Graeme H. Smith

SN 2002cx: The Most Peculiar Known Type Ia Supernova

ABSTRACT We present photometric and spectroscopic observations of supernova (SN) 2002cx, which reveal it to be unique among all observed Type Ia supernovae (SNe Ia). SN 2002cx exhibits an SN 1991T–like premaximum spectrum, an SN 1991bg–like luminosity, and expansion velocities roughly half those of normal SNe Ia. Photometrically, SN 2002cx has a broad peak in the R band and a plateau phase in the I band, and slow late‐time decline. The B−V color evolution is nearly normal, but the V−R and V−I colors are very red. Early‐time spectra of SN 2002cx evolve very quickly and are dominated by lines from Fe‐group elements; features from intermediate‐mass elements (Ca, S, Si) are weak or absent. Mysterious emission lines are observed around 7000 A at about 3 weeks after maximum brightness. The nebular spectrum of SN 2002cx is also unique, consisting of narrow iron and cobalt lines. The observations of SN 2002cx are inconsistent with the observed spectral/photometric sequence and provide a major challenge to our und...

THE ABUNDANCES OF NEUTRON-CAPTURE SPECIES IN THE VERY METAL-POOR GLOBULAR CLUSTER M15: A UNIFORM ANALYSIS OF RED GIANT BRANCH AND RED HORIZONTAL BRANCH STARS

The globular cluster M15 is unique in its display of star-to-star variations in the neutron-capture elements. Comprehensive abundance surveys have been previously conducted for handfuls of M15 red giant branch (RGB) and red horizontal branch (RHB) stars. No attempt has been made to perform a single, self-consistent analysis of these stars, which exhibit a wide range in atmospheric parameters. In the current effort, a new comparative abundance derivation is presented for three RGB and six RHB members of the cluster. The analysis employs an updated version of the line transfer code MOOG, which now appropriately treats coherent, isotropic scattering. The apparent discrepancy in the previously reported values for the metallicity of M15 RGB and RHB stars is addressed and a resolute disparity of Δ(RHB – RGB) ≈ 0.1 dex in the iron abundance was found. The anti-correlative behavior of the light neutron-capture elements (Sr, Y, Zr) is clearly demonstrated with both Ba and Eu, standard markers of the s- and r-process, respectively. No conclusive detection of Pb was made in the RGB targets. Consequently for the M15 cluster, this suggests that the main component of the s-process has made a negligible contribution to those elements normally dominated by this process in solar system material. Additionally for the M15 sample, a large Eu abundance spread is confirmed, which is comparable to that of the halo field at the same metallicity. These abundance results are considered in the discussion of the chemical inhomogeneity and nucleosynthetic history of M15.

MULTI-ELEMENT ABUNDANCE MEASUREMENTS FROM MEDIUM-RESOLUTION SPECTRA. IV. ALPHA ELEMENT DISTRIBUTIONS IN MILKY WAY SATELLITE GALAXIES*

We derive the star formation histories of eight dwarf spheroidal (dSph) Milky Way satellite galaxies from their alpha element abundance patterns. Nearly 3000 stars from our previously published catalog (Paper II) comprise our data set. The average [alpha/Fe] ratios for all dSphs follow roughly the same path with increasing [Fe/H]. We do not observe the predicted knees in the [alpha/Fe] vs. [Fe/H] diagram, corresponding to the metallicity at which Type Ia supernovae begin to explode. Instead, we find that Type Ia supernova ejecta contribute to the abundances of all but the most metal-poor ([Fe/H] < -2.5) stars. We have also developed a chemical evolution model that tracks the star formation rate, Types II and Ia supernova explosions, and supernova feedback. Without metal enhancement in the supernova blowout, massive amounts of gas loss define the history of all dSphs except Fornax, the most luminous in our sample. All six of the best-fit model parameters correlate with dSph luminosity but not with velocity dispersion, half-light radius, or Galactocentric distance.

