arXiv subjects

G. Altavilla

Publications and source records attributed to G. Altavilla.

At least 55 records · Page 3Linked to original sources

arXiv2009

SN 1999ga: a low-luminosity linear type II supernova?

Type II-linear supernovae are thought to arise from progenitors that have lost most of their H envelope by the time of the explosion, and they are poorly understood because they are only occasionally discovered. It is possible that they are intrinsically rare, but selection effects due to their rapid luminosity evolution may also play an important role in limiting the number of detections. In this context, the discovery of a subluminous type II-linear event is even more interesting. We investigate the physical properties and characterise the explosion site of the type II SN 1999ga, which exploded in the nearby spiral galaxy NGC 2442. Spectroscopic and photometric observations of SN 1999ga allow us to constrain the energetics of the explosion and to estimate the mass of the ejected material, shedding light on the nature of the progenitor star in the final stages of its life. The study of the environment in the vicinity of the explosion site provides information on a possible relation between these unusual supernovae and the properties of the galaxies hosting them. Despite the lack of early-time observations, we provide reasonable evidence that SN 1999ga was probably a type II-linear supernova that ejected a few solar masses of material, with a very small amount of radioactive elements of the order of 0.01 solar masses.

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arXiv2007

Supernova rates from the Southern inTermediate Redshift ESO Supernova Search (STRESS)

To measure the supernova (SN) rates at intermediate redshift we performed the Southern inTermediate Redshift ESO Supernova Search (STRESS). Unlike most of the current high redshift SN searches, this survey was specifically designed to estimate the rate for both type Ia and core collapse (CC) SNe. We counted the SNe discovered in a selected galaxy sample measuring SN rate per unit blue band luminosity. Our analysis is based on a sample of ~43000 galaxies and on 25 spectroscopically confirmed SNe plus 64 selected SN candidates. Our approach is aimed at obtaining a direct comparison of the high redshift and local rates and at investigating the dependence of the rates on specific galaxy properties, most notably their colour. The type Ia SN rate, at mean redshift z=0.3, amounts to 0.22^{+0.10+0.16}_{-0.08 -0.14} h_{70}^2 SNu, while the CC SN rate, at z=0.21, is 0.82^{+0.31 +0.30}_{-0.24 -0.26} h_{70}^2 SNu. The quoted errors are the statistical and systematic uncertainties. With respect to local value, the CC SN rate at z=0.2 is higher by a factor of ~2 already at redshift, whereas the type Ia SN rate remains almost constant. This implies that a significant fraction of SN Ia progenitors has a lifetime longer than 2-3 Gyr. We also measured the SN rates in the red and blue galaxies and found that the SN Ia rate seems to be constant in galaxies of different colour, whereas the CC SN rate seems to peak in blue galaxies, as in the local Universe. SN rates per unit volume were found to be consistent with other measurements showing a steeper evolution with redshift for CC SNe with respect to SNe Ia. Finally we have exploited the link between star formation (SF) and SN rates to predict the evolutionary behaviour of the SN rates and compare it with the path indicated by observations.

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arXiv2007

The early spectral evolution of SN 2004dt

Aims. We study the optical spectroscopic properties of Type Ia Supernova (SN Ia) 2004dt, focusing our attention on the early epochs. Methods. Observation triggered soon after the SN 2004dt discovery allowed us to obtain a spectrophotometric coverage from day -10 to almost one year (~353 days) after the B band maximum. Observations carried out on an almost daily basis allowed us a good sampling of the fast spectroscopic evolution of SN 2004dt in the early stages. To obtain this result, low-resolution, long-slit spectroscopy was obtained using a number of facilities. Results. This supernova, which in some absorption lines of its early spectra showed the highest degree of polarization ever measured in any SN Ia, has a complex velocity structure in the outer layers of its ejecta. Unburnt oxygen is present, moving at velocities as high as ~16,700 km/s, with some intermediate-mass elements (Mg, Si, Ca) moving equally fast. Modeling of the spectra based on standard density profiles of the ejecta fails to reproduce the observed features, whereas enhancing the density of outer layers significantly improves the fit. Our analysis indicates the presence of clumps of high-velocity, intermediate-mass elements in the outermost layers, which is also suggested by the spectropolarimetric data.

