arXiv subjects

R. C. Smith

Publications and source records attributed to R. C. Smith.

176 records · Page 10Linked to original sources

arXiv2002

The Type Ia Supernova 1999aw: a Probable 1999aa-like Event In a Low-Luminosity Host Galaxy

SN 1999aw was discovered during the first campaign of the Nearby Galaxies Supernova Search (NGSS) project. This luminous, slow-declining (Delta m_{15} (B) = 0.81 \pm 0.03) Type Ia supernova was noteworthy in at least two respects. First, it occurred in an extremely low luminosity host galaxy that was not visible in the template images, nor in initial subsequent deep imaging. Secondly, the photometric and spectral properties of this supernova indicate that it very likely was similar to the subclass of Type Ia supernovae whose prototype is SN 1999aa. This paper presents the BVRI and J_{s} HK_{s} lightcurves of SN 1999aw (through 100 days past maximum light), as well as several epochs of optical spectra. From these data we calculate the bolometric light curve, and give estimates of the luminosity at maximum light and the initial ^{56}Ni mass. In addition, we present deep BVI images obtained recently with the Baade 6.5-meter telescope at Las Campanas Observatory which reveal the remarkably low-luminosity host galaxy.

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arXiv2002

Optical Photometry of the Type Ia SN 1999ee and the Type Ib/c SN 1999ex in IC 5179

We present UBVRIz lightcurves of the Type Ia SN 1999ee and the Type Ib/c SN 1999ex, both located in the galaxy IC 5179. SN 1999ee has an extremely well sampled lightcurve spanning from 10 days before Bmax through 53 days after peak. Near maximum we find systematic differences ~0.05 mag in photometry measured with two different telescopes, even though the photometry is reduced to the same local standards around the supernova using the specific color terms for each instrumental system. We use models for our bandpasses and spectrophotometry of SN 1999ee to derive magnitude corrections (S-corrections) and remedy this problem. This exercise demonstrates the need of accurately characterizing the instrumental system before great photometric accuracies of Type Ia supernovae can be claimed. It also shows that this effect can have important astrophysical consequences since a small systematic shift of 0.02 mag in the B-V color can introduce a 0.08 mag error in the extinction corrected peak B magnitudes of a supernova and thus lead to biased cosmological parameters. The data for the Type Ib/c SN 1999ex present us with the first ever observed shock breakout of a supernova of this class. These observations show that shock breakout occurred 18 days before Bmax and support the idea that Type Ib/c supernovae are due to core collapse of massive stars rather than thermonuclear disruption of white dwarfs.

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arXiv2001

The HI Environment of Three Superbubbles in the Large Magellanic Cloud

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 HI aperture synthesis imaging of the environment around three similar, optically-selected superbubble nebulae in the Large Magellanic Cloud. The resulting HI maps show that the ambient gas distribution around these superbubbles differ to an extreme: DEM L25 shows no neutral shell component, but is nestled within an HI hole; DEM L50 shows a massive neutral shell component, but is otherwise within an HI void; and DEM L301 shows no correspondence at all between the optical nebula and HI distribution. There is also poor correspondence between the HI 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.

