Sheila J. Kannappan

Ubvri light curves of 44 type ia supernovae

We present UBVRI photometry of 44 Type Ia supernovae (SNe Ia) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SNe Ia to date, nearly doubling the number of well-observed, nearby SNe Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SNe Ia observed at high redshift. The decline rate of SN Ia U-band light curves correlates well with the decline rate in other bands, as does the U - B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ~40% intrinsic scatter compared to the B band.

Photometry and Spectroscopy of GRB 030329 and Its Associated Supernova 2003dh: The First Two Months

We present extensive optical and infrared photometry of the afterglow of gamma-ray burst (GRB) 030329 and its associated supernova (SN) 2003dh over the first two months after detection (2003 March 30-May 29 UT). Optical spectroscopy from a variety of telescopes is shown and, when combined with the photometry, allows an unambiguous separation between the afterglow and SN contributions. The optical afterglow of the GRB is initially a power-law continuum but shows significant color variations during the first week that are unrelated to the presence of an SN. The early afterglow light curve also shows deviations from the typical power-law decay. An SN spectrum is first detectable ~7 days after the burst and dominates the light after ~11 days. The spectral evolution and the light curve are shown to closely resemble those of SN 1998bw, a peculiar Type Ic SN associated with GRB 980425, and the time of the SN explosion is close to the observed time of the GRB. It is now clear that at least some GRBs arise from core-collapse SNe.

The Type Ia Supernova 1998bu in M96 and the Hubble Constant

We present optical and near-infrared photometry and spectroscopy of the Type Ia SN 1998bu in the Leo I Group galaxy M96 (NGC 3368). The data set consists of 356 photometric measurements and 29 spectra of SN 1998bu between UT 1998 May 11 and July 15. The well-sampled light curve indicates the supernova reached maximum light in B on UT 1998 May 19.3 (JD 2450952.8 ± 0.8) with B = 12.22 ± 0.03 and V = 11.88 ± 0.02. Application of a revised version of the Multicolor Light Curve Shape (MLCS) method yields an extinction toward the supernova of AV = 0.94 ± 0.15 mag, and indicates the supernova was of average luminosity compared to other normal Type Ia supernovae. Using the HST Cepheid distance modulus to M96 and the MLCS fitted parameters for the supernova, we derive an extinction-corrected absolute magnitude for SN 1998bu at maximum, MV = -19.42 ± 0.22. Our independent results for this supernova are consistent with those of Suntzeff et al. Combining SN 1998bu with three other well-observed local calibrators and 42 supernovae in the Hubble flow yields a Hubble constant, H0 = 64 -->img1.gif km s-1 Mpc-1, where the error estimate incorporates possible sources of systematic uncertainty including the calibration of the Cepheid period-luminosity relation, the metallicity dependence of the Cepheid distance scale, and the distance to the LMC.

PHYSICAL SOURCES OF SCATTER IN THE TULLY-FISHER RELATION

We analyze residuals from the Tully-Fisher relation for the emission-line galaxies in the Nearby Field Gal- axy Survey, a broadly representative survey designed to fairly sample the variety of galaxy morphologies and environments in the local universe for luminosities from MB ¼� 15 to � 23. For a subsample consisting of the spiral galaxies brighter than M i R ¼� 18, we find strong correlations between Tully-Fisher residuals and both BR color and EW(H� ). The extremes of the correlations are populated by Sa galaxies, which show consistently red colors, and spiral galaxies with morphological peculiarities, which are often blue. If we apply an EW(H� )-dependent or BR color-dependent correction term to the Tully-Fisher relation, the scatter in the relation no longer increases from R to B to U but instead drops to a nearly constant level in all bands, close to the scatter we expect from measurement errors. We argue that these results probably reflect corre- lated offsets in luminosity and color as a function of star formation history. Broadening the sample in mor- phology and luminosity, we find that most nonspiral galaxies brighter than M i ¼� 18 follow the same correlations between Tully-Fisher residuals and BR color and EW(H� ) as do spirals, albeit with greater scatter. However, the color and EW(H� ) correlations do not apply to galaxies fainter than M i R ¼� 18 or to emission-line S0 galaxies with anomalous gas kinematics. For the dwarf galaxy population, the parameters controlling Tully-Fisher residuals are instead related to the degree of recent evolutionary disturbance: overlu- minous dwarfs have higher rotation curve asymmetries, brighter U-band effective surface brightnesses, and shorter gas consumption timescales than their underluminous counterparts. As a result, sample selection strongly affects the measured faint-end slope of the Tully-Fisher relation, and a sample limited to include only passively evolving, rotationally supported galaxies displays a break toward steeper slope at low luminosities.

