Showing papers by "M. S. Oey published in 2002"

A New Spectral Classification System for the Earliest O Stars: Definition of Type O2

Abstract: High-quality, blue-violet spectroscopic data are collected for 24 stars that have been classified as type O3 and that display the hallmark N IV and N V lines. A new member of the class is presented; it is the second known in the Cyg OB2 association, and only the second in the northern hemisphere. New digital data are also presented for several of the other stars. Although the data are inhomogeneous, the uniform plots by subcategory reveal some interesting new relationships. Several issues concerning the classification of the hottest O-type spectra are discussed, and new digital data are presented for the five original O3 dwarfs in the Carina Nebula, in which the N IV, N V features are very weak or absent. New spectral types O2 and O3.5 are introduced here as steps toward resolving these issues. The relationship between the derived absolute visual magnitudes and the spectroscopic luminosity classes of the O2–O3 stars shows more scatter than at later O types, at least partly because some overluminous dwarfs are unresolved multiple systems, and some close binary systems of relatively low luminosity and mass emulate O3 supergiant spectra. However, it also appears that the behavior of He II λ4686, the primary luminosity criterion at later O types, responds to other phenomena in addition to luminosity at spectral types O2–O3. There is evidence that these spectral types may correspond to an immediate pre-WN phase, with a correspondingly large range of luminosities and masses. A complete census of spectra classified into the original O3 subcategories considered here (not including intermediate O3/WN types or O3 dwarfs without N IV, N V features) totals 45 stars; 34 of them belong to the Large Magellanic Cloud and 20 of the latter to 30 Doradus.

Galactic porosity and a star formation threshold for the escape of ionizing radiation from galaxies

Abstract: The spatial distribution of star formation within galaxies strongly affects the resulting feedback processes. Previous work has considered the case of a single, concentrated nuclear starburst, and also that of distributed single supernovae (SNe). Here, we consider interstellar medium (ISM) structuring by SNe originating in spatially distributed clusters having a cluster membership spectrum given by the observed H II region luminosity function. We show that, in this case, the volume of H I cleared per SN is considerably greater than in either of the two cases considered hitherto. We derive a simple relationship between the 'porosity' of the ISM and the star formation rate (SFR), and deduce a critical SFR c r i t , at which the ISM porosity is unity. This critical value describes the case in which the SN mechanical energy output over a time-scale (t e ) is comparable with the ISM 'thermal' energy contained in random motions; t e is the duration of SN mechanical input per superbubble. This condition also defines a critical gas consumption time-scale t e x h , which for a Salpeter initial mass function and random velocities of ≃10 km s - 1 is roughly 10 1 0 yr. We draw a link between porosity and the escape of ionizing radiation from galaxies, arguing that high escape fractions are expected if SFR ≥ SFR c r i t . The Lyman break galaxies, which are presumably subject to infall on a time-scale

Galactic porosity and a star formation threshold for the escape of ionising radiation from galaxies

Abstract: The spatial distribution of star formation within galaxies strongly affects the resulting feedback processes. Previous work has considered the case of a single, concentrated nuclear starburst, and also that of distributed single supernovae (SNe). Here, we consider ISM structuring by SNe originating in spatially distributed clusters having a cluster membership spectrum given by the observed HII region luminosity function. We show that in this case, the volume of HI cleared per SN is considerably greater than in either of the two cases considered hitherto. We derive a simple relationship between the ``porosity'' of the ISM and the star formation rate (SFR), and deduce a critical SFR_crit, at which the ISM porosity is unity. This critical value describes the case in which the SN mechanical energy output over a timescale t_e is comparable with the ISM ``thermal'' energy contained in random motions; t_e is the duration of SN mechanical input per superbubble. This condition also defines a critical gas consumption timescale t_exh, which for a Salpeter IMF and random velocities of \simeq 10 km s-1 is roughly 10e10 years. We draw a link between porosity and the escape of ionising radiation from galaxies, arguing that high escape fractions are expected if SFR >~ SFR_crit. The Lyman Break Galaxies, which are presumably subject to infall on a timescale < t_exh, meet this criterion, as is consistent with the significant leakage of ionising photons inferred in these systems. We suggest the utility of this simple parameterisation of escape fraction in terms of SFR for semi-empirical models of galaxy formation and evolution and for modeling mechanical and chemical feedback effects.

