arXiv subjects

D. Branch

Publications and source records attributed to D. Branch.

At least 19 recordsLinked to original sources

arXiv2010

Studying the small scale ISM structure with supernovae

AIMS. In this work we explore the possibility of using the fast expansion of a Type Ia supernova photosphere to detect extra-galactic ISM column density variations on spatial scales of ~100 AU on time scales of a few months. METHODS. We constructed a simple model which describes the expansion of the photodisk and the effects of a patchy interstellar cloud on the observed equivalent width of Na I D lines. Using this model we derived the behavior of the equivalent width as a function of time, spatial scale and amplitude of the column density fluctuations. RESULTS. The calculations show that isolated, small (<100 AU) clouds with Na I column densities exceeding a few 10^11 cm^-2 would be easily detected. In contrast, the effects of a more realistic, patchy ISM become measurable in a fraction of cases, and for peak-to-peak variations larger than ~10^12 cm^-2 on a scale of 1000 AU. CONCLUSIONS. The proposed technique provides a unique way to probe the extra-galactic small scale structure, which is out of reach for any of the methods used so far. The same tool can also be applied to study the sub-AU Galactic ISM structure.

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arXiv2009

Type Ia supernova science 2010-2020

In the next decade Type Ia supernovae (SNe Ia) will be used to test theories predicting changes in the Dark Energy equation of state with time. Ultimately this requires a dedicated space mission like JDEM. SNe Ia are mature cosmological probes --- their limitations are well characterized, and a path to improvement is clear. Dominant systematic errors include photometric calibration, selection effects, reddening, and population-dependent differences. Building on past lessons, well-controlled new surveys are poised to make strides in these areas: the Palomar Transient Factory, Skymapper, La Silla QUEST, Pan-STARRS, the Dark Energy Survey, LSST, and JDEM. They will obviate historical calibrations and selection biases, and allow comparisons via large subsamples. Some systematics follow from our ignorance of SN Ia progenitors, which there is hope of determining with SN Ia rate studies from 0<z<4. Aside from cosmology, SNe Ia regulate galactic and cluster chemical evolution, inform stellar evolution, and are laboratories for extreme physics. Essential probes of SNe Ia in these contexts include spectroscopy from the UV to the IR, X-ray cluster and SN remnant observations, spectropolarimetry, and advanced theoretical studies. While there are an abundance of discovery facilities planned, there is a deficit of follow-up resources. Living in the systematics era demands deep understanding rather than larger statistics. NOAO ReSTAR initiative to build 2-4m telescopes would provide necessary follow-up capability. Finally, to fully exploit LSST, well-matched wide-field spectroscopic capabilities are desirable.

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arXiv2008

Multi-layered Spectral Formation in SNe Ia Around Maximum Light

We use the radiative transfer code PHOENIX to study the line formation of the wavelength region 5000-7000 Angstroms. This is the region where the SNe Ia defining Si II feature occurs. This region is important since the ratio of the two nearby silicon lines has been shown to correlate with the absolute blue magnitude. We use a grid of LTE synthetic spectral models to investigate the formation of line features in the spectra of SNe Ia. By isolating the main contributors to the spectral formation we show that the ions that drive the spectral ratio are Fe III, Fe II, Si II, and S II. While the first two strongly dominate the flux transfer, the latter two form in the same physical region inside of the supernova. We also show that the naive blackbody that one would derive from a fit to the observed spectrum is far different than the true underlying continuum.

