arXiv subjects

Schuyler D. Van Dyk

Publications and source records attributed to Schuyler D. Van Dyk.

At least 127 records · Page 7Linked to original sources

arXiv2004

MERLIN and VLA Observations of VII Zw 19: Distant Cousin of M82

We have observed the starburst dwarf galaxy VII Zw 19 at subarcsecond resolution with the MERLIN and MERLIN+EVN networks at 18 cm, with MERLIN at 6 cm, and with the VLA in A array at 6 and 2 cm. The galaxy is detected at all VLA wavelengths and is resolved. It is also resolved at 18 cm by MERLIN, but is not detected by the EVN nor by MERLIN at 6 cm. VII Zw 19 has a complex structure of nonthermal radio emission at 18 cm extended over ~1200-1800 pc (4-6''). That the EVN did not detect this emission indicates that there are no obvious point sources that could be radio supernovae. The 2 cm emission, predominantly thermal free-free emission, has a markedly different spatial distribution from the nonthermal emission. The radio colors show that the galaxy contains numerous supernova remnants, as well as many young HII regions, many of which are optically thick at 6 cm. Two extended regions of 2 cm emission have little 18 cm flux and are probably emission from young star clusters. VII Zw 19 resembles M82 in its radio and infrared spectrum, but the starburst region in VII Zw 19 is twice the size of the one in M82 and twice as luminous.

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arXiv2003

Optical Photometry and Spectroscopy of the SN 1998bw-like Type Ic Supernova 2002ap

We present optical photometric and spectral data of the peculiar Type Ic SN 2002ap. Photometric coverage includes UBVRI bands from 2002 January 30, the day after discovery, through 2002 December 12. There are 5 early-time spectra and 8 in the nebular phase. We determine that SN 2002ap is similar to SN 1997ef and the GRB-associated SN 1998bw with respect to spectral and photometric characteristics. The nebular spectra of SN 2002ap present the largest Mg I] λ4571 to [O I] λλ6300, 6364 ratio of any supernova spectra yet published, suggesting that the progenitor of SN 2002ap was a highly stripped star. Comparing the nebular spectra of SN 1985F and SN 2002ap, we notice several similar features, casting the classification of SN 1985F as a normal Type Ib supernova in doubt. We also present nebular modeling of SN 2002ap and find that the object ejected >~ 1.5 M_{sun} of material within the outer velocity shell of the nebula (~5500 km/s) and synthesized ~0.09 M_{sun} of 56Ni.

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arXiv2003

Radio Emission from SN 2001gd in NGC 5033

We present the results of monitoring the radio emission from the Type IIb supernova SN 2001gd between 2002 February 8 and 2002 October 28. Most of the data were obtained using the Very Large Array at the five wavelengths of $λλ$1.3 cm (22.4 GHz), 2.0 cm (14.9 GHz), 3.6 cm (8.44 GHz), 6.2 cm (4.86 GHz), and 21 cm (1.4 GHz). Observations were also made with Giant Meterwave Radio Telescope at $λ$21 cm (1.4 GHz). The object was discovered optically well after maximum light, making any determination of the early radio evolution difficult. However, subsequent observations indicate that the radio emission has evolved regularly in both time and frequency and is well described by the SN shock/circumstellar medium interaction model.

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arXiv2003

On the Progenitor of Supernova 2001du in NGC 1365

Hubble Space Telescope (HST) WFPC2 archival images obtained years prior to the explosion of the Type II-plateau supernova (SN) 2001du in NGC 1365 previously have been analyzed to isolate the progenitor star. The SN site was located using precise astrometry applied to the HST images, with significant uncertainty, leaving three possible progenitor candidates. Images of the fading SN have recently become publicly available in the HST archive, allowing us to pinpoint the SN's exact location on the pre-explosion images. We show that the SN occurred in very close proximity to one of the blue candidate stars, but we argue that this star is not the actual progenitor. Instead, the progenitor was not detected on the pre-SN images, and we constrain the progenitor's mass to be less than 13 (+7, -4) Msuns. This is consistent with previous constraints on the progenitor masses of other Type II-plateau supernovae (SNe), suggesting that such SNe arise from the iron core collapse of massive stars at the lower extreme of the possible mass range.

