Working Group Members:

• N.K. Rao, IIA, Bangalore

• A. Saha, NOAO

• T. P. Prabhu, IIA, Bangalore

• G. C. Anupama, IIA, Bangalore

Supernovae studies science areas:

The discovery that the Universe is accelerating was first gleaned from the Hubble diagram of type Ia supernovae (SNeIa), which are the brightest standard candles that can be seen to redshifts of 2 and beyond. Even after the WMAP and other CMB results, the SNeIa Hubble diagram remains the most persuasive evidence of an accelerating Universe, and thus in turn, of some form of Dark Energy. It is widely held that the observations of large numbers of SNeIa to redshifts of ~2 are needed in order to beat down the statistical errors, so that the Hubble diagram is defined in enough detail to set interesting limits on the nature of Dark Energy, and to differentiate between various theoretical models of the same. In order to achieve these goals, plans are afoot to build large facilities, both on the ground and in space, which will address this very important problem.

While the SNeIa route is the most obvious approach, several detailed aspects of such experiments are still problem areas:

1. SNeIa show intrinsic variation in peak luminosity from object to object. Attempts to correlate the peak brightness against observed characteristics of individual SNeIa have been by and large successful, but different research groups have not converged (see Reindl et al. 2005, ApJ 624, 532 and references therein).
2. The detailed explosion mechanism, or for that matter even the stellar progenitors, have not been identified with any real defree of confidence. So a theory of the physical processes that produce the observed range of peak luminosities is lacking. Only 3 or 4 groups are actively working on the problem, which requires detailed modelling of non-LTE expanding atmospheres.
3. At progressively higher redshifts, any given observer passband is seeing rest frame radiation that was emitted correspondingly blue-wards. Since our empirical knowledge of SNeIa luminosities and characteristics is from optical and near IR observations of relatively nearby objects -- at high redshifts we must observe progressively further in the IR. At z~2, the restframe V-band is at 1.6 microns, and I-band is past the K-band. To observe SNeIa at such redshifts, one must reach faint levels not achievable from the ground due to high sky background: thus necessitating very expensive large IR space telescopes.
4. In order to derive the rest-frame brightness from IR observations, one must be able to calibrate the measuring instrument adequately. About 1 or better accuracy is needed in the IR observations in physical units (i.e ergs/cm-sq/Hz) to be useful for characterizing Dark Energy. This is several factors beyond the current state of IR absolute flux calibration.

How can extensive UV observations of nearby SNeIa help with the above issues?
 
I. The intrinsic variation in peak luminosity increases as one goes to bluer wavelengths. Thus UV light curves may provide the best handle on object by object variations in the peak luminosity. In other words, the UV light curves may indicate how much the B or V band luminosity of a particular SNIa departs from the mean for this class of objects. At any rate, the UV data are likely to set important limits on the possible intrinsic variation in individual SNeIa luminosities: either they will allow us to improve how well we can predict the brightness of an individual SNIa, or we will have to live with the current uncertainty. This degree to which we can make this prediction is the "floor" of the noise in the Hubble diagram. Any experiment that wishes to characterize Dark Energy from the Hubble diagram must know what this floor is, in order to determine how many SNeIa must be observed at any redshift in order to beat the random noise. If UV observation help to make better predictions, it will drastically reduce the number of SNeIa that we need to observe at large z. A factor of 2 improvement in noise will result in a factor of 4 smaller distant SNeIa sample requirement.

II. The longer wavelength baseline offers better separation of extinction versus variations in the intrinsic colors of SNeIa.

III. It **may** shed light on the explosion mechanism, since this is substantial additional information. Opinion about this is mixed in the theoretical community: some say since the UV comes from the outer skin of the explosion shell, it is unlikely to probe anything that reflects the explosion. Others say that especially if UV spectra can be obtained, it may reveal important clues from the chemical signature.

IV. Perhaps most important is that a library of light curves of nearby (z < 0.1) SNeIa in UV pass-bands made in a well controlled flux calibration environment will be ideal templates for observing SNeIa at higher redshifts. For instance, if luminosity is known at 2000 Å, then at z~2, the corresponding pass-band is only at 6000 Å, where the distant SN can be observed adequately from the ground, and where also the state of absolute flux calibration is adequate for addressing the Dark Energy problem. The space observations, now in the UV instead of the IR, are of BRIGHT SNeIa, and achievable with only a small space telescope.

Return to top