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HD 184100


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Photospheric-Phase Spectropolarimetry and Nebular-Phase Spectroscopy of the Peculiar Type Ic Supernova 2002ap
The early-time optical spectrum of the Type Ic supernova (SN) 2002ap wascharacterized by unusually broad features, leading some authors todesignate it a ``hypernova.'' We present optical spectropolarimetry ofthis object 16 and 37 days after the estimated date of explosion. Aftercorrecting for interstellar polarization, we find evidence for a highlevel of intrinsic continuum polarization at both epochs: p>~1.3% onday 16 and p>~1.0% on day 37. Prominent line polarization is alsoseen, especially in the trough of the Ca II near-infrared triplet duringthe second epoch. When interpreted in terms of the oblate,electron-scattering model atmospheres of Höflich, our results implyan asymmetry of at least 20% (day 16) and 17% (day 37). The data suggesta fair degree of axisymmetry, although the polarization angle of thedominant polarization axes are different by about 55° between thetwo epochs, implying a complex morphology for the thinning ejecta. Inparticular, there exists some spectropolarimetric evidence for adifferent distribution of Ca relative to iron-group elements. We alsopresent flux spectra of SN 2002ap taken 131 and 140 days after theexplosion. The spectra are characterized by a very weak continuum andbroad emission lines, indicating that SN 2002ap has entered the nebularphase. The spectral features are substantially similar to those of``normal'' SNe Ic in the nebular phase, and the emission lines are notsignificantly broader. However, some of the broad lines are very sharplypeaked and may possess a narrow component (probably unresolved by ourspectra, FWHM<~400 km s-1) that is redshifted by ~580 kms-1 with respect to the systemic velocity of the hostgalaxy.

The Metamorphosis of SN 1998bw
We present and discuss the photometric and spectroscopic evolution ofthe peculiar SN 1998bw, associated with GRB 980425, through an analysisof optical and near-IR data collected at ESO-La Silla. The spectroscopicdata, spanning the period from day -9 to day +376 (relative to Bmaximum), have shown that this supernova (SN) was unprecedented,although somewhat similar to SN 1997ef. Maximum expansion velocities ashigh as 3×104 km s-1 to some extent mask itsresemblance to other Type Ic SNe. At intermediate phases, betweenphotospheric and fully nebular, the expansion velocities(~104 km s-1) remained exceptionally high comparedto those of other recorded core-collapse SNe at a similar phase. Themild linear polarization detected at early epochs suggests the presenceof asymmetry in the emitting material. The degree of asymmetry, however,cannot be decoded from these measurements alone. The He I 1.083 and2.058 μm lines are identified, and He is suggested to lie in an outerregion of the envelope. The temporal behavior of the fluxes and profilesof emission lines of Mg I] λ4571, [O I] λλ6300,6364, and a feature ascribed to Fe are traced to stimulate futuremodeling work. The uniqueness of SN 1998bw became less obvious once itentered the fully nebular phase (after 1 yr), when it was very similarto other Type Ib/c-IIb objects, such as the Type Ib SN 1996N and theType IIb SN 1993J, even though SN 1998bw was 1.4 mag brighter than SN1993J and 3 mag brighter than SN 1996N at a comparable phase. Thelate-phase optical photometry, which extends up to 403 days after Bmaximum, shows that the SN luminosity declined exponentially butsubstantially faster than the decay rate of 56Co. Theultraviolet-optical-infrared bolometric light curve, constructed usingall available optical data and the early JHK photometry presented inthis work, shows a slight flattening starting on about day +300. Sinceno clear evidence of ejecta-wind interaction was found in the late-timespectroscopy (see also the work of Sollerman and coworkers), this may bedue to the contribution of the positrons since most γ-rays escapethermalization at this phase. A contribution from the superposed H IIregion cannot, however, be excluded. Based on observations collected atESO-La Silla.

Supernova 1998bw in ESO 184-G82
IAUC 6969 available at Central Bureau for Astronomical Telegrams.

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Observation and Astrometry data

Constellation:ぼうえんきょう座
Right ascension:19h35m30.59s
Declination:-53°07'10.2"
Apparent magnitude:8.341
Distance:416.667 parsecs
Proper motion RA:3.9
Proper motion Dec:-19
B-T magnitude:9.975
V-T magnitude:8.476

Catalogs and designations:
Proper Names
HD 1989HD 184100
TYCHO-2 2000TYC 8774-1412-1
USNO-A2.0USNO-A2 0300-35911545
HIPHIP 96358

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