on HI reionization can test the amount of small-scale power available to the formation of the first objects, and provide a constraint on values of $\sigma_8$ less than or about 0.7. Since current UV observations indicate an epoch of reionization for HeII at $z$ of about 3, HeII may reionize more than once. Measurements of the HeII Gunn-Peterson effect in the intergalactic medium at redshifts exceeding about 3 may reveal the significance of Pop III stars for HeII reionization, particularly in void regions that may contain relic ionization from early Pop III stellar activity.Comment: 29 pages, 4 figures. Accepted for publication in Ap.J. (Feb. 20, 2003 issue; v. 584); minor revisions, results unchange

Detecting Primordial Stars

We discuss the expected properties of the first stellar generations in the Universe. We find that it is possible to discern truly primordial populations from the next generation of stars by measuring the metallicity of high-z star forming objects. The very low background of the future James Webb Space Telescope (JWST) will enable it to image and study first-light sources at very high redshifts, whereas its relatively small collecting area limits its capability in obtaining spectra of z~10-15 first-light sources to either the bright end of their luminosity function or to strongly lensed sources. With a suitable investment of observing time JWST will be able to detect individual Population III supernovae, thus identifying the very first stars that formed in the Universe.Comment: [8 pages, 5 figures] Invited Talk, to appear in IMF@50: The Stellar Initial Mass Function Fifty Years Later, eds E. Corbelli, F. Palla, and H. Zinnecker (Dordrecht: Kluwer

Inferring physical conditions in interstellar clouds of H_2

We have developed a code that models the formation, destruction, radiative transfer, and vibrational/rotational excitation of H_2 in a detailed fashion. We discuss how such codes, together with FUSE observations of H_2 in diffuse and translucent lines of sight, may be used to infer various physical parameters. We illustrate the effects of changes in the major physical parameters (UV radiation field, gas density, metallicity), and we point out the extent to which changes in one parameter may be mirrored by changes in another. We provide an analytic formula for the molecular fraction, f_H2, as a function of cloud column density, radiation field, and grain formation rate of H_2. Some diffuse and translucent lines of sight may be concatenations of multiple distinct clouds viewed together. Such situations can give rise to observables that agree with the data, complicating the problem of uniquely identifying one set of physical parameters with a line of sight. Finally, we illustrate the application of our code to an ensemble of data, such as the FUSE survey of H_2 in the Large and Small Magellanic Clouds (LMC/SMC), in order to constrain the elevated UV radiation field intensity and reduced grain formation rate of H_2 in those low- metallicity environments.Comment: 33 pages (aastex, manuscript), 9 figures (3 color). accepted to Ap

Linguistic distance to English impedes research performance

Today, scientific knowledge is predominantly disseminated in English. We show that global universities’ research performance, as measured by publications in top journals, declines as the differences between their local language and English increase. This effect is robust to controls for university factors like proportion of international staff and faculty-to-student ratio, as well as country-level factors like economic development, youth academic achievement, university degree rate, politics, culture, trade with and geographic distance to English-speaking countries, among others. This quantification of the research performance penalties induced by linguistic distance from the lingua franca may inform policy makers who must balance trade-offs between embracing English against cultural and local labor market pressures to orient around the local language

Cosmological Effects of the First Stars: Evolving Spectra of Population III

The first stars hold intrinsic interest for their uniqueness and for their potentially important contributions to galaxy formation, chemical enrichment, and feedback on the intergalactic medium (IGM). Although the sources of cosmological reionization are unknown at present, the declining population of large bright quasars at redshifts z \u3e 3 implies that stars are the leading candidates for the sources that reionized the hydrogen in the IGM by z ~ 6. The metal-free composition of the first stars restricts the stellar energy source to proton-proton burning rather than the more efficient CNO cycle. Consequently, they are hotter, smaller, and have harder spectra than their present-day counterparts of finite metallicity. We present new results from a continuing study of metal-free stars from a cosmological point of view. We have calculated evolving spectra of Population III clusters, derived from a grid of zero-metallicity stellar evolutionary tracks. We find that H-ionizing photon production from metal-free stellar clusters takes twice as long as that of Population II to decline to 1/10 its peak value. In addition, metal-free stars produce substantially more photons than Population II in the He II (E \u3e 4 ryd) continuum. We suggest that large Ly? equivalent widths (WLy? \u3e 400 ?) may provide a means of detecting metal-free stellar populations at high redshift and that He II recombination lines (??1640, 4686) may confirm identifications of Population III. While Population III clusters are intrinsically bluer than their Population II counterparts, nebular continuum emission makes up this difference and may confuse attempts to discern Population III stars with broadband colors. In a companion paper, we explore the consequences of evolving spectra of Population III for the reionization of the IGM in both H and He