arXiv subjects

Nino Panagia

Publications and source records attributed to Nino Panagia.

105 records · Page 6Linked to original sources

arXiv2000

Preliminary Spectral Analysis of the Type II Supernova 1999em

We have calculated fast direct spectral model fits to two early-time spectra of the Type-II plateau SN 1999em, using the SYNOW synthetic spectrum code. The first is an extremely early blue optical spectrum and the second a combined HST and optical spectrum obtained one week later. Spectroscopically this supernova appears to be a normal Type II and these fits are in excellent agreement with the observed spectra. Our direct analysis suggests the presence of enhanced nitrogen. We have further studied these spectra with the full NLTE general model atmosphere code PHOENIX. While we do not find confirmation for enhanced nitrogen (nor do we rule it out), we do require enhanced helium. An even more intriguing possible line identification is complicated Balmer and He I lines, which we show falls naturally out of the detailed calculations with a shallow density gradient. We also show that very early spectra such as those presented here combined with sophisticated spectral modeling allows an independent estimate of the total reddening to the supernova, since when the spectrum is very blue, dereddening leads to changes in the blue flux that cannot be reproduced by altering the ``temperature'' of the emitted radiation. These results are extremely encouraging since they imply that detailed modeling of early spectra can shed light on both the abundances and total extinction of SNe II, the latter improving their utility and reliability as distance indicators.

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arXiv2000

Analysis of the Type IIn Supernova 1998S: Effects of Circumstellar Interaction on Observed Spectra

We present spectral analysis of early observations of the Type IIn supernova 1998S using the general non-local thermodynamic equilibrium atmosphere code \tt PHOENIX}. We model both the underlying supernova spectrum and the overlying circumstellar interaction region and produce spectra in good agreement with observations. The early spectra are well fit by lines produced primarily in the circumstellar region itself, and later spectra are due primarily to the supernova ejecta. Intermediate spectra are affected by both regions. A mass-loss rate of order $\dot M \sim 0.0001-0.001$\msol yr$^{-1}$ is inferred for a wind speed of 100-1000 \kmps. We discuss how future self-consistent models will better clarify the underlying progenitor structure.

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arXiv2000

HST Spectroscopy of Spot 1 on the Circumstellar Ring of SN 1987A

We present ultraviolet and optical spectra of the first bright spot (PA = 29 degrees) on Supernova 1987A's equatorial circumstellar ring taken with the Space Telescope Imaging Spectrograph. We interpret this spot as the emission produced by radiative shocks that occur where the supernova blast wave strikes an inward protrusion of the ring. The observed line widths and intensity ratios indicate the presence of radiative shocks with velocities ranging from 100 to 250 km s^-1 entering dense (> 10^4 cm^-3) gas. These observations, and future observations of the development of the spectra and line profiles, provide a unique opportunity to study the hydrodynamics of radiative shocks.

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arXiv2000

Pre-Supernova Evolution of Massive Stars

We present the preliminary results of a detailed theoretical investigation on the hydrodynamical properties of Red Supergiant (RSG) stars at solar chemical composition and for stellar masses ranging from 10 to 20 M_sun. We find that the main parameter governing their hydrodynamical behavior is the effective temperature, and indeed when moving from higher to lower effective temperatures the models show an increase in the dynamical perturbations. Also, we find that RSGs are pulsationally unstable for a substantial portion of their lifetimes. These dynamical instabilities play a key role in driving mass loss, thus inducing high mass loss rates (up to several 1e-4 M_sun/yr) and considerable variations of the mass loss activity over timescales of the order of 10,000 years. Our results are able to account for the variable mmass loss rates as implied by radio observations of type II supernovae, and we anticipate that comparisons of model predictions with observed circumstellar phenomena around SNII will provide valuable diagnostics about their progenitors and their evolutionary histories.