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arXiv2002

A Search for Core-Collapse Supernova Progenitors In Hubble Space Telescope Images

Identifying the massive progenitor stars that give rise to core-collapse supernovae (SNe) is one of the main pursuits of supernova and stellar evolution studies. Using ground-based images of recent, nearby SNe obtained primarily with KAIT, astrometry from 2MASS, and archival images from HST, we have attempted the direct identification of the progenitors of 16 SNe II and SNe Ib/c. We may have identified the progenitors of the SNe II 1999br, 1999ev, and 2001du as supergiant stars with M^0_V ~ -6 mag in all three cases. We may have also identified the progenitors of the SNe Ib 2001B and 2001is as very luminous supergiants with M^0_V ~ -8 to -9 mag, and possibly the progenitor of the SN Ic 1999bu as a supergiant with M^0_V ~ -7.5 mag. Additionally, we have recovered at late times SNe 1999dn, 2000C, and 2000ew, although none of these had detectable progenitors on pre-SN images. In fact, for the remaining SNe only limits can be placed on the absolute magnitude and color (when available) of the progenitor. The detected SN II progenitors and limits are consistent with red supergiants as progenitor stars, although possibly not as red as we had expected. Our results for the SNe Ib/c do not strongly constrain either Wolf-Rayet stars or massive interacting binary systems as progenitors.

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arXiv2002

A Study of the Type II-Plateau Supernova 1999gi, and the Distance to its Host Galaxy, NGC 3184

We present optical spectra and photometry sampling the first six months after discovery of supernova (SN) 1999gi in NGC 3184. SN 1999gi is shown to be a Type II-plateau event with a photometric plateau lasting until about 100 days after discovery. The reddening values resulting from five independent techniques are all consistent with an upper bound of E(B-V) < 0.45 mag established by comparing the early-time color of SN 1999gi with that of an infinitely hot blackbody, and yield a probable reddening of E(B-V) = 0.21 +/- 0.09 mag. Using the expanding photosphere method (EPM), we derive a distance to SN 1999gi of 11.1^{+2.0}_{-1.8} Mpc and an explosion date of 1999 December 5.8^{+3.0}_{-3.1}, or 4.1^{+3.0}_{-3.1} days prior to discovery. This distance is consistent with a recent Tully-Fisher distance derived to NGC 3184 (D ~ 11.59 Mpc), but is somewhat closer than the Cepheid distances derived to two galaxies that have generally been assumed to be members of a small group containing NGC 3184 (NGC 3319, D = 13.30 +/- 0.55 Mpc, and NGC 3198, D = 13.80 +/- 0.51 Mpc). We reconsider the upper mass limit (9^{+3}_{-2} M_{sun}) recently placed on the progenitor star of SN 1999gi by Smartt et al. (2001, 2002) in light of these results. Following the same procedures, but using the new data presented here, we arrive at a less restrictive upper mass limit of 15^{+5}_{-3} M_{sun} for the progenitor. The increased upper limit results mainly from the larger distance derived through the EPM than was assumed by the Smartt et al. analyses, which relied on less precise (and less recent) distance measurements to NGC 3184.

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arXiv2002

Possible Recovery of SN 1961V In Hubble Space Telescope Archival Images

SN 1961V in NGC 1058 was originally classified by Fritz Zwicky as a ``Type V'' supernova. However, it has been argued that SN 1961V was not a genuine supernova, but instead the superoutburst of an eta Carinae-like luminous blue variable star. In particular, Filippenko et al. (1995, AJ, 110, 2261) used pre-refurbishment HST WFPC images and the known radio position of SN 1961V to conclude that the star survived the eruption and is likely coincident with a V \~ 25.6 mag, V-I ~ 1.9 mag object. Recently, Stockdale et al. (2001, AJ, 122, 283) recovered the fading SN 1961V at radio wavelengths and argue that its behavior is similar that of some Type II supernovae. We have analyzed post-refurbishment archival HST WFPC2 data and find that the new radio position is still consistent with the Filippenko et al. object, which has not changed in brightness or color, but is also consistent with an adjacent, fainter (I ~ 24.3 mag) and very red (V-I > 1.0 mag) object. We suggest that this fainter object could be the survivor of SN 1961V. Forthcoming HST observations may settle this issue.