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arXiv2001

The Distance to SN 1999em from the Expanding Photosphere Method

We present optical and IR spectroscopy of the first two months of evolution of the Type II SN 1999em. We combine these data with high-quality optical/IR photometry beginning only three days after shock breakout, in order to study the performance of the ``Expanding Photosphere Method'' (EPM) in the determination of distances. With this purpose we develop a technique to measure accurate photospheric velocities by cross-correlating observed and model spectra. The application of this technique to SN 1999em shows that we can reach an average uncertainty of 11% in velocity from an individual spectrum. Our analysis shows that EPM is quite robust to the effects of dust. In particular, the distances derived from the VI filters change by only 7% when the adopted visual extinction in the host galaxy is varied by 0.45 mag. The superb time sampling of the BVIZJHK light-curves of SN 1999em permits us to study the internal consistency of EPM and test the dilution factors computed from atmosphere models for Type II plateau supernovae. We find that, in the first week since explosion, the EPM distances are up to 50% lower than the average, possibly due the presence of circumstellar material. Over the following 65 days, on the other hand, our tests lend strong credence to the atmosphere models, and confirm previous claims that EPM can produce consistent distances without having to craft specific models to each supernova. This is particularly true for the VI filters which yield distances with an internal consistency of 4%. From the whole set of BVIZJHK photometry, we obtain an average distance of 7.5+/-0.5 Mpc, where the quoted uncertainty (7%) is a conservative estimate of the internal precision of the method obtained from the analysis of the first 70 days of the supernova evolution.

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arXiv2001

Large Scale Diffuse X-ray Emission from the Large Magellanic Cloud

X-ray mosaics of the Large Magellanic Cloud (LMC) taken with the ROSAT Position Sensitive Proportional Counter (PSPC) have revealed extensive diffuse X-ray emission, indicative of hot >= 10^6 K gas associated with this irregular galaxy on scales from ~10 pc to >= 1000 pc. We have selected regions of large-scale (d >= 600 pc) diffuse X-ray emission, such as supergiant shells, the LMC Spur, and the LMC Bar, and examined the physical conditions of the hot gas associated with them. We find that for these objects the plasma temperatures range from kT ~0.15 - 0.60 keV and the derived electron densities range from n_e ~0.005 - 0.03 cm^-3. Furthermore, we have examined the fraction of diffuse X-ray emission from the LMC and compared it to the total X-ray emission. We find that discrete sources such as X-ray binaries and supernova remnants (SNRs) account for ~41% and ~21% of the X-ray emission from the LMC, respectively. In contrast, diffuse X-ray emission from the field and from supergiant shells account for ~30% and ~6% of the total X-ray emission, respectively.

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arXiv2001

Calibrating nebular diagnostics of T_* and abundance

We obtained nebular spectroscopy of LMC HII regions having classified stellar populations, thereby strongly constraining the ionization input parameters. Using photoionization models, we then evaluate the performance of nebular diagnostics of T_* and abundance. We introduce [Ne III]/H-beta as a nebular diagnostic of the ionizing stellar T_*. In contrast to the widely-used η-prime parameter, [Ne III]/H-beta has greater sensitivity to mid and early O-stars, and is robust to nebular morphology and the presence of shocks. We present a preliminary calibration of both T_* diagnostics for LMC metallicity. We also introduce S234 = ([S II] + [S III] + [S IV])/H-beta as a diagnostic of S abundance. S234 is much less sensitive to the nebular ionization parameter than is S23 or R23. The intensity of [S IV]10.5 microns is easily estimated from the optical and near-IR line ratios. We present calibrations of S23 and S234 that are reliable at metallicities Z < 0.5 Z_sol.

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arXiv1999

Calibration of Nebular Emission-Line Diagnostics: I. Stellar Effective Temperatures

We present a detailed comparison of optical H II region spectra to photoionization models based on modern stellar atmosphere models. We examine both spatially resolved and integrated emission-line spectra of the HII regions DEM L323, DEM L243, DEM L199, and DEM L301 in the Large Magellanic Cloud. The published spectral classifications of the dominant stars range from O7 to WN3, and morphologies range from Stromgren sphere to shell structure. Two of the objects include SNR contamination. The overall agreement with the predictions is generally within 0.2 dex for major diagnostic line ratios. An apparent pattern in the remaining discrepancies is that the predicted T_e is ~1000 K hotter than observed. (Abridged) Our analysis of the complex DEM L199 allows a nebular emission-line test of unprecedented detail for WR atmospheres. Surprisingly, we find no nebular He II 4686 emission, despite the fact that both of the dominant WN3 stars should be hot enough to fully ionize He I in their atmospheres. We confirm that the η-prime emission-line parameter is not as useful as hoped for determining the ionizing stellar effective temperature, T*. Both empirically and theoretically, we find that it is insensitive for T* >40 kK, and that it also varies spatially. The shock-contaminated objects show that η-prime will also yield a spuriously high T* in the presence of shocks. It is furthermore sensitive to shell morphology. We suggest [Ne III]/Hb as an additional probe of T*. Although it is abundance-dependent, [Ne III]/Hb has higher sensitivity to T*, is independent of morphology, and is insensitive to shocks in our objects. We attempt a first empirical calibration of these nebular diagnostics of T*.

