N. Panagia’s research while affiliated with Space Telescope Science Institute and other places

Aims. We study the accretion properties of pre-main sequence (PMS) low-mass stars in the LH 91 association within the Large Magellanic Clouds. Methods. Using optical multiband photometry obtained with the Hubble Space Telescope, we identify 75 candidates showing H α excess emission above the 3 σ level with equivalent width EWH α ⩾ 10 Å. We estimate the physical parameters (effective temperature, luminosity, age, mass, accretion luminosity, and mass accretion rate) of the PMS stellar candidates. Results. The age distribution suggests a period of active star formation ranging from a few million years up to ∼60 Myr with a gap between ∼5 Myr and 10 Myr. The masses of the PMS candidates span from 0.2 M ⊙ for the cooler objects to 1.0 M ⊙ with a median of ∼0.80 M ⊙ . The median value of the accretion luminosity of our 75 PMS stars is about 0.12 L ⊙ , the median value of the mass accretion rate is about 4.8 × 10 ⁻⁹ M ⊙ yr ⁻¹ with higher values for the younger population (∼1.2 × 10 ⁻⁸ M ⊙ yr ⁻¹ ), and lower values for the older candidates (∼4.7 × 10 ⁻⁹ M ⊙ yr ⁻¹ ). We compare our results with findings for LH 95, the closest region to LH 91 for which accretion properties of PMS candidates were previously derived. An interesting qualitative outcome is that LH 91 seems to be in a more evolved stage. Moreover, we find that the PMS candidates are distributed homogeneously, without any evidence of clumps around more massive stars.

We study the accretion properties of pre-main sequence low-mass stars in the LH 91 association within the Large Magellanic Clouds. Using optical multiband photometry obtained with the Hubble Space Telescope, we identify 75 candidates showing H$\alpha$ excess emission above the 3$\sigma$ level with equivalent width $EW_{H\alpha}$ $\ge$ 10 \AA. We estimate the physical parameters (effective temperature, luminosity, age, mass, accretion luminosity and mass accretion rate) of the pre-main sequence stellar candidates. The age distribution suggests a star formation ranging from few Myr up to $\sim$ 60 Myr with a gap between $\sim$ 5 Myr and 10 Myr. The masses of the PMS candidates span from 0.2 $M_{\odot}$ for the cooler objects to 1.0 $M_{\odot}$ with a median of $\sim$ 0.80 $M_{\odot}$. The median value of the accretion luminosity of our 75 PMS stars is about 0.12 $L_{\odot}$, the median value of the mass accretion rate is about 4.8 $\times$ $10^{-9}$ $M_{\odot} yr^{-1}$ with higher values for the younger population ($\sim$ 1.2 $\times$ $10^{-8}$ $M_{\odot} yr^{-1}$), and lower values for the older candidates ($\sim$ 4.7 $\times$ $10^{-9}$ $M_{\odot} yr^{-1}$). We compare our results with the findings on LH 95, the closer region to LH 91 for which accretion properties of PMS candidates were derived. An interesting qualitative outcome is that LH 91 seems to be in a more evolved stage. Moreover, we find that the PMS candidates are distributed homogeneously, without any evidence of clumps around more massive stars.

Observations with the Wide Field Camera on HST were examined, and 27 Cepheids were discovered. Photometry and period analysis have produced unambiguous light curves free of alias. The observed P-L relation has a slope consistent with seminal calibration studies of Galactic and LMC Cepheids. An apparent distance modulus (in V) of 28.47 ± 0.10 to IC 4182 is derived. This implies that Mv(max) for SNIa 1937C is -19.92 ± 0.2 mag, independent of everything except differential absorption between the Cepheids and the supernova. Using this to calibrate Hubble diagrams for SNIa by several authors, and allowing for a 1- σ uncertainty in the absolute magnitude from the calibration of only one SNIa, we obtain H 0 = 45 ± 14kms ⁻¹ Mpc ⁻¹ .

Three supernovae have so far been detected in the radio range shortly after their optical outbursts. All are Type IIs. A fourth supernova, a Type I, is being monitored for radio emission but, at an age of approximately one year, has not yet been detected. For two of the supernovae, extensive data are presented on their “light curves” and spectra and models which have been suggested in the literature are discussed.

