A&A 368, 391–397 (2001) Astronomy DOI: 10.1051/0004-6361:20000104 & c ESO 2001 Astrophysics The brightness of SN 1991T and the uniformity of decline-rate and colour corrected absolute magnitudes of supernovae Ia? T. Richtler1,J.B.Jensen2,??, J. Tonry3,B.Barris3, and G. Drenkhahn4 1 Universidad de Concepci´on, Departamento de F´ısica, Casilla 160-C, Concepci´on, Chile 2 Gemini Observatory, 670 N. A`ohoku Place Hilo, HI 96720, USA e-mail: [email protected] 3 Institute for Astronomy, University of Hawaii 2680 Woodlawn Drive, Honolulu, HI 96822, USA 4 Max-Planck-Institut f¨ur Astrophysik, Postfach 1317, 85741 Garching bei M¨unchen, Germany e-mail: [email protected] Received 7 August 2000 / Accepted 14 November 2000 Abstract. We present a distance to NGC 4527, the host galaxy of the type Ia SN 1991T, measured by surface brightness fluctuations. This supernova has been labelled “peculiar” both on the grounds of its spectroscopic behaviour and its apparent overluminosity with respect to other supernovae. The distance modulus to NGC 4527 and thus to SN 1991T is 30.26 0.09. This relatively short distance largely removes the discrepancy with other Ia supernovae having similar light-curve characteristics and also removes the motivation for interpreting SN 1991T as a super-Chandrasekhar explosion. However, the reddening uncertainty results in significant uncertainty of the absolute magnitudes. We show further that, although the intrinsic absolute magnitudes among Ia supernovae are quite different, their brightness, corrected for decline-rate and colour, shows a high degree of uniformity, including underluminous events like SN 1991bg and SN 1997cn. In particular, the present data do not support a significant difference between corrected SNe Ia luminosities in late-type and early-type host galaxies. Key words. supernovae: general – supernovae: individual: SN 1991T – galaxies: individual: NGC 4527 – distance scale 1. Introduction at high redshifts, selection effects towards the bright end of the supernovae luminosity distribution are probably A precondition for the successful use of Ia supernovae present, the apparently overluminous supernovae are of at high redshifts for constraining cosmological models is particular interest. Obviously, the distance to the host the understanding of low-redshift supernovae, in partic- galaxy is the key parameter. For example, in the case of ular their luminosities and their dependence on light- SN 1994D in NGC 4526, it could be shown that this ap- curve parameters such as the decline-rate (Phillips 1993). parently overluminous Ia event took place at a shorter Besides the decline rate, it is also the colour at maximum, distance than initially assumed, and thus its luminos- for which one has to correct (Tripp 1998; Drenkhahn & ity is consistent with other type-Ia SNe (Drenkhahn & Richter 1999). The Cal´an-Tololo sample (Hamuy et al. Richtler 1999). 1996) and the Harvard sample (Riess et al. 1999) demon- Another more extreme example of overluminosity is strated that most Ia supernovae, after appropriate correc- SN 1991T in NGC 4527. Besides its allegedly high luminos- tions, exhibit a surprisingly high degree of uniformity in ity, it also exhibited spectroscopic peculiarities. (Fischer their maximum brightness. et al. 1999) discuss the present knowledge about SN 1991T A few supernovae, however, seem to deviate in the and also offer an interpretation of these abnormal spec- sense that they are either too bright or too dim. Since tral features. Their analysis suggested that SN 1991T was a super-Chandrasekhar explosion following the merging of Send offprint requests to: T. Richtler, two white dwarfs. e-mail: [email protected] ? Based on observations with the NASA/ESA Hubble Space Although there is general agreement that NGC 4527 is Telescope, obtained at the Space Telescope Science Institute, located on the near side of the Virgo cluster, there is no which is operated by AURA, Inc., under NASA contract NAS individual distance determination besides a Tully-Fisher 5-26555. distance, where it is known that large deviations can oc- ?? Gemini Science Fellow. cur. Fischer et al. (1999) assumed its distance to be equal Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20000104 392 T. Richtler et al.: The brightness of SN 1991T to the distance of two neighboring spiral galaxies, which in the individual case to distinguish between an intrin- also have been hosts for Ia SNe, NGC 4496 (SN 1960F), sic red colour and reddening. We also point out that only and NGC 4536 (SN 1981B). the galactic foreground reddening can be accounted for, The aim of our paper is to present an individual dis- because internal reddenings are not available. However, if tance to SN 1991T, based