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arXiv2007

SN 2003du: 480 days in the Life of a Normal Type Ia Supernova

An extensive set of optical and NIR photometry and low-resolution spectra the Type Ia supernova (SN Ia) 2003du was obtained using a number of facilities. The observations started 13 days before B-band maximum light and continued for 480 days with exceptionally good time sampling. The optical photometry was calibrated through the S-correction technique. The UBVRIJHK light curves and the color indices of SN 2003du closely resemble those of normal SNe Ia. SN 2003du reached a B-band maximum of 13.49 (+/-0.02) mag on JD2452766.38 (+/-0.5). We derive a B-band stretch parameter of 0.988 (+/-0.003), which corresponds to dM15=1.02 (+/-0.05), indicative of a SN Ia of standard luminosity. The reddening in the host galaxy was estimated by three methods, and was consistently found to be negligible. We estimate a distance modulus of 32.79 (+/-0.15) mag to the host galaxy, UGC 9391. The peak UVOIR bolometric luminosity of 1.35(+/-0.20) 10^43 erg/s and Arnett's rule implies that M(Ni56)=0.68 (+/-0.14)M_sun of Ni56 was synthesized during the explosion. Modeling of the UVOIR bolometric light curve also indicates M(Ni56) in the range 0.6-0.8 M_sun. The spectral evolution of SN 2003du at both optical and NIR wavelengths also closely resembles normal SNe Ia. In particular, the Si II ratio at maximum R(Si II)=0.22 (+/-0.02) and the time evolution of the blueshift velocities of the absorption line minima are typical. The pre-maximum spectra of SN 2003du showed conspicuous high-velocity features in the Ca II H&K doublet and infrared triplet, and possibly in Si II 6355, lines. We compare the time evolution of the profiles of these lines with other well-observed SNe Ia and we suggest that the peculiar pre-maximum evolution of Si II 6355 line in many SNe Ia is due to the presence of two blended absorption components.

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arXiv2006

Type Ia SNe along redshift: the R(Si II) ratio and the expansion velocities in intermediate z supernovae

We study intermediate--z SNe Ia using the empirical physical diagrams which enable to learn about those SNe explosions. This information can be very useful to reduce systematic uncertainties of the Hubble diagram of SNe Ia up to high z. The study of the expansion velocities and the measurement of the ratio $\mathcal{R}$(\SiII) allow to subtype those SNe Ia as done for nearby samples. The evolution of this ratio as seen in the diagram $\mathcal{R}$(\SiII)--(t) together with $\mathcal{R}$(\SiII)$_{max}$ versus (B-V)$_{0}$ indicate consistency of the properties at intermediate z compared with local SNe. At intermediate--z, the expansion velocities of Ca II and Si II are similar to the nearby counterparts. This is found in a sample of 6 SNe Ia in the range 0.033$\leq z \leq$0.329 discovered within the {\it International Time Programme} (ITP) of {\it Cosmology and Physics with SNe Ia} during the spring of 2002. Those supernovae were identified using the 4.2m William Herschel Telescope. Two SNe Ia at intermediate z were of the cool FAINT type, one being a SN1986G--like object highly reddened. The $\mathcal{R}$(\SiII) ratio as well as subclassification of the SNe Ia beyond templates help to place SNe Ia in their sequence of brightness and to distinguish between reddened and intrinsically red supernovae. This test can be done with very high z SNe Ia and it will help to reduce systematic uncertainties due to extinction by dust. It should allow to map the high-z sample into the nearby one.

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arXiv2006

Anomalous extinction behaviour towards the Type Ia SN 2003cg

We present optical and near-infrared photometry and spectroscopy of the Type Ia SN 2003cg, which exploded in the nearby galaxy NGC 3169. The observations cover a period between -8.5 and +414 days post-maximum. SN 2003cg is a normal but highly-reddened Type Ia event. Its B magnitude at maximum B_max = 15.94+/-0.04 and Delta m_15(B)_obs = 1.12+/-0.04 (Delta m_15(B)_intrinsic = 1.25+/-0.05). Allowing R_v to become a free parameter within the Cardelli et al. (1989) extinction law, simultaneous matches to a range of colour curves of normal SNe Ia yielded E(B-V) = 1.33+/-0.11, and R_v = 1.80+/-0.19. While the value obtained for R_v is small, such values have been invoked in the past, and may imply a grain size which is small compared with the average value for the local ISM.

