arXiv subjects

N. Panagia

Publications and source records attributed to N. Panagia.

At least 19 recordsLinked to original sources

arXiv2024

On the universal validity of Case B recombination theory

In an ongoing search for low-mass extreme emission line galaxies, we identified a galaxy with a Ha/Hb Balmer line ratio of 2.620 +- 0.078. Ha/Hb Balmer ratios lower than the dust-free Case~B value appear relatively frequently in extreme emission line galaxies. These low values suggest that the Case~B assumption may not be valid in these objects. After ruling out the possibility that the low Ha/Hb ratio is due to systematic errors introduced by observational effects, we use constraints from the total Hb luminosity, the [OIII]/[OII] line ratio and the Balmer line equivalent widths, to suggest that the gas is optically thick to both Ha and Lya photons, and the geometry and orientation of the scattering gas causes Ha photons to be preferentially removed from the line of sight with respect to higher order Balmer series photons. Finally, we use data from the SDSS survey to show that Balmer self-absorption may be more important than previously assumed in high excitation emission line galaxies, where Lya pumping of the hydrogen excited state can be effective. If not recognized, Balmer self-absorption could lead to inaccurate estimates of galaxy physical properties. As an example, the effect of dust extinction could be over-estimated, for spherically symmetric scattering medium, or under-estimated, for a not spherically-symmetric distribution.

astro-ph.GA↗

arXiv2022

Pre-main Sequence stars in LH91

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$α$ excess emission above the 3$σ$ level with equivalent width $EW_{Hα}$ $\ge$ 10 Å. 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.

astro-ph.SR↗

arXiv2017

A Tale of Three Cities: OmegaCAM discovers multiple sequences in the color-magnitude diagram of the Orion Nebula Cluster

As part of the Accretion Discs in H$α$ 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.

astro-ph.SR↗

arXiv2016

Hubble Tarantula Treasury Project V. The star cluster Hodge 301: the old face of 30 Doradus

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 $δ$E(B$-$V)$\approx 0.04$ mag.

astro-ph.SR↗

arXiv2015

Hubble Tarantula Treasury Project. III. Photometric Catalog and Resulting Constraints on the Progression of Star Formation in the 30 Doradus Region

We present and describe the astro-photometric catalog of more than 800,000 sources found in the Hubble Tarantula Treasury Project (HTTP). HTTP is a Hubble Space Telescope (HST) Treasury program designed to image the entire 30 Doradus region down to the sub-solar (~0.5 solar masses) mass regime using the Wide Field Camera 3 (WFC3) and the Advanced Camera for Surveys (ACS). We observed 30 Doradus in the near ultraviolet (F275W, F336W), optical (F555W, F658N, F775W), and near infrared (F110W, F160W) wavelengths. The stellar photometry was measured using point-spread function (PSF) fitting across all the bands simultaneously. The relative astrometric accuracy of the catalog is 0.4 mas. The astro-photometric catalog, results from artificial star experiments and the mosaics for all the filters are available for download. Color-magnitude diagrams are presented showing the spatial distributions and ages of stars within 30 Dor as well as in the surrounding fields. HTTP provides the first rich and statistically significant sample of intermediate and low mass pre-main sequence candidates and allows us to trace how star formation has been developing through the region. The depth and high spatial resolution of our analysis highlight the dual role of stellar feedback in quenching and triggering star formation on the giant HII region scale. Our results are consistent with stellar sub-clustering in a partially filled gaseous nebula that is offset towards our side of the Large Magellanic Cloud.

astro-ph.GA↗

arXiv2015

A Semi-Analytical Line Transfer (SALT) model to interpret the spectra of galaxy outflows

We present a Semi-Analytical Line Transfer model, SALT, to study the absorption and re-emission line profiles from expanding galactic envelopes. The envelopes are described as a superposition of shells with density and velocity varying with the distance from the center. We adopt the Sobolev approximation to describe the interaction between the photons escaping from each shell and the remaining of the envelope. We include the effect of multiple scatterings within each shell, properly accounting for the atomic structure of the scattering ions. We also account for the effect of a finite circular aperture on actual observations. For equal geometries and density distributions, our models reproduce the main features of the profiles generated with more complicated transfer codes. Also, our SALT line profiles nicely reproduce the typical asymmetric resonant absorption line profiles observed in star-forming/starburst galaxies whereas these absorption profiles cannot be reproduced with thin shells moving at a fixed outflow velocity. We show that scattered resonant emission fills in the resonant absorption profiles, with a strength that is different for each transition. Observationally, the effect of resonant filling depends on both the outflow geometry and the size of the outflow relative to the spectroscopic aperture. Neglecting these effects will lead to incorrect values of gas covering fraction and column density. When a fluorescent channel is available, the resonant profiles alone cannot be used to infer the presence of scattered re-emission. Conversely, the presence of emission lines of fluorescent transitions reveals that emission filling cannot be neglected.