As part of the Accretion Discs in H$\alpha$ with OmegaCAM (ADHOC) survey, we imaged in r, i and H-alpha a region of 12x8 square degrees around the Orion Nebula Cluster. Thanks to the high-quality photometry obtained, we discovered three well-separated pre-main sequences in the color-magnitude diagram. The populations are all concentrated towards the cluster's center. Although several explanations can be invoked to explain these sequences we are left with two competitive, but intriguing, scenarios: a population of unresolved binaries with an exotic mass ratio distribution or three populations with different ages. Independent high-resolution spectroscopy supports the presence of discrete episodes of star formation, each separated by about a million years. The stars from the two putative youngest populations rotate faster than the older ones, in agreement with the evolution of stellar rotation observed in pre-main sequence stars younger than 4 Myr in several star forming regions. Whatever the final explanation, our results prompt for a revised look at the formation mode and early evolution of stars in clusters.

Based on color-magnitude diagrams (CMDs) from the Hubble Space Telescope Hubble Tarantula Treasury Project (HTTP) survey, we present the star formation history (SFH) of Hodge~301, the oldest star cluster in the Tarantula Nebula. The HTTP photometry extends faint enough to reach, for the first time, the cluster pre-main sequence (PMS) turn-on, where the PMS joins the main sequence. Using the location of this feature, along with synthetic CMDs generated with the latest PARSEC models, we find that Hodge~301 is older than previously thought, with an age between 26.5 and 31.5 Myr. From this age, we also estimate that between 38 and 61 supernovae Type-II exploded in the region. The same age is derived from the main sequence turn-off, whereas the age derived from the post-main sequence stars is younger and between 20 and 25 Myr. Other relevant parameters are a total stellar mass of $\approx 8800\,\pm 800$M$_{\odot}$ and average reddening E(B$-$V) $\approx 0.22-0.24$ mag, with a differential reddening $\delta$E(B$-$V)$\approx 0.04$ mag.

Based on color-magnitude diagrams (CMDs) from the Hubble Space Telescope Hubble Tarantula Treasury Project (HTTP) survey, we present the star formation history (SFH) of Hodge~301, the oldest star cluster in the Tarantula Nebula. The HTTP photometry extends faint enough to reach, for the first time, the cluster pre-main sequence (PMS) turn-on, where the PMS joins the main sequence. Using the location of this feature, along with synthetic CMDs generated with the latest PARSEC models, we find that Hodge~301 is older than previously thought, with an age between 26.5 and 31.5 Myr. From this age, we also estimate that between 38 and 61 supernovae Type-II exploded in the region. The same age is derived from the main sequence turn-off, whereas the age derived from the post-main sequence stars is younger and between 20 and 25 Myr. Other relevant parameters are a total stellar mass of $\approx 8800\,\pm 800$M$_{\odot}$ and average reddening E(B$-$V) $\approx 0.22-0.24$ mag, with a differential reddening $\delta$E(B$-$V)$\approx 0.04$ mag.

An analysis of type Ia supernova (SNIa) events in early type galaxies from the Cappellaro et al. [6] database provides strong evidence that the rate of type Ia supernovae (SNe) in radio-loud galaxies is about 4 times higher than the rate measured in radio-quiet galaxies, i.e. SNIa-rate(radio-loud) SNe per century and per 10 ¹⁰ (SNU) as compared to SNIa-rate(radio-quiet)= 0.11 ± 0.03 SNU. The exact value of the enhancement is still rather uncertain, but is likely to be in the range ~ 2 – 7. We discuss the possible causes of this result and we conclude that the enhancement of the SNIa explosion rate in radio-loud galaxies has the same common origin as their being strong radio sources, but that there is no causality link between the two phenomena. We argue that repeated episodes of interaction and/or mergers of early type galaxies with dwarf companions are responsible for inducing both strong radio activity in ~14% of early type galaxies, and the ~ 1 Gyr old stellar population needed to supply an adequate number SNIa progenitors.

The high resolution obtained through the use of VLBI gives an unique opportunity to directly observe the interaction of an expanding radio supernova with its surrounding medium. We present here results from our VLBI observations of the young supernovae SN 1979C, SN 1986J, and SN 2001gd.