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arXiv2008

Massive stars exploding in a He-rich circumstellar medium. I. Type Ibn (SN 2006jc-like) events

We present new spectroscopic and photometric data of the type Ibn supernovae 2006jc, 2000er and 2002ao. We discuss the general properties of this recently proposed supernova family, which also includes SN 1999cq. The early-time monitoring of SN 2000er traces the evolution of this class of objects during the first few days after the shock breakout. An overall similarity in the photometric and spectroscopic evolution is found among the members of this group, which would be unexpected if the energy in these core-collapse events was dominated by the interaction between supernova ejecta and circumstellar medium. Type Ibn supernovae appear to be rather normal type Ib/c supernova explosions which occur within a He-rich circumstellar environment. SNe Ibn are therefore likely produced by the explosion of Wolf-Rayet progenitors still embedded in the He-rich material lost by the star in recent mass-loss episodes, which resemble known luminous blue variable eruptions. The evolved Wolf-Rayet star could either result from the evolution of a very massive star or be the more evolved member of a massive binary system. We also suggest that there are a number of arguments in favour of a type Ibn classification for the historical SN 1885A (S-Andromedae), previously considered as an anomalous type Ia event with some resemblance to SN 1991bg.

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arXiv2006

Hydrogen issue in Core Collapse Supernovae

We discuss results of analyzing a time series of selected photospheric-optical spectra of core collapse supernovae (CCSNe). This is accomplished by means of the parameterized supernovae synthetic spectrum (SSp) code ``SYNOW''. Special attention is addressed to traces of hydrogen at early phases, especially for the stripped-envelope SNe (i.e. SNe Ib-c). A thin low mass hydrogen layer extending to very high ejection velocities above the helium shell, is found to be the most likely scenario for Type Ib SNe.

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arXiv2006

Chang-Refsdal Microlensed Type Ia Supernova Light Curves

A description of the light curves of microlensed Type Ia supernovae (SNe Ia) as extended and expanding sources is presented. We give examples of what microlensing by stellar-mass Chang-Refsdal lenses can do to a small percentage of supernova light curves. We find that in addition to overall brightening, significant changes in light--curve shapes can also occur. Peaks can be distorted, plateaus can appear, and even secondary peaks can be formed. The effects of both the relative motion of the lens and the supernova and the expansion of the supernova are given and compared. The effects of relative motion are more pronounced when a distant supernova ($z_s \sim 1$) impacts well within the Einstein ring of a nearby microlens ($z_d \sim 0.05$) and are less important for more distant deflectors. We also find that the increase in shear that comes with increased deflector distance tends to reduce the time variability of microlensing. We briefly discuss the probability of observing these effects.

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arXiv2006

Hydrogen and helium traces in type Ib-c supernovae

The spectroscopic properties of a selected optical photospheric spectra of core collapse supernovae (CCSNe) are investigated.Special attention is devoted to traces of hydrogen at early phases. The generated spectra are found to match the observed ones reasonably well, including a list of only 23 candidate ions. Guided by SN Ib 1990I, the observed trough near 6300Åis attributed to H$α$ in almost all Type Ib events, although in some objects it becomes too weak to be discernible, especially at later phases. Alternative line identifications are discussed. Differences in the way hydrogen manifests its presence within CCSNe are highlighted. In Type Ib SNe, the H$α$ contrast velocity (i.e. line velocity minus the photospheric velocity) seems to increase with time at early epochs, reaching values as high as 8000 km s$^{-1}$ around 15-20 days after maximum and then remains almost constant. The derived photospheric velocities, indicate a lower velocity for Type II SNe 1987A and 1999em as compared to SN Ic 1994I and SN IIb 1993J, while Type Ib events display a somewhat larger variation. The scatter, around day 20, is measured to be $\sim$5000 km s$^{-1}$. Following two simple approaches, rough estimates of ejecta and hydrogen masses are given. A mass of hydrogen of approximately 0.02 $M_\odot$ is obtained for SN 1990I, while SNe 1983N and 2000H ejected $\sim$0.008 $M_\odot$ and $\sim$0.08 $M_\odot$ of hydrogen, respectively. SN 1993J has a higher hydrogen mass, $\sim 0.7$ $M_\odot$ with a large uncertainty. A low mass and thin hydrogen layer with very high ejection velocities above the helium shell, is thus the most likely scenario for Type Ib SNe. Some interesting and curious issues relating to oxygen lines suggest future investigations.