The AIMSS project - I:bridging the star cluster-galaxy divide

We describe the structural and kinematic properties of the first compact stellar systems discovered by the Archive of Intermediate Mass Stellar Systems project. These spectroscopically confirmed objects have sizes (∼6 < Re [pc] < 500) and masses (∼2 × 106 < M*/M⊙ < 6 × 109) spanning the range of massive globular clusters, ultracompact dwarfs (UCDs) and compact elliptical galaxies (cEs), completely filling the gap between star clusters and galaxies. Several objects are close analogues to the prototypical cE, M32. These objects, which are more massive than previously discovered UCDs of the same size, further call into question the existence of a tight mass–size trend for compact stellar systems, while simultaneously strengthening the case for a universal ‘zone of avoidance’ for dynamically hot stellar systems in the mass–size plane. Overall, we argue that there are two classes of compact stellar systems (1) massive star clusters and (2) a population closely related to galaxies. Our data provide indications for a further division of the galaxy-type UCD/cE population into two groups, one population that we associate with objects formed by the stripping of nucleated dwarf galaxies, and a second population that formed through the stripping of bulged galaxies or are lower mass analogues of classical ellipticals. We find compact stellar systems around galaxies in low- to high-density environments, demonstrating that the physical processes responsible for forming them do not only operate in the densest clusters.

Systematic Uncertainties in Stellar Mass Estimation for Distinct Galaxy Populations

We show that different stellar mass estimation methods yield overall mass scales that disagree by factors up to ~2 for the z = 0 galaxy population and, more importantly, relative mass scales that sometimes disagree by factors 3 between distinct classes of galaxies (spiral/irregular types, classical E/S0s, and E/S0s whose colors reflect recent star formation). This comparison considers stellar mass estimates based on (1) two different calibrations of the correlation between K-band mass-to-light ratio and B - R color and (2) detailed fitting of UBRJHK photometry and optical spectrophotometry using two different population synthesis models (Bruzual-Charlot and Maraston), with the same initial mass function in all cases. We also compare stellar+gas masses with dynamical masses. This analysis offers only weak arguments for preferring a particular stellar mass estimation method, given the plausibility of real variations in dynamical properties and dark matter content. These results help to calibrate the systematic uncertainties inherent in mass-based evolutionary studies of galaxies, including comparisons of low- and high-redshift galaxies.

The ubiquity and dual nature of ultra‐compact dwarfs

We present the discovery of several Ultra Compact Dwarfs (UCDs) located in field/group environments. Examination of these objects, plus literature UCDs, confirms the existence of two distinct formation channels for these compact stellar systems. We find that the UCDs we have discovered around the group elliptical NGC3923 (and most UCDs in general) have properties consistent with their being the most luminous members of the host galaxy’s globular cluster (GC) system. As with GCs they are therefore likely to be the product of the most violent epochs of galaxy formation. We describe UCDs of this type as giant GCs (GGCs). In contrast, the UCD we have found associated with the isolated S0 NGC4546 is clearly the result of the stripping of a nucleated companion galaxy. The young age (�3.4 Gyr) of the UCD, the lack of a correspondingly young GC population, the apparently short dynamical friction decay timescale (�0.5 Gyr) of the UCD, and the presence of a counterrotating gas disc in the host galaxy (co-rotating with respect to the UCD) together suggest that this UCD is the liberated nucleus remaining after the recent stripping of a companion by NGC4546. We infer that the presence of UCDs of either category (GGC’s formed in major star forming events, or stripped nuclei formed in minor mergers) can provide a useful probe of the assembly history of the host galaxy. We suggest a general scheme that unifies the formation of GCs, UCDs, and galaxy nuclei. In this picture “normal” GCs are a composite population, composed of GCs formed in situ, GCs acquired from accreted galaxies, and a population of lower mass stripped dwarf nuclei masquerading as GCs. Above a “scaling onset mass” of 2×10 6 M� (MV � –10), UCDs emerge together with a mass-size relation and a likely mass-metallicity relation (the “blue tilt”). In the mass range up to 7×10 7 M� (MV � –13) UCDs comprise a composite population of GGCs and stripped nuclei. Interestingly, dwarf nuclei have similar colours to blue GCs and UCDs across the scaling onset mass, smoothly extending the blue tilt, while nuclei of more massive galaxies and a prominent minority of UCDs extend the red locus of GCs. Above 7×10 7 M� , UCDs must be almost exclusively stripped nuclei, as no sufficiently rich GC systems exist to populate such an extreme of the GCLF.

