My Ph.D work was dedicated mainly to the study of Core Collapse SNe (CCSNe), both their observational properties and their physical interpretation. I have been particularly involved in the study of SN 1999em in NGC 1637, a type II Plateau event (SN IIP), possibly the most well studied event among CCSNe after SN 1987 A. In fact a large study, covering more than 600 days after outburst, photometrically and spectroscopically, has been presented (Elmhamdi A. et al. 2003, MNRAS, 338, 939). The widely spaced and critical observations of this object have provided a unique opportunity to test our knowledges of the physics of CCSNe and as well an understanding of the different parameters that characterize the pre-SN evolution. In particular we have argued for dust condensation, spectroscopically and photometrically, between days 465 and 510. Support for our finding of dust formation comes from the more recent late optical photometric observations sampling 679 to 738 days after explosion date presented by Leonard et al. (2003; astro-ph/0305259). We have also pointed out the differences between SN 1999em compared to SN 1987 A in manifesting the same events (e.g. Bochum event, asymmetry, dust formation), and have discussed the physical interpretations of such diversities. The bolometric properties of the SN have also been presented and discussed, providing an estimate of the ejected 56Ni mass, which was lower ( rv 0.02 M8 ) compared to a typical value for SNe IIP (rv 0.07 M8 ). An estimate of the oxygen mass has also been provided. Interestingly, two correlations are hinted at when analysing the observational behaviour of SN 1999em and other IIP events, namely, a curious flattening in the light curves, just after the steep decline from the plateau phase, clearer for the blue bands. This behaviour is also reported in other events, having all lower ejected 56Ni mass (SNe 1997D, 1991G and 1999eu), indicating this behaviour to be a common feature in faint SNe IIP. In addition the duration of the flattening period seems to be correlated to the amount of ejected 56Ni. The second hinted correlation is between the measured Ha luminosities and 56Ni mass when compared to SN 1987 A at later phases. Indeed this second hinted correlation was the real incentive for a following project in which our main goal was to check the possibility of using Ha luminosity at the nebular epoch as a tracer of 56Ni mass in SN e IIP. My second project was a study of a sample of type IIP SN e on the basis of available photometry and spectra, especially at latter epochs (Elmhamdi A., Chugai N. N. and Danziger I. J. 2003, A&A, 404, 1077). The main goal of the work was to check the idea of using Ha luminosity as tracer of 56Ni mass in type IIP SNe. We make use of collected published photometry while the utilized spectra have been taken from Asiago/ESO Catalogue. Additional spectra were provided kindly by R. Stathakis. Once we fix the points related to the extinction and distance, which are crucial when dealing with a SN-sample study, we proceed with computing the amounts of ejected 56Ni photometrically using the absolute V-light curve of SN 1987 A as template (in the 120-400 days time range). We found a range from lower values for SN 1999eu and SN 1997D to a higher one for SN 1992H, with an average of about 0.05 M8 . Analysing the absolute light curves of the events, we introduced a new parameter, called "steepness" and dubbed S, which describes the shape of the light curves and provides a way to measure the decay rate at the inflection point. A confirmation of the correlation between the 56Ni mass and plateau Mv found by Hamuy (2003, ApJ, 582, 905) is evident, while an interesting by-product of the sample photometry analysis is the demonstrated correlation between 56Ni mass and the steepness parameter S. The correlation is such that the steeper the decline at the inflection point the lower is the mass of 56 Ni. This correlation is interesting in the sense that, if confirmed, will provide distance and extinction independent estimates of the 56Ni mass in SN e IIP. vVe then applied a two-zone model of the Ha luminosity in SN IIP to explore the sensitivity of the Ha behaviour to variation of model parameters. The primary purpose of the upgraded model is to specify better the early nebular phase compared to the previous version ( Chugai 1990, Sv AL, 16, 457). We found that if mass, energy and mixing conditions do not vary strongly among SNe IIP (less than factor 1.4) then with an accuracy better than 10% Ha luminosity is proportional to 56Ni mass during the 200 - 400 days after explosion. Ha luminosities were then used to derive 56Ni masses. This was done employing two approaches: first, using the Ha light curve in SN 1987 A as template and, second, applying the model computations. Both approaches agree within 15% unless we are dealing with extreme cases such as SN 1970G (type IIP /L) and underluminous SN 1997D. In both these cases we should possess additional information about ejecta mass and energy to derive the 56Ni mass from Ha. The 56Ni mass values derived from the photometry and Ha luminosity agree within 20%, which thus gives us confidence that Ha is a good indicator of the amount of 56Ni in SNe IIP. Simultaneously, this consistency suggests that parameters of SNe IIP (mass, energy and mixing) are not very different. In fact this is consistent with the uniformity of plateau luminosities and plateau lengths of SNe IIP. Worth noting is the simple approach of using Ha light curve of SN 1987 A as a template to estimate the 56Ni mass. Indeed this simple approach has been applied for three SNe for which we have late spectra but no photometry (SNe 1995ad, 1995V and 1995W), where we obtained reasonable values, demonstrating the usefulness of the method. The last part of my Ph.D program was devoted to the study and analysis of observational properties of some type lb/ c SN e. Indeed, an analysis of photometry and spectra of the type lb SN 19901 have been completed (A. Elmhamdi & I. J. Danziger 2003; In preparation). We investigate many particular observational aspects this event showed during its evolution. SN 1990I is found to show higher expansion velocities compared to the type lb sample studied by Branch et al. (2002, ApJ, 566, 1005). We show evidence of helium lines in the ejecta. This event provides further understanding of some features that may or may not be common in this class of objects (i.e. possible dust condensation, observational evidence of asymmetry), and as well gives a check on the present believable physical scenario behind CCSN e that lack hydrogen lines in their spectra at the level of the progenitor stars. We give an estimate of the ejecta and 56Ni masses (M(56 Ni) = 0.11 M0 and Mej = 3.7 M0 ) by applying a simplified 1-ray deposition model to the recovered quasi-bolometric "EV RI" light curve. This latter shows a change of slope at late phases, with an e-folding time of 60 ±2 d in the [50 : 200] d time interval, clearly faster than the one of 56Co decay (i.e. 111.3 d), suggestive of the 1-rays escape with lower deposition, consistent with the low mass nature of the ejecta. At early phases ([30: 100] d), the pseudo-light curves of SNe 1990I and 1993J are found to display a high degree of similarity. After day 200 they behave differently: while SN 1993J tends to flatten, SN 1990I demonstrates a dramatic fall with a deficit in luminosity estimated around day 308 to be about 503. Spectroscopically, SN 1990I shows evidence for asymmetry and clumping. A blueshift in some nebular lines is reported, which we interpret as due to dust condensation in the ejecta when combined with the rapid and sudden drop in both the pseudo-bolometric light curve and ( B V) colour around day 250. In an appendix I outline a possible method for determining 0 /Fe yields from CCSNe. At the moment the uncertainties in estimates of oxygen masses are too large to draw safe conclusions about progenitor masses.
|Titolo:||Physical Interpretations of observations of Core Collapse SNe|
|Relatore/i esterni:||Danziger, John I.|
|Data di pubblicazione:||17-ott-2003|
|Appare nelle tipologie:||8.1 PhD thesis|