We present a comprehensive study of the observational dependence of the mass-loss rate in stationary stellar winds of hot massive stars on the metal content of their atmospheres. The metal content of stars in the Magellanic Clouds is discussed, and a critical assessment is given of state-of-the-art mass-loss determinations of OB stars in these two satellite systems and the Milky-Way. Assuming a powerlaw dependence of mass loss on metal content, u M ∝ Z m , and adopting a theoretical relation between the terminal flow velocity and metal content, v∞ ∝ Z 0.13 (Leitherer et al. 1992, ApJ, 401, 596), we find m = 0.83 ± 0.16 for non-clumped outflows from an analysis of the wind momentum luminosity relation (WLR) for stars more luminous than 10 5.2 L� . Within the errors, this result is in agreement with the prediction m = 0.69 ± 0.10 by Vink et al. (2001, A&A, 369, 574). Absolute empirical values for the mass loss, based on Hα and ultraviolet (UV) wind lines, are found to be a factor of two higher than predictions in this high luminosity regime. If this difference is attributed to inhomogeneities in the wind, and this clumping does not impact the predictions, this would imply that luminous O and early-B stars have clumping factors in their Hα and UV line forming regions of about a factor of four. For lower luminosity stars, the winds are so weak that their strengths can generally no longer be derived from optical spectral lines (essentially Hα) and one must currently rely on the analysis of UV lines. We confirm that in this low-luminosity domain the observed Galactic WLR is found to be much steeper than expected from theory (although the specific sample is rather small), leading to a discrepancy between UV mass-loss rates and the predictions by a factor 100 at luminosities of L ∼ 10 4.75 L� , the origin of which is unknown. We emphasize that even if the current mass-loss rates of hot luminous stars would turn out to be overestimated as a result of wind clumping, but the degree of clumping would be rather independent of metallicity, the scalings derived in this study are expected to remain correct.

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The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars

Using integrated optical spectrophotometry for 412 star-forming galaxies at z ~ 0, and fiber-aperture spectrophotometry for 120,846 SDSS galaxies at z ~ 0.1, we investigate the Hα λ6563, Hβ λ4861, [O II] λ3727, and [O III] λ5007 nebular emission lines and the U-band luminosity as quantitative star formation rate (SFR) indicators. We demonstrate that the extinction-corrected Hα λ6563 luminosity is a reliable SFR tracer even in highly obscured star-forming galaxies. We find that variations in dust reddening dominate the systematic uncertainty in SFRs derived from the observed Hβ, [O II], and U-band luminosities, producing a factor of ~1.7, ~2.5, and ~2.1 scatter in the mean transformations, respectively. We show that [O II] depends weakly on variations in oxygen abundance over a wide range in metallicity, 12 + log(O/H) = 8.15-8.7 (Z/Z☉ = 0.28-1.0), and that in this metallicity interval galaxies occupy a narrow range in ionization parameter (-3.8 log U -2.9). We show that the scatter in [O III] λ5007 as a SFR indicator is a factor of 3-4 due to its sensitivity to metal abundance and ionization. We develop empirical SFR calibrations for Hβ and [O II] parameterized in terms of the B-band luminosity, which remove the systematic effects of reddening and metallicity and reduce the SFR scatter to ±40% and ±90%, respectively, although individual galaxies may deviate substantially from the median relations. Finally, we compare the z ~ 0 relations between blue luminosity and reddening, ionization, and [O II]/Hα ratio against measurements at z ~ 1 and find broad agreement. We emphasize, however, that optical emission-line measurements including Hα for larger samples of intermediate- and high-redshift galaxies are needed to test the applicability of our locally derived SFR calibrations to distant galaxies.