Sn 2005cg: explosion physics and circumstellar interaction of a normal type ia supernova in a low-luminosity host

We present the spectral evolution, light curve, and corresponding interpretation for the “normal-bright” Type Ia Supernova 2005cg discovered by ROTSE-IIIc. The host is a low-luminosity (Mr = -16.75), blue galaxy with strong indications of active star formation and an environment similar to that expected for SNe Ia at high redshifts. Early-time (t � -10 days) optical spectra obtained with the HET reveal an asymmetric, triangularshaped Si II absorption feature at about 6100 A with a sharp transition to the continuum at a blue shift of about 24,000 km s -1 . By 4 days before maximum, the Si II absorption feature becomes symmetric with smoothly curved sides. Similar Si II profile evolution has previously been observed in other supe rnovae, and is predicted by some explosion models, but its significance has not been fu lly recognized. Although the spectra predicted by pure deflagration and delayed detonation models are simil ar near maximum light, they predict qualitatively different chemical abundances in the outer layers and thus give qualitatively different spectra at the earliest phases. The Si line observed in SN 2005cg at early times requires the presence of burning products at high velocities and the triangular shape is likely to be formed in an extended region of slowly declining Si abundance that characterizes delayed detonation models. The spectra show a high-velocity Ca II IR feature that coincides in velocity space with the Si II cutoff. This supports the interpretation that the Ca II is formed when the outer layers of the SN ejecta sweep up about 5× 10 -3 M⊙ of material within the progenitor system. We compare our results with other “Branch-normal” SNe Ia with early time spectra, namely SN 2003du, 1999ee and 1994D. Although the expansion velocities based on their Si II absorption minima differ, all show triangular-shaped profiles and velocity cutoffs between 23,000 and 25,000 km s -1 , which are consistent with the Doppler shifts of their respective high-velocity Ca II IR features. SN 1990N-like objects, however, showed distinctly different behavior that may suggest separate progenitor sub-classes. Subject headings:Supernovae, SN 2005cg, deflagration, detonation, cosmolog y, star formation

A broad search for counterrotating gas and stars: Evidence for mergers and accretion

We measure the frequency of bulk gas-stellar counterrotation in a sample of 67 galaxies drawn from the Nearby Field Galaxy Survey, a broadly representative survey of the local galaxy population down to MB ~ -15. We detect four counterrotators among 17 E/S0s with extended gas emission (24 %). In contrast, we find no clear examples of bulk counterrotation among 38 Sa–Sbc spirals, although one Sa does show peculiar gas kinematics. This result implies that, at 95% confidence, no more than 8% of Sa–Sbc spirals are bulk counterrotators. Among types Sc and later, we identify only one possible counterrotator, a Magellanic irregular. We use these results together with the physical properties of the counterrotators to constrain possible origins for this phenomenon.

Angular size measurements of carbon Miras and S-type stars

In our continuing investigation of highly evolved stars, we report new interferometric angular diameter observations of 5 carbon and 4 S-type Mira Variable stars, and 4 non-Mira S Stars. From the data effective temperatures and linear radii are calculated.