The H I Environment of Three Superbubbles in the Large Magellanic Cloud

Abstract: The ambient interstellar environment of wind- and supernova-driven superbubbles strongly affects their evolution, but its properties are rarely well determined. We have therefore obtained H I aperture synthesis imaging of the environment around three similar optically selected superbubble nebulae in the Large Magellanic Cloud. The resulting H I maps show that the ambient gas distributions around these superbubbles differ to an extreme: DEM L25 shows no neutral shell component but is nestled within an H I hole; DEM L50 shows a massive neutral shell component but is otherwise within an H I void; and DEM L301 shows no correspondence at all between the optical nebula and H I distribution. There is also poor correspondence between the H I and optical kinematics. These results strongly caution against inferring properties of the ambient neutral environment of individual superbubbles without direct observations. Finally, all three objects show some evidence of shock activity.

Structure and dynamics of candidate O star bubbles in N44

Abstract: Dynamical studies of superbubbles and Wolf-Rayet ring nebulae show discrepancies from the standard adiabatic model for windblown bubbles. We therefore study the physical properties and kinematics of three candidate bubbles blown by single O stars to evaluate whether these discrepancies are also found in these simpler objects. Our sample candidates are N44 F, N44 J, and N44 M, in the outskirts of the H II complex N44 in the Large Magellanic Cloud. We have obtained ground-based and Hubble Space Telescope emission-line images and high-dispersion echelle spectra for these objects. From the Hα luminosities and the [O III]/Hα ratios of these nebulae, we estimate the spectral types of the ionizing stars to be O7 V, O9.5 V, and O9.5 V for N44 F, N44 J, and N44 M, respectively. We find that the observed expansion velocity of 12 km s-1 for N44 F is consistent with the stellar wind luminosity expected from the central ionizing star, as predicted by the standard bubble model. The observed upper limits for the expansion velocities of N44 J and N44 M are also compatible with the expected values, within the uncertainties. We also report the discovery in N44 F of strongly defined dust columns, similar to those seen in the Eagle Nebula. The photoevaporation of these dense dust features may be kinematically important and may actually govern the evolution of the shell. The inclusion of photoevaporation processes may thus undermine the apparent agreement between the observed bubble dynamics and the simple adiabatic models.

The Wind of the B[e] Supergiant Hen S22 Viewed through a Reflection Nebula in DEM L 106

Abstract: Narrow-band HST WFPC2 images reveal a bow-shock-like halo around the HII region N30B toward the B[e] supergiant Hen S22 located within the larger DEM L 106 nebula in the Large Magellanic Cloud. High-dispersion spectra of N30B show a narrow H-alpha emission component from the ionized gas; the velocity variations indicate a gas flow of -5 to -10 km/s in the vicinity of the HII regions, which is resultant from interactions with Hen S22's stellar wind and responsible for the bow-shock morphology. Spectra of N30B's halo show broad H-alpha profiles extending over >1000 km/s, similar to that of Hen S22, indicating that the halo is a reflection nebula of Hen S22. Broad-band morphologies of N30B's halo are also consistent with the reflection nebula interpretation. We use dust-scattering properties and the observed brightnesses of the reflection nebula and Hen S22 to constrain the reflection geometry. The reflected stellar H-alpha emission and absorption vary across the reflection nebula as a result of viewing S22's anisotropic wind across varying angles. This reflection nebula, together with the edge-on orientation of Hen S22's disk, provides an invaluable opportunity to study the disk and polar winds of a B[e] supergiant.