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arXiv2005

SN 1998A: Explosion of a Blue Supergiant

We present spectroscopic and photometric observations of the peculiar Type II supernova (SN) 1998A. The light curves and spectra closely resemble those of SN 1987A, suggesting that the SN 1998A progenitor exploded when it was a compact blue supergiant. However, the comparison with SN 1987A also highlights some important differences: SN 1998A is more luminous and the spectra show bluer continua and larger expansion velocities at all epochs. These observational properties indicate that the explosion of SN 1998A is more energetic than SN 1987A and more typical of SNe II. Comparing the observational data to simulations, we deduce that the progenitor of SN 1998A was a massive star (~ 25 Mo) with a small pre-supernova radius (< 6 x 10^{12} cm). The Ba II lines, unusually strong in SN 1987A and some faint II--P events, are almost normal in the case of SN 1998A, indicating that the temperature plays a key role in determining their strength.

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arXiv2005

High-Velocity Features: a ubiquitous property of Type Ia SNe

Evidence of high-velocity features such as those seen in the near-maximum spectra of some Type Ia Supernovae (eg SN 2000cx) has been searched for in the available SNIa spectra observed earlier than one week before B maximum. Recent observational efforts have doubled the number of SNeIa with very early spectra. Remarkably, all SNeIa with early data (7 in our RTN sample and 10 from other programmes) show signs of such features, to a greater or lesser degree, in CaII IR, and some also in SiII 6255A line. High-velocity features may be interpreted as abundance or density enhancements. Abundance enhancements would imply an outer region dominated by Si and Ca. Density enhancements may result from the sweeping up of circumstellar material by the highest velocity SN ejecta. In this scenario, the high incidence of HVFs suggests that a thick disc and/or a high-density companion wind surrounds the exploding white dwarf, as may be the case in Single Degenerate systems. Large-scale angular fluctuations in the radial density and abundance distribution may also be responsible: this could originate in the explosion, and would suggest a deflagration as the more likely explosion mechanism. CSM-interaction and surface fluctuations may coexist, possibly leaving different signatures on the spectrum. In some SNe the HVFs are narrowly confined in velocity, suggesting the ejection of blobs of burned material.

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arXiv2005

Supernova search at intermediate z. III. Expansion velocities of the ejecta

We discuss the expansion velocities of different elements in the ejecta of the intermediate-z SNe Ia discovered as a part of the International Time Programme (ITP) project ``Omega and Lambda from Supernovae and the Physics of Supernova Explosions'' at the European Northern Observatory (ENO). The expansion velocities measured for each normal SNIa are found to be within the typical velocity dispersion for their epoch. Meanwhile, the subluminous SN 2002lk SiII expansion velocity is significantly higher than that of SN 1991bg shortly after maximum. The observed phase was younger in SN2002lk than in the local subluminous SNIa SN1991bg.

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arXiv2005

Supernova search at intermediate z. I. Spectroscopic analysis

We study 8 supernovae discovered as part of the International Time Programme (ITP) project ``Omega and Lambda from Supernovae and the Physics of Supernova Explosions'' at the European Northern Observatory (ENO). The goal of the project is to increase the sample of intermediate redshift (0.1<z<0.4) SNe Ia for testing properties of SNe Ia along z and for enlarging the sample in the Hubble diagram up to large z.

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arXiv2005

Supernova search at intermediate z. II. Host galaxy morphology

We discuss the host galaxy morphology of the 8 SNe discovered as a part of the International Time Programme (ITP) project ``Omega and Lambda from Supernovae, and the Physics of Supernovae Explosions'' at the European Northern Observatory (ENO). Identification of the SN host galaxy types was done exploiting both imaging and spectroscopy. A peculiar SNIa at z= 0.033 is found in a spiral galaxy, as most other SNeIa with z between 0.1 and 0.4. A complete account of these studies will be given elsewhere.

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arXiv2005

The diversity of Type Ia Supernovae: evidence for systematics?