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arXiv2002

Narrow Lines in Type II Supernovae -- Probing the Circumstellar Nebulae of the Progenitors

We have carried out a high-dispersion (R~30,000) echelle spectroscopic survey of 16 Type II supernovae (SNe) to search for narrow emission lines from circumstellar nebulae ejected by their massive progenitors. Circumstellar nebulae, if detected, provide invaluable opportunities to probe SN progenitors. Of the 16 SNe observed, SN ejecta are clearly detected in 4 SNe and possibly in another 2 SNe, interstellar gas is detected in 12 SNe, and circumstellar material is detected only in SN 1978K and SN 1998S. In the case of SN 1978K we are able to place an upper limit of ~2.2 pc for the size of the circumstellar ejecta nebula and note that this is more consistent with the typical sizes observed for ejecta nebulae around luminous blue variables rather than Wolf-Rayet stars. In the case of SN 1998S, our observations of the narrow lines \~1 year after the SN explosion show variations compared to early epochs. The nebular lines we observe from SN 1998S originate from either the low--density, outer region of a circumstellar nebula or have become dominated by an interstellar component.

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arXiv2002

A Hubble Space Telescope Snapshot Survey of Nearby Supernovae

We present photometry of 12 recent supernovae (SNe) recovered in a {\it Hubble Space Telescope} Snapshot program, and tie the measurements to earlier ground-based observations, in order to study the late-time evolution of the SNe. Many of the ground-based measurements are previously unpublished, and were made primarily with a robotic telescope, the Katzman Automatic Imaging Telescope. Evidence for circumstellar interaction is common among the core-collapse SNe. Late-time decline rates for Type IIn SNe are found to span a wide range, perhaps due to differences in circumstellar interaction. An extreme case, SN IIn 1995N, declined by only 1.2 mag in $V$ over about 4 years following discovery. Template images of some SNe must therefore be obtained many years after the explosion, if contamination from the SN itself is to be minimized. Evidence is found against a previous hypothesis that the Type IIn SN 1997bs was actually a superoutburst of a luminous blue variable star. The peculiar SN Ic 1997ef, a "hypernova," declined very slowly at late times. The decline rate of the SN Ia 2000cx decreased at late times, but this is unlikely to have been caused by a light echo.

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arXiv2001

BVRI Photometry of Supernovae

We present optical photometry of one Type IIn supernova (1994Y) and nine Type Ia supernovae (1993Y, 1993Z, 1993ae, 1994B, 1994C, 1994M, 1994Q, 1994ae, and 1995D). SN 1993Y and SN 1993Z appear to be normal SN Ia events with similar rates of decline, but we do not have data near maximum brightness. The colors of SN 1994C suggest that it suffers from significant reddening or is intrinsically red. The light curves of SN 1994Y are complicated; they show a slow rise and gradual decline near maximum brightness in $VRI$ and numerous changes in the decline rates at later times. SN 1994Y also demonstrates color evolution similar to that of the SN IIn 1988Z, but it is slightly more luminous and declines more rapidly than SN 1988Z. The behavior of SN1994Y indicates a small ejecta mass and a gradual strengthening of the H$α$ emission relative to the continuum.