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arXiv2005

Using Line Profiles to Test the Fraternity of Type Ia Supernovae at High and Low Redshifts

Using archival data of low-redshift (z < 0.01) Type Ia supernovae (SN Ia) and recent observations of high-redshift (0.16 < z <0.64; Matheson et al. 2005) SN Ia, we study the "uniformity'' of the spectroscopic properties of nearby and distant SN Ia. We find no difference in the measures we describe here. In this paper, we base our analysis solely on line-profile morphology, focusing on measurements of the velocity location of maximum absorption (vabs) and peak emission (vpeak). We find that the evolution of vabs and vpeak for our sample lines (Ca II 3945, Si II 6355, and S II 5454, 5640) is similar for both the low- and high-redshift samples. We find that vabs for the weak S II 5454, 5640 lines, and vpeak for S II 5454, can be used to identify fast-declining [dm15 > 1.7] SN Ia, which are also subluminous. In addition, we give the first direct evidence in two high-z SN Ia spectra of a double-absorption feature in Ca II 3945, an event also observed, though infrequently, in low-redshift SN Ia spectra (6/22 SN Ia in our local sample). We report for the first time the unambiguous and systematic intrinsic blueshift of peak emission of optical P-Cygni line profiles in Type Ia spectra, by as much as 8000 km/s. All the high-z SN Ia analyzed in this paper were discovered and followed up by the ESSENCE collaboration, and are now publicly available.

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arXiv2005

SN 1998A: Explosion of a Blue Supergiant

We present spectroscopic and photometric observations of the peculiar Type II supernova (SN) 1998A. The light curves and spectra closely resemble those of SN 1987A, suggesting that the SN 1998A progenitor exploded when it was a compact blue supergiant. However, the comparison with SN 1987A also highlights some important differences: SN 1998A is more luminous and the spectra show bluer continua and larger expansion velocities at all epochs. These observational properties indicate that the explosion of SN 1998A is more energetic than SN 1987A and more typical of SNe II. Comparing the observational data to simulations, we deduce that the progenitor of SN 1998A was a massive star (~ 25 Mo) with a small pre-supernova radius (< 6 x 10^{12} cm). The Ba II lines, unusually strong in SN 1987A and some faint II--P events, are almost normal in the case of SN 1998A, indicating that the temperature plays a key role in determining their strength.

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arXiv2004

High velocity features in the spectra of the Type Ia SN 1999ee: a property of the explosion or evidence of circumstellar interaction?

The near-maximum spectra of the Type Ia SN 1999ee are reviewed. Two narrow absorption features corresponding to the strongest component of the CaII IR triplet appear in the spectra from 7 days before to 2 days after B-band maximum, at a high velocity (~22,000 km/s). Before these features emerge, the CaII IR triplet has an anomalously high velocity, indicating that the narrow features were still blended with the main, photospheric component. High-velocity CaII absorption has been observed in other SNe Ia, but usually detached from the photospheric component. Furthermore, the SiII 6355A line is observed at a comparably high velocity (~20,000 km/s) 9 and 7 days before B maximum, but then it suddenly shifts to much lower velocities. Synthetic spectra are used to reproduce the data under various scenarios. An abundance enhancement requires an outer region dominated by Si and Ca, the origin of which is not easy to explain in terms of nuclear burning. A density enhancement leads to a good reproduction of the spectral evolution if a mass of ~0.10 Msun is added at velocities between 16,000 and 28,000 km/s, and it may result from a perturbation, possibly angular, of the explosion. An improved match to the CaII IR triplet at the earliest epoch can be obtained if the outermost part of this modified density profile is assumed to be dominated by H (~0.004 Msun above 24,000 km/s). Line broadening is then the result of increased electron scattering. This H may be the result of interaction between the ejecta and circumstellar material.