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arXiv2000

Supernovae

The properties of supernovae (SNe) are reviewed. It is shown that the observed characteristics of the morphological classes of SNe (types Ia, Ib/c, II) can be explained in terms of two basic explosion mechanisms, i.e. core collapse of massive stars and thermonuclear explosion of low/moderate mass stars. The study of SNe can provide valuable insight in the late phases of the evolution of their progenitors and, therefore, can constrain the theory of stellar evolution quite tightly. Also, bright SNe of type Ia can be used to probe the Universe up to high redshifts, allowing us to measure cosmological constants and to gain valuable information on the formation and evolution of galaxies. The case of SN 1987A is discussed in some detail: it appears that in this event a number of predictions were astonishingly well verified but still many aspects were completely at variance with "common wisdom" expectations, indicating that the SN phenomenon is still not fully understood.

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arXiv2000

Observational Constraints to the Evolution of Massive Stars

We consider some aspects of the evolution of massive stars which can only be elucidated by means of "indirect" observations, i.e. measurements of the effects of massive stars on their environments. We discuss in detail the early evolution of massive stars formed in high metallicity regions as inferred from studies of HII regions in external galaxies.

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arXiv2000

Radio Supernovae and GRB 980425

Study of radio supernovae (RSNe) over the past 20 years includes two dozen detected objects and more than 100 upper limits. From this work we are able to identify classes of radio properties, demonstrate conformance to and deviations from existing models, estimate the density and structure of the circumstellar material and, by inference, the evolution of the presupernova stellar wind, and reveal the last stages of stellar evolution before explosion. It is also possible to detect ionized hydrogen along the line of sight, to demonstrate binary properties of the stellar system, and to show clumpiness of the circumstellar material. More speculatively, it may be possible to provide distance estimates to radio supernovae. The interesting and unusual radio supernova SN 1998bw, which is thought to be related to the gamma-ray burst GRB 980425, is discussed in particular detail. Its radio properties are compared and contrasted with those of other known RSNe.

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arXiv2000

Young Stellar Populations Around SN1987A

We present the results of a study of the stellar population in a region of 30 pc radius around SN1987A, based on an analysis of multi-band HST-WFPC2 images. The effective temperature, radius and, possibly, reddening of each star were determined for 21,955 stars by fitting the measured broad band magnitudes to the ones calculated with model atmospheres. In addition, we have identified all stars with Halpha equivalent widths in excess of 8 A, amounting to a total of 492 stars. An inspection to the HR diagram reveals the presence of several generations of young stars, with ages between 1 and 150 Myrs, superposed on a much older field population (0.6-6 Gyrs). A substantial fraction of young stars have ages around 12 Myrs which is the stellar generation coeval to SN 1987A progenitor. The youngest stars in the field appear to be strong-line T Tauri stars. This constitute the first positive detection of low mass (about 1-2 Mo) Pre Main Sequence (PMS) stars outside the Milky Way. Their positions in the HR diagram appear to require that star formation in the LMC occurs with accretion rates about 10 times higher than in the Milky Way, i.e. ~10^{-4} Mo/yr. SN1987A appears to belong to a loose, young cluster 12+/-2 Myrs old, in which the slope of the present mass function is almost identical to Salpeter's for masses above 3 Mo, but becomes much flatter for lower masses. On a large scale, we find that the spatial distribution of massive stars and low-mass PMS stars are conclusively different. This results casts doubts on the validity of an IMF concept on a small scale (say, less than 10 pc). A preliminary analysis, done for the whole field as a single entity, shows that the IMF slope for the young population present over the entire region is steeper than Gamma ~ -1.7.

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arXiv1999

HST observations of the LMC field around SN87A: distance determinations with Red Clump and Tip of the Red Giant Branch stars

We have used HST-WFPC2 multiband observations of a field around SN 1987A in the Large Magellanic Cloud to measure its distance from the Sun. The observations allowed us to carefully determine the interstellar extinction along the line of sight to a large number of stars and to measure the LMC distance by using two stellar distance indicators: the Red Clump and the Tip of the Red Giant Branch. From an application of the Red Clump method we obtain a distance modulus (m-M)o,rc(LMC)=18.59+-0.04+-0.08 mag (statistical plus systematic error), in good agreement with the distance derived by using the Tip of the Red Giant Branch stars, namely (m-M)o,trgb(LMC)=18.69+-0.25+-0.06 mag (statistical plus systematic error). Both values agree well with the distance to the SN 1987A as determined from a study of its inner ring fluorescent echo ((m-M)(SN87A)=18.55+-0.05 mag, Panagia 1998), thus excluding distance moduli lower than 18.43 to a 99.7% significance level. Differences with respect to previous results obtained using the same distance indicators are discussed.