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arXiv2001

Evidence for Blow-out in the Low-mass Dwarf Galaxy Holmberg I

We present radio and optical observations of Holmberg I (HoI), a member of the M81 group of galaxies (distance 3.6Mpc). HoI is a low-mass, low surface- brightness dwarf galaxy. High-resolution multi-array VLA HI observations reveal a supergiant shell (diameter: 1.7 kpc) which covers about half the optical extent of HoI and which comprises 75% of the total HI content (total HI mass: 1.1 10^8 M_o). We set a tentative upper limit to the dark matter content of < 3.1 10^8 M_o. The HI data are complemented by deep, optical UBV(RI)_c and Halpha observations obtained at the Calar Alto 2.2m telescope. We find M_HI/L_B = 1.1 M_o/L_B_o. The total visible (stars plus gas) mass of HoI adds up to 2.4 10^8 M_o. This leads to a total mass of < 5.5 10^8 M_o. The origin of HoI's peculiar HI morphology is discussed in terms of a supergiant shell created by strong stellar winds and supernova explosions (energy: equiv. 20-260 type II SN; age: 80+-20 Myr). The morphological center of HoI is offset by 0.75 kpc with respect to the dynamical center. Within the interior of the shell the light distribution is exponential with a rather shallow gradient and blue optical colors. Beyond a radius corresponding to an HI column density of 10^21 cm^-2, the putative star formation (SF) threshold, the disk becomes considerably redder and the slope for the exponential light distribution steepens. It is speculated that a generation of stars (age 15-30 Myr) is the result of secondary SF on the rim of the shell. Based on the global morphology and velocity dispersion as well as the location of the HII regions we find evidence for ram pressure within the M81 group. Finally, we discuss the likelihood of HoI having lost some of its interstellar material to the intergalactic medium (''blow-out'' scenario).

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arXiv2001

MSX, 2MASS, and the LMC: A Combined Near and Mid Infrared View

The Large Magellanic Cloud (LMC) has been observed by both the Midcourse Space Experiment (MSX) in the mid-infrared and the Two Micron All Sky Survey (2MASS) in the near-infrared. We have performed a cross-correlation of the 1806 MSX catalog sources and nearly 1.4 million 2MASS catalogued point and extended sources and find 1664 matches. Using the available color information, we identify a number of stellar populations and nebulae, including main sequence stars, giant stars, red supergiants, carbon- and oxygen-rich asymptotic giant branch (AGB) stars, planetary nebulae, H II regions, and other dusty objects likely associated with early-type stars. 731 of these sources have no previous identification. We compile a listing of all objects, which includes photometry and astrometry. The 8.3 micron MSX sensitivity is the limiting factor for object detection: only the brighter red objects, specifically the red supergiants, AGB stars, planetary nebulae and HII regions, are detected in the LMC. The remaining objects are likely in the Galactic foreground. The spatial distribution of the infrared LMC sources may contribute to understanding stellar formation and evolution and the overall galactic evolution. We demonstrate that a combined mid- and near-infrared photometric baseline provides a powerful means of identifying new objects in the LMC for future ground-based and space-based follow-up observations.

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arXiv2001

Optical and Ultraviolet Spectroscopy of SN 1995N: Evidence for Strong Circumstellar Interaction

Optical and ultraviolet observations of the Type IIn supernova 1995N at epochs between 321 and 1799 days after the explosion show three distinct velocity components. The narrow lines come from circumstellar gas and show both low and high ionization. This component has a low filling factor, and is photoionized by X-rays from the shock. The intermediate component, which is dominated by newly processed oxygen, originates in a shell with velocity of 2500-5000 km/s, and most likely comes from the ejecta. The hydrogen- and helium-dominated gas has a low ionization, a high density, and velocities that extend out to ~10,000 km/s. Strong signatures of Ly-alpha-pumped fluorescence lines of Fe II are seen in the near-infrared and ultraviolet. The He/H ratio, ~ 0.3 by number, and the nitrogen overabundance provide strong evidence for CNO burning products. The fluxes of the broad hydrogen and helium lines decrease considerably faster than the oxygen lines. The H-alpha line profile shows strong evolution, with the red wing decreasing faster than the blue. Possible scenarios, involving either a clumpy circumstellar medium, or an aspherical distribution of the surrounding gas, are discussed based on the line profiles and physical conditions. Finally, we propose that Type IIn supernovae have their origin in red supergiants in a superwind phase.