The photometric and spectroscopic properties of 26 well observed Type Ia Supernovae (SNeIa) were analyzed with the aim to explore SNIa diversity. The sample includes (Branch-)normal SNe as well as extreme events like SNe 1991T and 1991bg, while the truly peculiar SNIa, SN2000cx and SN2002cx are not included in our sample . A statistical treatment reveals the existence of three different groups. The first group (FAINT) consists of faint SNeIa similar to SN1991bg, with low expansion velocities and rapid evolution of SiII velocity. A second group consists of ``normal'' SNeIa, also with high temporal velocity gradient (HVG), but with brighter mean absolute magnitude =-19.3 and higher expansion velocities than the FAINT SNe. The third group includes both ``normal'' and SN1991T-like SNeIa: these SNe populate a narrow strip in the SiII velocity evolution plot, with a small velocity gradient (SVG), but have absolute magnitudes similar to HVGs. While the FAINT and HVG SNeIa together seem to define a relation between RSi(II) and Dm15(B), the SVG ones either do not conform with that relation or define a new, looser one. The RSi(II) pre-maximum evolution of HVGs is strikingly different from that of SVGs. The impact of this evidence on the understanding of SNIa diversity, in terms of explosion mechanisms, degree of ejecta mixing, and ejecta-CSM interaction, is discussed.

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arXiv2004

Death rate of massive stars at redshift ~0.3

We report the first result of a supernova search program designed to measure the evolution of the supernova rate with redshift. To make the comparison with local rates more significant we copied, as much as possible, the same computation recepies as for the measurements of local rates. Moreover, we exploited the multicolor images and the photometric redshift technique to characterize the galaxy sample and accurately estimate the detection efficiency. Combining our data with the recently published meaurements of the SN Ia rate at different redshifts, we derived the first, direct measurement of the core collapse supernova rate at z = 0.26 as r_cc = 1.45^{+0.55}{-0.45} h^2 SNu [h=H_0/75]. This is a factor three {+/- 50%)} larger than the local estimate. The increase for a look back time of "only" 2.8 Gyr is more rapid than predicted by most of the published models of the SN rate evolution. Core-collapse SN rates measure the death rate of massive star and, because of the short time scale of evolution, can be translated in a measurement of the ongoing SFR. Assuming a Salpeter IMF and the standard scenario for core-collapse progenitors we derived to an estimate of the star formation rate at redshift 3.1^{+1.1}{-1.0} 10^-2 h^3 M_sun yr^-1 Mpc^{-3} which compare very well with a recent estimate based on the measurement of the Halpha luminosity density at the same redshift.

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arXiv2004

Cepheid calibration of type Ia Supernovae and the Hubble constant

We investigate how a different calibration of the Cepheid Period-Luminosity (PL) relation taking into account the metallicity corrections, affects the absolute magnitude calibration of Supernovae (SNe) Ia and, in turn, the determination of the Hubble constant H_0. We exploit SN Ia light curves from literature and previously unpublished data, to build the M_B -- Delta m_{15}(B) relation and we calibrate the zero point by means of 9 type Ia SNe with Cepheid measured distances. This relation was then used to build the Hubble diagram and in turn to derive H_0. In the attempt to correct for the host galaxy extinction, we found that the data seems to suggest a value for the total to selective absorption ratio, R_B=3.5, which is smaller than the standard value for our own Galaxy R_B=4.315. Depending on different metallicity corrections for the Cepheids P-L relation, values of R_B and SN sample selection criteria, we found that the values of the Hubble constant H_0 is in the range 68--74 km s^{-1} Mpc^{-1}, with associated uncertainties of the order of 10%. Unpublished photometry is also presented for 18 SNe of our sample (1991S, 1991T, 1992A, 1992K, 1993H, 1993L, 1994D, 1994M, 1994ae, 1995D, 1995ac, 1995bd, 1996bo, 1997bp, 1997br, 1999aa, 1999dk, 2000cx), which are the results of a long standing effort for supernova monitoring at ESO - La Silla and Asiago Observatories.