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arXiv1999

HST study of the Stellar Populations within 30 pc of SN 1987A

We have studied the stellar populations in a region of 30 pc around SN 1987A in the Large Magellanic Cloud using multi-band HST-WFPC2 images. The effective temperature, luminosity and reddening of each detected star were determined by fitting the measured broad band magnitudes to the ones calculated with model atmospheres. The resulting HR diagram reveals the presence of stars with ages between 1 and 150 Myrs, superposed on a much older field population. The youngest stars in the field appear to be T Tauri stars, characterized by strong Halpha excesses. The Star Formation Rate has increased monotonically in the last 8 Gyrs and the star formation activity is still very high at present. Young stars of low and high mass have different spatial distributions. The latter ones are strongly concentrated in the vicinities of the Supernova, whereas the previous ones are more evenly distributed on the field. Hence, the Mass Function varies on a scale of a few parsecs. However, averaging over the entire area, one can define an Initial Mass Function, well fitted by a power-law with a slope alpha<-2.55. The uncertainty on the IMF due to incompleteness in identifying T Tauri stars is discussed.

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arXiv1998

Detection of Pre-Shock Dense Circumstellar Material of SN 1978K

The supernova SN 1978K has been noted for its lack of emission lines broader than a few thousand km/s since its discovery in 1990. Modeling of the radio spectrum of the peculiar SN 1978K indicates the existence of HII absorption along the line of sight. To determine the nature of this absorbing region, we have obtained a high-dispersion spectrum of SN 1978K at the wavelength range 6530--6610 Å. The spectrum shows not only the moderately broad H-alpha emission of the supernova ejecta but also narrow nebular H-alpha and [N II] emission. The high [N II]6583/H-alpha ratio, 0.8-1.3, suggests that this radio absorbing region is a stellar ejecta nebula. The expansion velocity and emission measure of the nebula are consistent with those seen in ejecta nebulae of luminous blue variables. Previous low-dispersion spectra have detected a strong [N II]5755 line, indicating an electron density of 3-12x10^5 cm^{-3}. We argue that this stellar ejecta nebula is probably part of the pre-shock dense circumstellar envelope of SN 1978K. We further suggest that SN 1997ab may represent a young version of SN 1978K.

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arXiv1998

New HST Observations of High Velocity Lyman Alpha and Balmer Alpha in SNR 1987A

We describe and model high velocity (~15,000 km/s) Lyman alpha and Balmer alpha emission from supernova remnant 1987A seen in September and October 1997 with the Space Telescope Imaging Spectrograph. Part of this emission comes from a reverse shock located at ~75% of the radius of the inner boundary of the inner circumstellar ring and confined within 30 degrees of the equatorial plane. Departure from axisymmetry in the Lyman alpha and Balmer alpha emission correlates with that seen in nonthermal radio emission and reveals an asymmetry in the circumstellar gas distribution. We also see diffuse high velocity Lyman alpha emission from supernova debris inside the reverse shock that may be due to excitation by nonthermal particles accelerated by the shock.