Calibrating nebular diagnostics of t* and abundance

Abstract: Resumen en: We obtained nebular spectroscopy of LMC H II regions having classi ed stellar populations, thereby strongly constraining the ionization input parameters...

Structure and Dynamics of Candidate O Star Bubbles in N44

Abstract: Dynamical studies of superbubbles and Wolf-Rayet ring nebulae show discrepancies from the standard, adiabatic model for wind-blown bubbles. We therefore study the physical properties and kinematics of three candidate bubbles blown by single O stars, to evaluate whether these discrepancies are also found in these simpler objects. Our sample candidates are N44F, N44J, and N44M, in the outskirts of the HII complex N44 in the Large Magellanic Cloud. We have obtained ground-based and HST emission-line images and high dispersion echelle spectra for these objects. From the Halpha luminosities and the [OIII]/Halpha ratios of these nebulae, we estimate the spectral types of the ionizing stars to be O7V, O9.5V and O9.5V for N44F, N44J, and N44M, respectively. We find that the observed expansion velocity of 12 km/s for N44F is consistent with the stellar wind luminosity expected from the central ionizing star, as predicted by the standard bubble model. The observed upper limits for the expansion velocities of N44J and N44M are also compatible with the expected values, within the uncertainties. We also report the discovery in N44F of strongly-defined dust columns, similar to those seen in the Eagle Nebula. The photoevaporation of these dense dust features may be kinematically important and may actually govern the evolution of the shell. The inclusion of photoevaporation processes may thus undermine the apparent agreement between the observed bubble dynamics and the simple adiabatic models.

Discussion: Star Formation Within Galaxies

Abstract: This Discussion session focused on star formation within galactic scales. We attempt to identify the dominant physical processes and parameters that characterize star formation, and to identify key questions that illuminate these phenomena. The Discussion was delineated by the following cycle of three questions: (A) Is the top of the HII LF physically distinct? (B) How does massive star feedback affect the ISM and star formation? (C) How do ISM properties affect the HII LF? Finally, is one of these three questions the fundamental one of the cycle? Corresponding answers emerged from the Discussion: (A) The HII LF to date is a continuous power law; (B) There are both positive and negative feedback effects, which are poorly understood; (C) The HII LF appears remarkably independent of ISM properties. Therefore, we suggest that the resultant fundamental question is: ``Is the HII LF and parent stellar cluster membership function universal?'' This is analogous to the related question of a universal stellar IMF. Understanding the relationship, if any, between the IMF, cluster membership function, and ISM properties may finally lead to a quantitative theory of star formation.

Dispersal of massive star products and consequences for galactic chemical evolution

Abstract: The processes that disperse the products of massive stars from their birth sites play a fundamental role in determining the observed abundances I discuss parameterizations for element dispersal and their roles in chemical evolution, with an emphasis on understanding present-day dispersion and homogeneity in metallicity Turbulence dominates mixing processes, with characteristic timescales of order 10^8 yr, implying significant dilution of metals into the ISM This permits a rough estimate of the metallicity distribution function of enrichment events Many systems, including the Milky Way and nearby galaxies, show metallicity dispersions that as yet appear consistent with pure inhomogeneous evolution There are also systems like I Zw~18 that show strong homogenization, perhaps tied to small galaxy size, high star formation rate, and/or superwinds

Metal Dispersal and the Number of Population III Stars

Abstract: Two fundamental constraints on the earliest star formation conditions in the Galaxy are an apparent empirical low-metallicity threshold of −4 ≲ [Fe/Hl, and an upper limit to the fraction of Population III halo stars of F III < 4 × 10−4. How do these observed constraints compare with predictions of simple models? This is investigated within the framework of element dispersal from clustered core-collapse SNe. Simple arguments considering turbulent mixing within multi-phase ISM suggest that the observed low-metallicity threshold is consistent with rough expected values. However, the observed limit on F III is two orders of magnitude larger than predictions from this simple, one-zone inhomogeneous chemical evolution.