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arXiv2004

GRB 020410: A Gamma-Ray Burst Afterglow Discovered by its Supernova Light

We present the discovery and monitoring of the optical transient (OT) associated with GRB 020410. The fading OT was found by Hubble Space Telescope (HST) observations taken 28 and 65 days after burst at a position consistent with the X-ray afterglow. Subsequent re-examination of early ground based observations revealed that a faint OT was present 6 hours after burst, confirming the source association with GRB 020410. A deep non-detection after one week requires that the OT re-brightened between day 7 and day 28, and further late time HST data taken approximately 100 days after burst imply that it is very red.We compare both the flux and color of the excess with supernova models and show that the data are best explained by the presence of a Type Ib/c supernova at a redshift z ~ 0.5, which occured roughly coincident with the day of GRB.

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arXiv2004

Direct Analysis of Spectra of the Peculiar Type Ia Supernova 2000cx

The Type Ia SN 2000cx exhibited multiple peculiarities, including a lopsided B-band light-curve peak that does not conform to current methods for using shapes of light curves to standardize SN Ia luminosities. We use the parameterized supernova synthetic-spectrum code SYNOW to study line identifications in the photospheric-phase spectra of SN 2000cx. Previous work established the presence of Ca II infrared-triplet features forming above velocity about 20,000 km/s, much higher than the photospheric velocity of about 10,000 km/s. We find Ti II features forming at the same high velocity. High-velocity line formation is partly responsible for the photometric peculiarities of SN 2000cx: for example, B-band flux blocking by Ti II absorption features that decreases with time causes the B light curve to rise more rapidly and decline more slowly than it otherwise would. SN 2000cx contains an absorption feature near 4530 A that may be H-beta, forming at the same high velocity. The lack of conspicuous H-alpha and P-alpha signatures does not necessarily invalidate the H-beta identification if the high-velocity line formation is confined to a clump that partly covers the photosphere and the H-alpha and P-alpha source functions are elevated relative to that of resonance scattering. The H-beta identification is tentative. If it is correct, the high-velocity matter must have come from a nondegenerate companion star.

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arXiv2003

Low Luminosity Type II Supernovae: Spectroscopic and Photometric Evolution

In this paper we present spectroscopic and photometric observations for four core collapse supernovae (SNe), namely SNe 1994N, 1999br, 1999eu and 2001dc. Together with SN 1997D, we show that they form a group of exceptionally low-luminosity events. These SNe have narrow spectral lines (indicating low expansion velocities) and low luminosities at every phase (significantly lower than those of typical core-collapse supernovae). The very low luminosity during the ^{56}Co radioactive decay tail indicates that the mass of ^{56}Ni ejected during the explosion is much smaller (M(Ni) ~ 2-8 x 10^{-3} Mo) than the average (M(Ni) ~ 6-10 x 10^{-2} Mo). Two supernovae of this group (SN 1999br and SN 2001dc) were discovered very close to the explosion epoch, allowing us to determine the lengths of their plateaux (~ 100 days) as well as establishiing the explosion epochs of the other, less-completely observed SNe. It is likely that this group of SNe represent the extreme low-luminosity tail of a single continuous distribution of SN II-P events. Their kinetic energy is also exceptionally low. Although an origin from low mass progenitors has also been proposed for low-luminosity core-collapse SNe, recent work provides evidence in favour of the high mass progenitor scenario. The incidence of these low--luminosity SNe could be as high as 4-5$ $ of all type II SNe.

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arXiv2003

Optical Spectra of the Type Ia Supernova 1998aq

We present 29 optical spectra of the normal Type Ia SN 1998aq, ranging from 9 days before to 241 days after the time of maximum brightness. This spectroscopic data set, together with photometric data presented elsewhere, makes SN 1998aq one of the best observed Type Ia supernova at optical wavelengths. We use the parameterized supernova synthetic-spectrum code SYNOW to study line identifications in the early photospheric-phase spectra. The results include evidence for lines of singly ionized carbon, at ejection velocities as low as 11,000 km/sec. Implications for explosion models are discussed.