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arXiv1998

The Opacity of Nearby Galaxies from Colors and Counts of Background Galaxies: I. The Synthetic Field Method and its Application to NGC 4536 and NGC 3664

We describe a new, direct method for determining the opacity of foreground galaxies which does not require any a priori assumptions about the spatial distribution or the reddening law of the obscuring material. The method is to measure the colors and counts of background galaxies which can be identified through the foreground system. The method is calibrated, and the effects of confusion and obscuration are decoupled by adding various versions of a suitable deep reference frame containing only field galaxies with known properties into the image of the foreground galaxy, and analyzing these ``synthetic field'' images in the same way as the real images. We test the method on HST WFPC2 archived images of two galaxies which are quite different: NGC 4536 is a large Sc spiral, and NGC 3664 is a small Magellanic irregular. The reference frames are taken from the Hubble Deep Field. From the background galaxy counts, NGC 4536 shows an extinction A_I ~ 1 mag in the northwestern arm region, and lower than 0.5 mag in the corresponding interarm region (no correction for inclination has been attempted). However, from the galaxy colors, the same reddening of E(V - I) ~ 0.2 is observed in both the arm and the interarm regions. In the interarm region, the combination of extinction and reddening can be explained by a diffuse component with a Galactic reddening law (R_V ~ 3). In the spiral arm, however, the same diffuse, low opacity component seems to coexist with regions of much higher opacity. Since the exposures are shorter the results for NGC 3664 are less clear, but also appear to be consistent with a two component distribution.

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arXiv1998

Radio Observations of SN 1980K: Evidence for Rapid Presupernova Evolution

New observations of SN 1980K made with the VLA at 20 and 6 cm from 1994 April through 1996 October show that the supernova (SN) has undergone a significant change in its radio emission evolution, dropping by a factor of ~2 below the flux density S \propto t^{-0.73} power-law decline with time t observed earlier. However, although S at all observed frequencies has decreased significantly, its current spectral index of α= -0.42\pm0.15 (S \propto ν^{+α}) is consistent with the previous spectral index of α=-0.60_{-0.07}^{+0.04}. It is suggested that this decrease in emission may be due to the SN shock entering a new region of the circumstellar material which has a lower density than that expected for a constant speed (w), constant mass-loss rate (Mdot) wind from the progenitor. If such an interpretation is correct, the difference in wind and shock speeds appears to indicate a significant evolution in the mass-loss history of the SN progenitor ~10^4 years before explosion, with a change in circumstellar density (\propto Mdot/w) occurring over a time span of \lesssim 4 kyr. Such features could be explained in terms of a fast ``blue-loop'' evolutionary phase of a relatively massive pre-SN progenitor star. If so, we may, for the first time, provide a stringent constraint on the mass of the SN progenitor based solely on the SN's radio emission.

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arXiv1996

Hubble Space Telescope Spectroscopic Observations of the Ejecta of SN 1987A at 2000 Days

We have used the Faint Object Spectrograph on the Hubble Space Telescope to observe the spectra of SN 1987A over the wavelength range 2000 -- 8000\ Å on dates 1862 and 2210 days after the supernova outburst. Even these pre-COSTAR observations avoid much of the contamination from the bright stars nearby and provide a very useful set of line strengths and shapes for analysis. The spectrum is formed in an unusual physical setting: cold gas which is excited and ionized by energetic electrons from the radioactive debris of the supernova explosion. The spectra of SN 1987A at this phase are surprisingly similar to those of the nova shells of CP Puppis and T Pyxidis decades after outburst. SN 1987A and the novae are characterized by emission from material with electron temperatures of only a few hundred degrees Kelvin, and show narrow Balmer continuum emission and strong emission lines from O$^+$. The Balmer continuum shape requires the electron temperature in the supernova ejecta to be as low as 500 K on day 1862 and 400 K on day 2210 after outburst. The \OIIUV\ doublet is surprisingly strong and is plausibly powered by collisional ionization of neutral oxygen to excited states of O$^+$. The line intensity ratio of the \OID\ doublet obtained from Gaussian fits of the line profiles is 1.8$\pm0.2$, contrary to the optically thin limit of 3. This ratio is {\it not} due to an optical depth effect, but rather is an artifact of assuming a Gaussian profile to fit the \OID\ doublet profile. Specifying the line ratio $R\, = \, F([{\rm OI}]6300)/F([{\rm OI}]6364)$ = 3 is consistent with the data and allows a calculation of the decomposed line profile. All the observed strong lines are found to be blueshifted by a similar amount

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