on surface brightness fluctua- reddening dominated the colour dependence of SN lumi- tions in the bulge of NGC 4527, which is more accurate nosities, we would expect distinctly larger colour coeffi- than what is available in the literature. cients in B and V . Before we present our method of distance determina- The coefficients in Table 1 result from a χ2-fit to the tion and its result, we recall briefly what is known about data of 41 (24 in the I band) SN Ia from Hamuy et al. the distance of this galaxy group and the luminosities of (1996) and Riess et al. (1999). The listed values are the the respective Ia SNe. best fits to the relation mmax + b · (∆m15 − ∆m15) 1.1. What is known about the distances of NGC 4527 +R · (Bmax − Vmax − (Bmax − Vmax)) (1) and NGC 4536? =5· log cz + Z NGC 4536, the host of SN 1981B, has a more or less with mmax being the foreground extinction corrected mag- well established distance, based on the PL-relation for nitudes in B, V and I, b the decline-rate coefficient, R the Cepheids observed by HST (Saha et al. 1996a; Saha et al. colour coefficient, and Z the zero-point of the Hubble di- 1996b). However, the Cepheid data are to a minor de- agram defined by the merging of both SNe samples. For gree subject to interpretational manipulation, depending the determination of the foreground extinction we used on details of the data reduction and also of the selec- the dust maps of Schlegel et al. (1998). tion of Cepheids used for the distance determination. Saha Table 2 lists the photometric parameters for our SNe et al. (1996b), for instance, quote a distance modulus to taken from Hamuy et al. (1996). The reddening values NGC 4536 of 31.07 mag, while Gibson et al. (2000) give and their uncertainties are those quoted by Fischer et al. 30.95 0.23 mag. (1999). Tully-Fisher (TF) distances have been measured both Using the correction terms for colour and decline-rate, for NGC 4527 and for NGC 4536. If we use the new TF and the Cepheid distance modulus for NGC 4536, one calibrations given by Sakai et al. (2000) and the TF data arrives at the corrected absolute maximum magnitudes given by Pierce (1994), we then get distance moduli (av- given in Table 3. eraged over B, R, I)of30.78 and 30.80 for NGC 4527 The Cal´an-Tololo and the Harvard sample demon- and NGC 4536, respectively. Pierce (1994) himself quotes strated that there is no observable offset between the 30.6and30.5, respectively. The differences are consistent corrected absolute magnitudes of SNe Ia in early-type with the dispersion of the TF relation which is about and late-type galaxies. Within the uncertainty limits, 0.3–0.4 mag. In this sense, the TF data are in agreement SN 1981B matches the SNe Ia in the Fornax cluster and with the Cepheid distance of NGC 4536. Moreover, the TF also to some of the Ia’s in other spiral galaxies, although distances cannot support any distance difference between uncertain photometry and uncertain extinction make the NGC 4527 and NGC 4536 and it appears reasonable to as- comparison more difficult (Drenkhahn & Richtler 1999). sume a distance modulus of 30.950.23 mag for NGC 4527 However, the brightness of SN 1991T is inconsistent with as well. that of SN 1981B or other supernovae, if the Cepheid dis- tance of NGC 4536 is also adopted for NGC 4527. Table 3 demonstrates that this discrepancy is not very dependent 2. Revised relations between SN Ia maximum on the adopted reddening, although the error bars overlap, brightness, decline rate and colour almost exclusively as a result of the reddening uncertainty It is now clear that the intrinsic maximum brightness (see Fischer et al. 1999 for a compilation of the relevant of SNe Ia vary by approximately 1 mag. Using both literature). the “Cal´an-Tololo” sample (Hamuy et al. 1996) and the Harvard sample (Riess et al. 1999), we below present 3. The distance to NGC 4527 from surface handy relations between maximum brightness (extinction brightness fluctuations corrected), decline rate and colour for the B, V ,andI bands. We define the colour as the difference between the We measured a new and accurate distance to NGC 4527 Bmax and Vmax maximum magnitudes, which is not nec- using the near-IR surface brightness fluctuation technique. essarily the B − V colour at, say, the B-maximum. These Two 128-second images of NGC 4527 taken with NICMOS relations are revised versions of those given in Drenkhahn on the Hubble Space Telescope were recovered from the & Richtler (1999), which only included the Cal´an-Tololo archive. These images were taken using the NIC2 camera sample. A similar analysis, based on the Cal´an-Tololo sam- using the F160W filter, and have a spatial resolution of ple alone, has been done by Tripp & Branch (1999).