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arXiv1999

A mystery solved: the mass ratio of the dwarf nova EM Cygni

We have discovered that the spectrum of the well-known dwarf nova EM Cyg is contaminated by light from a K2-5V star (in addition to the K-type mass donor star). The K2-5V star contributes approximately 16 per cent of the light from the system and if not taken into account has a considerable effect upon radial velocity measurements of the mass donor star. We obtain a new radial velocity amplitude for the mass donor star of K2 = 202 +/- 3 km/s, which compares with the value of K2 = 135 +/- 3 km/s obtained in Stover, Robinson & Nather's classic 1981 study of EM Cyg. The revised value of the amplitude combined with a measurement of rotational broadening of the mass donor vsini = 140 +/- 6 km/s, leads to a new mass ratio of q = M2/M1 = 0.88 +/- 0.05. This solves a long standing problem with EM Cyg because Stover et al.'s measurements indicated a mass ratio q > 1, a value which should have led to dynamically unstable mass transfer for the secondary mass deduced by Stover et al. The revised value of the mass ratio combined with the orbital inclination i = 67 +/- 2 degrees leads to masses of 0.99 +/- 0.12 Msun and 1.12 +/- 0.08 Msun for the mass donor and white dwarf respectively. The mass donor is evolved, since it has a later spectral type (K3) than its mass would imply. We discuss whether the K star could be physically associated with EM Cyg or not, and present the results of the spectroscopic study.

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arXiv1999

Measurements of Faint Supernovae

We summarize the current status of cosmological measurements using SNe Ia. Searches to an average depth of z~0.5 have found approximately 100 SNe Ia to date, and measurements of their light curves and peak magnitudes find these objects to be about 0.25mag fainter than predictions for an empty universe. These measurements imply low values for Omega_M and a positive cosmological constant, with high statistical significance. Searches out to z~1.0-1.2 for SNe Ia (peak magnitudes of I~24.5) will greatly aid in confirming this result, or demonstrate the existence of systematic errors. Multi-epoch spectra of SNe Ia at z~0.5 are needed to constrain possible evolutionary effects. I band searches should be able to find SNe Ia out to z~2. We discuss some simulations of deep searches and discovery statistics at several redshifts.

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arXiv1998

Supernova Limits on the Cosmic Equation of State

We use Type Ia supernovae studied by the High-Z Supernova Search Team to constrain the properties of an energy component which may have contributed to accelerating the cosmic expansion. We find that for a flat geometry the equation of state parameter for the unknown component, alpha_x=P_x/rho_x, must be less than -0.55 (95% confidence) for any value of Omega_m and is further limited to alpha_x<-0.60 (95%) if Omega_m is assumed to be greater than 0.1 . These values are inconsistent with the unknown component being topological defects such as domain walls, strings, or textures. The supernova data are consistent with a cosmological constant (alpha_x=-1) or a scalar field which has had, on average, an equation of state parameter similar to the cosmological constant value of -1 over the redshift range of z=1 to the present. Supernova and cosmic microwave background observations give complementary constraints on the densities of matter and the unknown component. If only matter and vacuum energy are considered, then the current combined data sets provide direct evidence for a spatially flat Universe with Omega_tot=Omega_m+Omega_Lambda = 0.94 +/- 0.26 (1-sigma).