The Type IC supernova 1994I in M51: detection of helium and spectral evolution

We present a series of spectra of SN 1994I in M51, starting 1 week prior to maximum brightness. The nebular phase began about 2 months after the explosion; together with the rapid decline of the optical light, this suggests that the ejected mass was small. Although lines of He I in the optical region are weak or absent, consistent with the Type Ic classification, we detect strong He I λ10830 absorption during the first month past maximum. Thus, if SN 1994I is a typical Type Ic supernova, the atmospheres of these objects cannot be completely devoid of helium. The emission-line widths are smaller than predicted by the model of Nomoto and coworkers, in which the iron core of a low-mass carbon-oxygen star collapses. They are, however, larger than in Type Ib supernovae.

Neutron-Capture Element Abundances in the Globular Cluster M15*

High-resolution, high signal-to-noise ratio, blue-violet spectra of three red giant branch tip stars in M15 have been obtained with the Keck I High-Resolution Echelle Spectrograph. These spectra have been analyzed to determine the abundances of several neutron-capture elements, including the radioactive chronometer element thorium. There are two principal results of this study. First, the abundances of the heavier (Z>/=56) elements for each of the three stars is well matched by a scaled solar system r-process abundance distribution. Second, a weighted mean-observed Th/Eu ratio for the stars implies an age for the neutron-capture material in M15 stars of 14+/-3 Gyr, in reasonable agreement with other recent age estimates for Galactic globular clusters.

A Hot Wind from the Classical T Tauri Stars: TW Hydrae and T Tauri

Spectroscopy of the infrared He I (10830 A) line with NIRSPEC on Keck and CSHELL at the IRTF, and of the ultraviolet C III (977 A) and O VI (1032 A) emission with FUSE, reveals that the classical T Tauri star TW Hydrae exhibits P Cygni profiles, line asymmetries, and absorption indicative of a continuous, fast (~400 km s-1), hot (~300,000 K) accelerating outflow with a mass-loss rate ~10-11 to 10-12 M☉ yr-1 or larger. Spectra of T Tau N appear consistent with such a wind. The source of the emission and outflow seems restricted to the stars themselves. Although the mass accretion rate is an order of magnitude less for TW Hya than for T Tau, the outflow reaches higher velocities at chromospheric temperatures in TW Hya. Winds from young stellar objects may be substantially hotter and faster than previously thought.

Proton Capture Chains in Globular Cluster Stars. III. Abundances of Giants in the Second-Parameter Globular Cluster NGC 7006*

High-resolution spectra have been obtained of six red giants in the globular cluster NGC 7006 using the HIRES instrument on the Keck I Telescope. The [iron-peak/Fe] and [α-nuclei/Fe] elemental ratios are similar to those found in other halo globular clusters. Significant, but modest, star-to-star abundance differences are found for the elements O, Na, and Al, with the [O/Fe] abundance ratio being anticorrelated with both [Na/Fe] and [Al/Fe]. These anticorrelations indicate that O → N processed material, within which Ne has also been converted to Na and Mg to Al by proton addition reactions, has been brought to the surface of the cluster giants via a deep-mixing mechanism that cycles material through the O → N burning zone just outside the main hydrogen-burning shell. Despite the significant [Al/Fe] differences among the NGC 7006 giants, there is very little accompanying difference in [Mg/Fe]. This result, however, is not inconsistent with a deep-mixing origin for the [Al/Fe] enhancements; because Mg is initially much more abundant than Al, conversion of only a small fraction of Mg into Al can still produce a large percentage increase in the Al abundance. Plots of [Na/Fe] versus [O/Fe] and [Al/Fe] versus [Mg/Fe] indicate that the amount of proton-capture–processed material that has been mixed into the envelopes of the NGC 7006 giants is less extreme than within some giants of the globular clusters M13 and M15. In particular, unlike the NGC 7006 giants, most giants in M13 and some in M15 exhibit significantly reduced [O/Fe] and [Mg/Fe] ratios and very high [Al/Fe]. This difference in the [Mg/Fe] behavior may be related to differences in horizontal-branch morphology between M13 and NGC 7006. Within some M13 giants, it is possible that deep mixing has accessed regions in which not only has Mg been converted to Al but significant amounts of helium have also been produced from hydrogen burning. Deep mixing in such stars may not only significantly reduce the surface [Mg/Fe] ratio but may also increase the [He/H] ratio by amounts that will affect the zero-age horizontal-branch (HB) position that a giant will eventually occupy. This could explain the extension of the M13 HB to very blue colors. By contrast, giants in NGC 7006 experience modest amounts of interior mixing that may be deep enough to detectably alter the surface [Al/Fe] ratio, but not the [Mg/Fe] and [He/H] ratios. These stars will have a relatively low [He/H] ratio and so will adopt relatively red positions when they evolve onto the horizontal branch; the HB of NGC 7006 does indeed comprise a large percentage of stars redder than the RR Lyrae gap. In this scenario, deep mixing within cluster red giants may be one of the second parameters that affects horizontal-branch morphology.

