D. A. Howell
A5080043647- Gamma-ray bursts and supernovae
- Astrophysical Phenomena and Observations
- Astrophysics and Cosmic Phenomena
- Pulsars and Gravitational Waves Research
- Stellar, planetary, and galactic studies
- Astronomy and Astrophysical Research
- Particle Detector Development and Performance
- Astro and Planetary Science
- Astronomical Observations and Instrumentation
- Superconducting Materials and Applications
- Geophysics and Gravity Measurements
- Solar and Space Plasma Dynamics
- Galaxies: Formation, Evolution, Phenomena
- Neutrino Physics Research
- Advanced Data Processing Techniques
- Methane Hydrates and Related Phenomena
- Adaptive optics and wavefront sensing
- Nuclear Physics and Applications
- Calibration and Measurement Techniques
- Geological and Geophysical Studies
- Atomic and Subatomic Physics Research
- CCD and CMOS Imaging Sensors
- Cosmology and Gravitation Theories
- Astrophysics and Star Formation Studies
- Radio Astronomy Observations and Technology
Las Cumbres Observatory Global Telescope Network
2016-2025
University of California, Santa Barbara
2016-2025
Flatiron Health (United States)
2022-2023
Flatiron Institute
2022-2023
Stony Brook University
2022-2023
University of Toronto
2004-2009
Oak Ridge National Laboratory
2009
Lockheed Martin (United States)
2009
University of Victoria
2007-2008
Institut National de Physique Nucléaire et de Physique des Particules
2007-2008
We present distance measurements to 71 high redshift type Ia supernovae discovered during the first year of 5-year Supernova Legacy Survey (SNLS). These events were detected and their multi-color light-curves measured using MegaPrime/MegaCam instrument at Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four one-square degree fields in bands, as part CFHT (CFHTLS). Follow-up spectroscopy was performed VLT, Gemini Keck telescopes confirm nature measure redshift. With this data...
Aims. We present cosmological constraints from a joint analysis of type Ia supernova (SN Ia) observations obtained by the SDSS-II and SNLS collaborations. The dataset includes several low-redshift samples (z< 0.1), all three seasons (0.05 <z< 0.4), years (0.2 1), it totals 740 spectroscopically confirmed supernovae with high-quality light curves.
After the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered a significant brightening of inner region NGC 2617, we began ~70 day photometric and spectroscopic monitoring campaign from X-ray through near-infrared (NIR) wavelengths. We report that 2617 went dramatic outburst, during which its flux increased by over an order magnitude followed increase optical/ultraviolet (UV) continuum almost magnitude. classified as Seyfert 1.8 galaxy in 2003, is now 1 due to appearance broad...
We report measurements of ΩM, ΩΛ, and w from 11 supernovae (SNe) at z = 0.36-0.86 with high-quality light curves measured using WFPC2 on the Hubble Space Telescope (HST). This is an independent set high-redshift SNe that confirms previous SN evidence for accelerating universe. The available photometry make it possible these alone to provide cosmological parameters comparable in statistical weight results. Combined earlier Supernova Cosmology Project data, new yield a measurement mass density...
The Palomar Transient Factory (PTF) is a fully-automated, wide-field survey aimed at systematic exploration of the optical transient sky. performed using new 8.1 square degree camera installed on 48-inch Samuel Oschin telescope Observatory; colors and light curves for detected transients are obtained with automated 60-inch telescope. PTF uses eighty percent 1.2-m fifty 1.5-m time. With an exposure 60-s reaches depth approximately 21.3 in g' 20.6 R (5 sigma, median seeing). Four major...
Las Cumbres Observatory Global Telescope (LCOGT) is a young organization dedicated to time-domain observations at optical and (potentially) near-IR wavelengths. To this end, LCOGT constructing worldwide network of telescopes, including the two 2 m Faulkes as many 17 × 1 23 40 cm telescopes. These telescopes initially will be outfitted for imaging (excepting telescopes) spectroscopy wavelengths between atmospheric UV cutoff roughly 1-μm limit silicon detectors. Since first LCOGT's are now...
We report on work to increase the number of well-measured Type Ia supernovae (SNe Ia) at high redshifts. Light curves, including signal-to-noise Hubble Space Telescope data, and spectra six SNe that were discovered during 2001, are presented. Additionally, for two with z > 1, we present ground-based J-band photometry from Gemini Very Large Telescope. These among most distant which near-IR observations have been obtained. add these together other data sets recently become available in...
We combine high redshift Type Ia supernovae from the first 3 years of Supernova Legacy Survey (SNLS) with other supernova (SN) samples, primarily at lower redshifts, to form a high-quality joint sample 472 SNe (123 low-$z$, 93 SDSS, 242 SNLS, and 14 {\it Hubble Space Telescope}). SN data alone require cosmic acceleration >99.9% confidence, including systematic effects. For dark energy equation state parameter (assumed constant out least $z=1.4$) in flat universe, we find $w =...