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arXiv2003

Supernova 2002bo: inadequacy of the single parameter description

We present optical/near-infrared photometry and spectra of the type Ia SN 2002bo spanning epochs from -13 days before maximum B-band light to +102 days after. The pre-maximum optical coverage is particularly complete. In some respects, SN 2002bo behaves as a typical "Branch normal" type Ia supernova (SN Ia) at optical and IR wavelengths. We find a B-band risetime of 17.9+-0.5 days, a Dm_{15}(B) of 1.13+-0.05, and a M_B=-19.41+-0.42. However, comparison with other type Ia supernovae having similar Delta m_{15}(B) values indicates that in other respects SN 2002bo is unusual. While the optical spectra of SN 2002bo are very similar to those of SN 1984A, lower velocities and a generally more structured appearance are found in SNe 1990N, 1994D and 1998bu. For supernovae having Dm_(15)(B) > 1.2, we confirm the variation of R(SiII) (Nugent et al. 1995) with Dm_(15)(B). However, for supernovae such as SN2002bo, with lower values of Dm_(15)(B) the relation breaks down. Moreover, the evolution of R(SiII) for SN 2002bo is strikingly different from that shown by other type Ia supernovae. The velocities of SN 2002bo and 1984A derived from SII 5640A, SiII 6355A and CaII H&K lines are either much higher and/or evolve differently from those seen in other normal SNe Ia events. We suggest that the unusually low temperature, the presence of high-velocity intermediate-mass elements and the low abundance of carbon at early times indicates that burning to Si penetrated to much higher layers than in more normal type Ia supernovae. This may be indicative of a delayed-detonation explosion.

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arXiv2003

Low Luminosity Type II Supernovae: Spectroscopic and Photometric Evolution

In this paper we present spectroscopic and photometric observations for four core collapse supernovae (SNe), namely SNe 1994N, 1999br, 1999eu and 2001dc. Together with SN 1997D, we show that they form a group of exceptionally low-luminosity events. These SNe have narrow spectral lines (indicating low expansion velocities) and low luminosities at every phase (significantly lower than those of typical core-collapse supernovae). The very low luminosity during the ^{56}Co radioactive decay tail indicates that the mass of ^{56}Ni ejected during the explosion is much smaller (M(Ni) ~ 2-8 x 10^{-3} Mo) than the average (M(Ni) ~ 6-10 x 10^{-2} Mo). Two supernovae of this group (SN 1999br and SN 2001dc) were discovered very close to the explosion epoch, allowing us to determine the lengths of their plateaux (~ 100 days) as well as establishiing the explosion epochs of the other, less-completely observed SNe. It is likely that this group of SNe represent the extreme low-luminosity tail of a single continuous distribution of SN II-P events. Their kinetic energy is also exceptionally low. Although an origin from low mass progenitors has also been proposed for low-luminosity core-collapse SNe, recent work provides evidence in favour of the high mass progenitor scenario. The incidence of these low--luminosity SNe could be as high as 4-5$ $ of all type II SNe.

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arXiv2002

An Intermediate Redshift Supernova Search at ESO: Reduction Tools and Efficiency Tests

We present the reduction and archiving tools developed for our search for supernovae at intermediate redshifts at ESO as well as the efficiency tests performed. The data reduction recipes developed for the SN candidates selection are described. All the variable sources detected are stored using a MySQL database which enables the identification of previously detected variable sources during past observational runs. Finally, experiments performed with artificial stars have shown that seeing plays a crucial role for the limiting magnitude of detection. Crucial is also the detection threshold used by Sextractor.

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arXiv2002

Peculiar, Low Luminosity Type II Supernovae: Low Energy Explosions in Massive Progenitors?

A number of supernovae, classified as Type II, show remarkably peculiar properties such as an extremely low expansion velocity and an extraordinarily small amount of $^{56}$Ni in the ejecta. We present a joint analysis of the available observations for two of these peculiar Type II supernovae, SN 1997D and SN 1999br, using a comprehensive semi-analytic method that can reproduce the light curve and the evolution of the line velocity and continuum temperature. We find that these events are under-energetic with respect to a typical Type II supernova and that the inferred mass of the ejecta is relatively large. We discuss the possibility that these supernovae originate from the explosion of a massive progenitor in which the rate of early infall of stellar material on the collapsed core is large. Events of this type could form a black hole remnant, giving rise to significant fallback and late-time accretion.

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