CN Abundance Variations on the Main Sequence of 47 Tucanae

We report on a deep spectroscopic survey for star-to-star CN variations along the main sequence (MS) of the globular cluster 47 Tuc with ESOs Very Large Telescope. We find a significant bimodal distribution in the S(3839) index for main-sequence stars in the mass range ~0.85 to 0.65 M⊙, or from the main-sequence turnoff down to ~2.5 mag below the main-sequence turnoff. An anticorrelation of CN and CH is evident on the MS. The result is discussed in the context of the ability of faint MS stars to alter their surface composition through internal evolutionary effects. We argue against internal stellar evolution as the only origin for the abundance spread in 47 Tuc; an external origin such as pollution seems to be more likely.

Direct Evidence for an Enhancement of Helium in Giant Stars in Omega Centauri

The double main sequence identified in the globular cluster Omega Centauri has been interpreted using isochrones to indicate a large variation in the abundance of helium. If true, a helium enhancement carries strong implications for the chemical and stellar evolutionary history of this cluster. However, only indirect measures currently support this conjecture. We report the discovery of a variation in the line strength of the near-infrared He I 10830 A transition in 12 similar red giants in Omega Centauri observed with PHOENIX on Gemini-S. Abundances of these stars derived from Magellan/MIKE spectra taken at the Las Campanas Observatory show that the helium transition is not detected in the most metal-poor population ([Fe/H] < -1.8), yet is present in the majority of stars with [Fe/H] ≥ ―1.8. These observations give the first direct evidence for an enhancement of helium in Omega Centauri. The appearance of helium appears better correlated with increased [AI/Fe] and [Na/Fe] abundances than as a function of [Fe/H], giving observational support to the presence of high-temperature H burning in a prior generation of stars.

GRB 071003: Broadband Follow-up Observations of a Very Bright Gamma-Ray Burst in a Galactic Halo

The optical afterglow of long-duration GRB 071003 is among the brightest yet to be detected from any GRB, with R ≈ 12 mag in KAIT observations starting 42 s after the GRB trigger, including filtered detections during prompt emission. However, our high-S/N afterglow spectrum displays only extremely weak absorption lines at what we argue is the host redshift of z = 1.60435, in contrast to the three other, much stronger Mg II absorption systems observed at lower redshifts. Together with Keck adaptive optics observations, which fail to reveal a host galaxy coincident with the burst position, our observations suggest a halo progenitor and offer a cautionary tale about the use of Mg II for GRB redshift determination. We present early- through late-time observations spanning the electromagnetic spectrum, constrain the connection between the prompt emission and early variations in the light curve (we observe no correlation), and discuss possible origins for an unusual, marked rebrightening that occurs a few hours after the burst: likely either a late-time refreshed shock or a wide-angle secondary jet. Analysis of the late-time afterglow is most consistent with a wind environment, suggesting a massive star progenitor. Together with GRB 070125, this may indicate that a small but significant portion of star formation in the early universe occurred far outside what we consider a normal galactic disk.