astro-ph.GA↗

arXiv2014

Mass accretion rates from multi-band photometry in the Carina Nebula: the case of Trumpler 14

We present a study of the mass accretion rates of pre-Main Sequence (PMS) stars in the cluster Trumpler 14 (Tr14) in the Carina Nebula. Using optical multi-band photometry we were able to identify 356 PMS stars showing H-alpha excess emission with equivalent width EW(H-alpha)>20Å. We interpret this observational feature as indication that these objects are still actively accreting gas from their circumstellar medium. From a comparison of the HR diagram with PMS evolutionary models we derive ages and masses of the PMS stars. We find that most of the PMS objects are younger than 10 Myr with a median age of ~3 Myr. Surprisingly, we also find that ~20% of the mass accreting objects are older than 10 Myr. For each PMS star in Trumpler 14 we determine the mass accretion rate ($\dot{M}_{acc}$) and discuss its dependence on mass and age. We finally combine the optical photometry with near-IR observations to build the spectral energy distribution (SED) for each PMS star in Tr14. The analysis of the SEDs suggests the presence of transitional discs in which a large amount of gas is still present and sustains accretion onto the PMS object at ages older than 10 Myr. Our results, discussed in light of recent recent discoveries with Herschel of transitional discs containing a massive gas component around the relatively old PSM stars TW Hydrae, 49 Ceti, and HD 95086, support a new scenario in which old and evolved debris discs still host a significant amount of gas.

astro-ph.SR↗

arXiv2013

Hα photometry of low mass stars in 47 Tucanae: chromospheric activity and exotica

We have used archival Hubble Space Telescope observations obtained with the Advanced Camera for Surveys to study the Hα emission properties of main sequence stars in the globular cluster 47 Tucanae. Using a combination of multi-band observations in the F606W, F814W and F658N bands, we search for stars showing Hα excess emission. An accurate photometric measurement of their Hα equivalent width allows us to identify objects with large Hα emission, which we attribute to mass accretion rather than enhanced chromospheric activity. The spatial position of some of these stars is coincident with that of known X-ray sources and their location in the colour-magnitude diagram allows us to classify them as active binaries or cataclysmic variables (CVs). We show that this method, commonly adopted to study accreting discs in young stellar objects, can be successfully used to identify and characterise candidate CVs.

astro-ph.SR↗

arXiv2013

Hubble Tarantula Treasury Project: Unraveling Tarantula's Web. I. Observational overview and first results

The Hubble Tarantula Treasury Project (HTTP) is an ongoing panchromatic imaging survey of stellar populations in the Tarantula Nebula in the Large Magellanic Cloud that reaches into the sub-solar mass regime (< 0.5 Mo). HTTP utilizes the capability of HST to operate the Advanced Camera for Surveys (ACS) and the Wide Field Camera 3 (WFC3) in parallel to study this remarkable region in the near-ultraviolet, optical, and near-infrared spectral regions, including narrow band H$α$ images. The combination of all these bands provides a unique multi-band view. The resulting maps of the stellar content of the Tarantula Nebula within its main body provide the basis for investigations of star formation in an environment resembling the extreme conditions found in starburst galaxies and in the early Universe. Access to detailed properties of individual stars allows us to begin to reconstruct the evolution of the stellar skeleton of the Tarantula Nebula over space and time with parcsec-scale resolution. In this first paper we describe the observing strategy, the photometric techniques, and the upcoming data products from this survey and present preliminary results obtained from the analysis of the initial set of near-infrared observations.