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arXiv2001

Radio Continuum Imaging of the Spiral Galaxy NGC 4258

We analyze 3."5 resolution, high sensitivity radio continuum images of the nearby spiral galaxy NGC 4258 at 6 and 20 cm derived from multiple observations used to monitor the radio supernova SN 1981K (Van Dyk et al. 1992, [ApJ, 396, 1995]). Seven bright H II region and five supernova remnant candidates are identified. Extinctions to the H II regions are estimated for the first time from a comparison of radio flux densities to new optical fluxes derived from H-alpha observations by Dutil & Roy (1999, [ApJ, 516, 62]). The bright end of the H II region luminosity function is established at each wavelength. The luminosity functions are best fit by power laws consistent with the shape of previously published radio and optical luminosity functions for a number of galaxies. The supernova remnants are all about 2--3 times the radio luminosity of Cas A. In addition, the galaxy's nucleus is shown to have been variable over the SN 1981K monitoring period. The spectral index (alpha) distribution of the anomalous radio arms is investigated and found to be relatively uniform at alpha = -0.65 +/- 0.10.

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arXiv2000

Radio Supernovae and GRB 980425

Study of radio supernovae (RSNe) over the past 20 years includes two dozen detected objects and more than 100 upper limits. From this work we are able to identify classes of radio properties, demonstrate conformance to and deviations from existing models, estimate the density and structure of the circumstellar material and, by inference, the evolution of the presupernova stellar wind, and reveal the last stages of stellar evolution before explosion. It is also possible to detect ionized hydrogen along the line of sight, to demonstrate binary properties of the stellar system, and to show clumpiness of the circumstellar material. More speculatively, it may be possible to provide distance estimates to radio supernovae. The interesting and unusual radio supernova SN 1998bw, which is thought to be related to the gamma-ray burst GRB 980425, is discussed in particular detail. Its radio properties are compared and contrasted with those of other known RSNe.

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arXiv1999

Discovery of Radio Outbursts in the Active Nucleus of M81

The low-luminosity active galactic nucleus of M81 has been monitored at centimeter wavelengths since early 1993 as a by-product of radio programs to study the radio emission from Supernova 1993J. The extensive data sets reveal that the nucleus experienced several radio outbursts during the monitoring period. At 2 and 3.6 cm, the main outburst occurred roughly in the beginning of 1993 September and lasted for approximately three months; at longer wavelengths, the maximum flux density decreases, and the onset of the burst is delayed. These characteristics qualitatively resemble the standard model for adiabatically expanding radio sources, although certain discrepancies between the observations and the theoretical predictions suggest that the model is too simplistic. In addition to the large-amplitude, prolonged variations, we also detected milder changes in the flux density at 3.6 cm and possibly at 6 cm on short (less than 1 day) timescales. We discuss a possible association between the radio activity and an optical flare observed during the period that the nucleus was monitored at radio wavelengths.

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arXiv1998

Late-Time Optical and UV Spectra of SN 1979C and SN 1980K

A low-dispersion Keck I spectrum of SN 1980K taken in August 1995 (t = 14.8 yr after explosion) and a November 1997 MDM spectrum (t = 17.0 yr) show broad 5500 km s^{-1} emission lines of Hα, [O I] 6300,6364 A, and [O II] 7319,7330 A. Weaker but similarly broad lines detected include [Fe II] 7155 A, [S II] 4068,4072 A, and a blend of [Fe II] lines at 5050--5400 A. The presence of strong [S II] 4068,4072 A emission but a lack of [S II] 6716,6731 A emission suggests electron densities of 10^{5-6} cm^{-3}. From the 1997 spectra, we estimate an Hαflux of 1.3 \pm 0.2 \times 10^{-15} erg cm^{-2} s^{-1} indicating a 25% decline from 1987--1992 levels during the period 1994 to 1997, possibly related to a reported decrease in its nonthermal radio emission.

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arXiv1998

Hubble Space Telescope WFPC2 Imaging of SN 1979C and Its Environment

The locations of supernovae in the local stellar and gaseous environment in galaxies contain important clues to their progenitor stars. As part of a program to study the environments of supernovae using Hubble Space Telescope (HST) imaging data, we have examined the environment of the Type II-L SN 1979C in NGC 4321 (M100). We place more rigorous constraints on the mass of the SN progenitor, which may have had a mass M \approx 17--18 M_sun. Moreover, we have recovered and measured the brightness of SN 1979C, m=23.37 in F439W (~B; m_B(max) = 11.6), 17 years after explosion. .

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