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arXiv1997

Constraints on Cosmological Models from Hubble Space Telescope Observations of High-z Supernovae

We have coordinated Hubble Space Telescope photometry with ground-based discovery for three supernovae: two SN Ia near z~0.5 (SN 1997ce, SN 1997cj) and a third event at z=0.97 (SN 1997ck). The superb spatial resolution of HST separates each supernova from its host galaxy and leads to good precision in the light curves. The HST data combined with ground-based photometry provide good temporal coverage. We use these light curves and relations between luminosity, light curve shape, and color calibrated from low-z samples to derive relative luminosity distances which are accurate to 10% at z~0.5 and 20% at z=1. The redshift-distance relation is used to place constraints on the global mean matter density, Omega_matter, and the normalized cosmological constant, Omega_Lambda. When the HST sample is combined with the distance to SN 1995K (z=0.48), analyzed by the same precepts, it suggests that matter alone is insufficient to produce a flat Universe. Specifically, for Omega_matter+Omega_Lambda=1, Omega_matter is less than 1 with >95% confidence, and our best estimate of Omega_matter is -0.1 +/- 0.5 if Omega_Lambda=0. Although the present result is based on a very small sample whose systematics remain to be explored, it demonstrates the power of HST measurements for high redshift supernovae.

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arXiv1997

Optical Light Curves of the Type Ia Supernovae 1990N and 1991T

We present UBVRI light curves for the bright Type Ia supernovae SN 1990N in NGC 4639 and SN 1991T in NGC 4527 based on photometry gathered in the course of the Calan/Tololo supernova program. Both objects have well-sampled light curves starting several days before maximum light and spanning well through the exponential tail. These data supercede the preliminary photometry published by Leibundgut et al (1991) and Phillips et al (1992). The host galaxies for these supernovae have (or will have) accurate distances based on the Cepheid period-luminosity relationship. The photometric data in this paper provide template curves for the study of general population of Type Ia supernova and accurate photometric indices needed for the Cepheid-supernova distance scale.

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arXiv1997

Time Dilation from Spectral Feature Age Measurements of Type Ia Supernovae

We have developed a quantitative, empirical method for estimating the age of Type Ia supernovae (SNe Ia) from a single spectral epoch. The technique examines the goodness of fit of spectral features as a function of the temporal evolution of a large database of SNe Ia spectral features. When a SN Ia spectrum with good signal-to-noise ratio over the rest frame range 3800 to 6800 A is available, the precision of a spectral feature age (SFA) is (1-sigma) ~ 1.4 days. SFA estimates are made for two spectral epochs of SN 1996bj (z=0.574) to measure the rate of aging at high redshift. In the 10.05 days which elapsed between spectral observations, SN 1996bj aged 3.35 $\pm$ 3.2 days, consistent with the 6.38 days of aging expected in an expanding Universe and inconsistent with no time dilation at the 96.4 % confidence level. The precision to which individual features constrain the supernova age has implications for the source of inhomogeneities among SNe Ia.

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arXiv1996

The Morphology of Type Ia Supernovae Light Curves

We present a family of six BVI template light curves for SNe Ia for days -5 and +80, based on high-quality data gathered at CTIO. These templates display a wide range of light curve morphologies, with initial decline rates of their B light curves between m15(B)=0.87 mag and 1.93 mag. We use these templates to study the general morphology of SNe Ia light curves. We find that several of the main features of the BVI templates correlate tightly with m15(B). In particular, the V light curves, which are probably a reasonably good approximation of the bolometric light curves, display an orderly progression in shapes between the most-luminous, slowest-declining events and the least-luminous, fastest-declining SNe. This supports the idea that the observed spectroscopic and photometric sequences of SNe Ia are due primarily to one parameter. Nevertheless, SNe with very similar initial decline rates do show significant differences in their light curve properties when examined in detail, suggesting the influence of one or more secondary parameters.

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