https://iopscience.iop.org/article/10.1086/500964/pdf

Optical Star Formation Rate Indicators

This spectroscopic study of NGC 4214 is part of a project to determine the extent to which the winds and supernovae from massive stars contribute to the short-term, localized chemical enrichment of the interstellar medium (ISM) in low-metallicity galaxies. Long-slit optical spectroscopy at 82 distinct spatial locations covering multiple starburst knots reveals no significant localized O, N, or He abundance differences that might be attributed to the winds of massive stars. We do find large-scale (200 pc) variations in the oxygen abundance. The southernmost, and probably youngest, starburst region exhibits higher O abundances by 0.095±0.019 dex and correspondingly lower N/O (0.108±0.038 dex) than the rest of the bright emission-line regions. This difference is consistent with O pollution at the locations of the young starburst, possibly from recent supernovae. The surveyed regions exhibit an anticorrelation between N/O and O/H consistent with O pollution, but no significant correlations between Ne/O and O/H. If, as expected from nucleosynthesis models, O and Ne are produced predominantly in the same short-lived massive stars, and N is produced predominantly in less massive, longer lived stars, then these trends are both consistent with the O pollution hypothesis. We show that observed internal N/O and O variations within NGC 4214, NGC 5253, and NGC 3125 are consistent with theoretical chemical evolution predictions during a phase of O production associated with massive stars early in a starburst. While uncertainties in the electron temperature, T(O++) are the dominant source of error, we show that the observed abundance trends are not consistent with those expected from temperature uncertainty effects. The N/O and He/H ratios are relatively insensitive to temperature errors and should serve as good indicators of abundance inhomogeneities wherever they may exist. We also present a self-consistent recomputation and tabulation of O, N/O, and He/H measurements in 60 metal-poor H II galaxies from the literature. Analysis of these data indicates that galaxies with strong Wolf-Rayet (W-R) features in their integrated spectrum exhibit identical N/O and He abundances to those galaxies lacking such features. Although abundance pollution from massive stars must occur on long timescales and global spatial scales, we take the absence of significant differences between W-R and non--W-R galaxies as evidence that W-R stars are not a significant contributor to abundance fluctuations on timescales comparable to the lifetimes of the H II regions. Thus, W-R galaxies need not be treated differently than non–W-R galaxies in studies of galactic chemical evolution and primordial abundance studies.

Elemental Abundance Variations and Chemical Enrichment from Massive Stars in Starbursts. I. NGC 4214

Six additional molecular clouds have been mapped in the nearby metal-poor dwarf irregular galaxy IC 10 using the Owens Valley Millimeter-Wave Interferometer. Since a good distance measurement is now available for IC 10, it is possible to measure accurately the CO-to-H2 conversion factor in this galaxy. The result for IC 10 is combined with published data for four other Local Group galaxies (M31, M33, NGC 6822, and the SMC) to trace the dependence of the CO-to-H2 conversion factor on the oxygen abundance using a well-defined sample of extragalactic giant molecular clouds. These data show conclusively that the CO-to-H2 conversion factor increases as the metallicity of the host galaxy decreases, with the conversion factor increasing by a factor of 4.6 for a factor of 10 decrease in metallicity. This new calibration of the effect of metallicity on the CO-to-H2 conversion factor will allow more accurate molecular gas masses to be obtained in galaxies and regions of galaxies with moderately low metallicity.

The Metallicity Dependence of the CO-to-H2 Conversion Factor from Observations of Local Group Galaxies

High-resolution CO maps of 9 molecular clouds in IC 10 are combined with the new measurement of the distance to this nearby metal poor galaxy to measure accurately the CO-to-H2 conversion factor. The result for IC 10 is combined with published data for four other Local Group galaxies (M31, M33, NGC 6822, the SMC) to trace the dependence of the CO-to-H2 conversion factor on oxygen abundance. These data show conclusively that the CO-to-H$_2$ conversion factor increases as the metallicity of the host galaxy decreases, with the conversion factor increasing by a factor of 4.6 for a factor of 10 decrease in metallicity.

B. L. Webster

Papers

A survey of chemical compositions of H II regions in the Magellanic Clouds