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arXiv2002

The Spectrum of the FeLoBAL Quasar FBQS 1214+2803: A Resonance-Scattering Interpretation

The usual interpetation of the spectrum of a BAL QSO is that a broad-band continuum from the central engine plus broad emission lines from a surrounding region (the BELR) emerges from near the center of the QSO, and broad absorption lines are superimposed in a separate outlying region (the BALR). For FBQS 1214+2803, designated as an FeLoBAL QSO because its spectrum contains numerous absorption features from excited states of Fe II, we explore an alternative interpretation based on resonance scattering. In this model line emission and absorption occur in the same line-forming region (the LFR). A resonance-scattering synthetic spectrum computed with the supernova synthetic-spectrum code SYNOW fits the spectrum of FBQS 1214+2802 rather well, so the resonance-scattering model merits further study. Some implications of the model and its possible applicability to other QSOs are briefly discussed.

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arXiv2002

Exceptionally Bright Type Ib Supernova 1991D

Photometric and spectroscopic observations of the peculiar Type Ib Supernova 1991D are presented. SN 1991D was exceptionally bright for a Type Ib supernova. The He I lines were rather weak and the velocity at the photosphere as a function of time was unusually low. Comparison of the observed and synthetic spectra indicates that either hydrogen was ejected with a minimum velocity of 12,000 km/s or the spectrum contained features caused by lines of Ne I. Light curve modelling suggests that the progenitor probably had a very large radius (~10^14 cm) and that a considerable amount of 56Ni was synthesized during the explosion (~0.7 Msolar). We suggest a progenitor model of SN 1991D that involves a binary system.

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arXiv2002

The Axially Symmetric Ejecta of Supernova 1987A

Extensive early observations proved that the ejecta of supernova 1987A (SN 1987A) are aspherical. Fifteen years after the supernova explosion, the Hubble Space Telescope has resolved the rapidly expanding ejecta. The late-time images and spectroscopy provide a geometrical picture that is consistent with early observations and suggests a highly structured, axially symmetric geometry. We present here a new synthesis of the old and new data. We show that the Bochum event, presumably a clump of $^{56}$Ni, and the late-time image, the locus of excitation by $^{44}$Ti, are most naturally accounted for by sharing a common position angle of about 14\degree, the same as the mystery spot and early speckle data on the ejecta, and that they are both oriented along the axis of the inner circumstellar ring at 45\degree to the plane of the sky. We also demonstrate that the polarization represents a prolate geometry with the same position angle and axis as the early speckle data and the late-time image and hence that the geometry has been fixed in time and throughout the ejecta. The Bochum event and the Doppler kinematics of the [Ca II]/[O II] emission in spatially resolved HST spectra of the ejecta can be consistently integrated into this geometry. The radioactive clump is deduced to fall approximately along the axis of the inner circumstellar ring and therefore to be redshifted in the North whereas the [Ca II]/[O II] 7300 Åemission is redshifted in the South. We present a jet-induced model for the explosion and argue that such a model can account for many of the observed asymmetries. In the jet models, the oxygen and calcium are not expected to be distributed along the jet, but primarily in an expanding torus that shares the plane and northern blue shift of the inner circumstellar ring.

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arXiv2000

Spectroscopically Peculiar Type Ia Supernovae and Implications for Progenitors

In a recent paper Li et al. (2000) reported that 36 percent of 45 Type Ia supernovae (SNe Ia) discovered since 1997 in two volume-limited supernova searches were spectroscopically peculiar, and they suggested that because this peculiarity rate is higher than that reported for an earlier observational sample by Branch et al. (1993), it is now more likely that SNe Ia are produced by more than one kind of progenitor. In this paper I discuss and clarify the differences between the results of Li et al. and Branch et al. and I suggest that multiple progenitor systems are now less likely than they were before.

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