The Palomar Transient Factory (PTF) is a wide-field experiment designed to investigate the optical transient and variable sky on time scales from minutes years. PTF uses CFH12k mosaic camera, with field of view 7.9 deg2 plate scale 1'' pixel-1, mounted Observatory 48 inch Samuel Oschin Telescope. operation strategy devised probe existing gaps in phase space search for theoretically predicted, but not yet detected, phenomena, such as fallback supernovae, macronovae,. Ia orphan afterglows...
We present an empirical model of Type Ia supernovae spectro-photometric evolution with time. The is built using a large data set including light-curves and spectra both nearby distant supernovae, the latter being observed by SNLS collaboration. derive average spectral sequence their main variability components color variation law. allows us to measure distance moduli in range 2500-8000 A calculable uncertainties, those arising from features. Thanks use high-redshift SNe rest-frame UV energy...
We present photometric properties and distance measurements of 252 high redshift Type Ia supernovae (0.15 < z 1.1) discovered during the first three years Supernova Legacy Survey (SNLS). These events were detected their multi-colour light curves measured using MegaPrime/MegaCam instrument at Canada-France-Hawaii Telescope (CFHT), by repeatedly imaging four one-square degree fields in bands. Follow-up spectroscopy was performed VLT, Gemini Keck telescopes to confirm nature measure redshifts....
We present observational constraints on the nature of dark energy using Supernova Legacy Survey three-year sample (SNLS3) Guy et al. and Conley use 472 Type Ia supernovae (SNe Ia) in this sample, accounting for recently discovered correlations between SN luminosity host galaxy properties, include effects all identified systematic uncertainties directly cosmological fits. Combining SNLS3 data with full WMAP7 power spectrum, Sloan Digital Sky luminous red a prior Hubble constant H0 from SHOES,...
(Abridged) Precision cosmology with Type Ia supernovae (SNe Ia) makes use of the fact that SN luminosities depend on their light-curve shapes and colours. Using Supernova Legacy Survey (SNLS) other data, we show there is an additional dependence global characteristics host galaxies: events same shape colour are, average, 0.08mag (~4.0sigma) brighter in massive galaxies (presumably metal-rich) low specific star-formation rates (sSFR). SNe a sSFR also have smaller slope ("beta") between...
We present the results of a Palomar Transient Factory (PTF) archival search for blue transients which lie in magnitude range between "normal" core-collapse and superluminous supernovae (i.e. with $-21\,{\leq}M_{R\,(peak)}\,{\leq}-19$). Of six events found after excluding all interacting Type~IIn Ia-CSM supernovae, three (PTF09ge, 09axc 09djl) are coincident centers their hosts, one (10iam) is offset from center, two (10nuj 11glr) precise can not be determined. All central have similar rise...
We report extensive observational data for five of the lowest redshift Super-Luminous Type Ic Supernovae (SL-SNe Ic) discovered to date, namely PTF10hgi, SN2011ke, PTF11rks, SN2011kf and SN2012il. Photometric imaging transients at +50 +230 days after peak combined with host galaxy subtraction reveals a luminous tail phase four these SL-SNe. A high resolution, optical near infrared spectrum from xshooter provides detection broad He I $\lambda$10830 emission line in (+50d) SN2012il, revealing...
(ABRIDGED) We show that Type Ia supernovae (SNe Ia) are formed within both very young and old stellar populations, with observed rates depend on the mass mean star-formation (SFRs) of their host galaxies. Models where SN rate depends solely galaxy ruled out 99% confidence. Our analysis is based 100 spectroscopically-confirmed SNe Ia, plus 24 photometrically-classified events, all from Supernova Legacy Survey (SNLS) distributed over 0.2
With the advent of large dedicated Type Ia supernova (SN Ia) surveys, K-corrections SNe and their uncertainties have become especially important in determination cosmological parameters. While are largely driven by SN broad-band colors, it is shown here that diversity spectral features can also be important. For an individual observation, statistical errors from inhomogeneity range 0.01 (where observed rest-frame filters aligned) to 0.04 misaligned). To minimize systematic caused assumed...
Stellar Explosions Stars that are born with masses greater than eight times of the Sun end their lives in luminous explosions known as supernovae. Over past decade, access to improved sky surveys has revealed rare types supernovae much more any those were before. Gal-Yam (p. 927 ) reviews these superluminous events and groups them into three classes share common observational physical characteristics. Gamma-ray bursts another type extreme explosive related death massive stars, which occur...
High-quality collections of Type II supernova (SN) light curves are scarce because they evolve for hundreds days, making follow-up observations time consuming and often extending over multiple observing seasons. In these difficulties, the diversity SNe is not fully understood. Here we present ultraviolet optical photometry 12 monitored by Las Cumbres Observatory Global Telescope Network during 2013 to 2014, compare them with previously studied having well-sampled curves. We explore SN...