astro-ph.GA↗

arXiv2013

The Hubble Space Telescope Treasury Program on the Orion Nebula Cluster

The Hubble Space Telescope (HST) Treasury Program on the Orion Nebula Cluster has used 104 orbits of HST time to image the Great Orion Nebula region with the Advanced Camera for Surveys (ACS), the Wide-Field/Planetary Camera 2 (WFPC2) and the Near Infrared Camera and Multi Object Spectrograph (NICMOS) instruments in 11 filters ranging from the U-band to the H-band equivalent of HST. The program has been intended to perform the definitive study of the stellar component of the ONC at visible wavelengths, addressing key questions like the cluster IMF, age spread, mass accretion, binarity and cirumstellar disk evolution. The scanning pattern allowed to cover a contiguous field of approximately 600 square arcminutes with both ACS and WFPC2, with a typical exposure time of approximately 11 minutes per ACS filter, corresponding to a point source depth AB(F435W) = 25.8 and AB(F775W)=25.2 with 0.2 magnitudes of photometric error. We describe the observations, data reduction and data products, including images, source catalogs and tools for quick look preview. In particular, we provide ACS photometry for 3399 stars, most of them detected at multiple epochs, WFPC2 photometry for 1643 stars, 1021 of them detected in the U-band, and NICMOS JH photometry for 2116 stars. We summarize the early science results that have been presented in a number of papers. The final set of images and the photometric catalogs are publicly available through the archive as High Level Science Products at the STScI Multimission Archive hosted by the Space Telescope Science Institute.

astro-ph.SR↗

arXiv2013

Five Supernova Survey Galaxies in the Southern Hemisphere: Supernova Ia Rates

Based on the database of 56 supernovae (SNe) events discovered in 3838 galaxies of the southern hemisphere, we compute the rate of SNe of different types along the Hubble sequence normalized to the optical and near-infrared (NIR) luminosities as well as to the stellar mass of the galaxies. We find that the rates of Type Ia SNe show a dependence on both morphology and colors of the galaxies, and therefore, on the star-formation activity. The rate of SNe Ia can be explained by assuming that at least 15% of Ia events in spiral galaxies originate in relatively young stellar populations. We also find that the rates show no modulation with nuclear activity or environment.

astro-ph.CO↗

arXiv2012

A double cluster at the core of 30 Doradus

Based on an analysis of data obtained with the Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) we report the identification of two distinct stellar populations in the core of the giant HII region 30Doradus in the Large Magellanic Cloud. The most compact and richest component coincides with the center of R136 and is ~1 Myr younger than a second more diffuse clump, located ~5.4 pc toward the northeast. We note that published spectral types of massive stars in these two clumps lend support to the proposed age difference. The morphology and age difference between the two sub-clusters suggests that an ongoing merger may occurring within the core of 30Doradus. This finding is consistent with the predictions of models of hierarchical fragmentation of turbulent giant molecular clouds, according to which star clusters would be the final products of merging smaller sub-structures.

astro-ph.GA↗

arXiv2012

Five supernova survey galaxies in the southern hemisphere. II. The supernova rates

Based on the database compiled in the first article of this series, with 56 SN events discovered in 3838 galaxies of the southern hemisphere, we compute the rate of supernovae (SNe) of different types along the Hubble sequence normalized to the optical and near-infrared luminosities as well as to the stellar mass of the galaxies. We find that the rates of all SN types show a dependence on both morphology and colors of the galaxies, and therefore, on the star-formation activity. The rate of core-collapse (CC) SNe is confirmed to be closely related to the Star Formation Rate (SFR) and only indirectly to the total mass of the galaxies. The rate of SNe Ia can be explained by assuming that at least 15% of Ia events in spiral galaxies originates in relatively young stellar populations. We find that the rates show no modulation with nuclear activity or environment. The ratio of SN rates between types Ib/c and II shows no trend with spiral type.

astro-ph.CO↗

arXiv2011

A minor body falling onto a neutron star as an explanation for the unusual gamma-ray burst GRB 101225A

The tidal disruption of a solar mass star around a supermassive black hole has been extensively studied analytically and numerically. In these events the star develops into an elongated banana-shaped structure. After completing an eccentric orbit, the bound debris fall onto the black hole, forming an accretion disk and emitting radiation. The same process may occur on planetary scales, if a minor body orbits too close to its star. In the Solar System, comets fall directly onto our Sun or onto planets. If the star is a compact object, the minor body can become tidally disrupted. Indeed, one of the first mechanisms invoked to produce strong gamma-ray emission involved accretion of comets onto neutron stars in our Galaxy. Here we report that the peculiarities of the `Christmas' gamma-ray burst (GRB 101225A) can be explained by a tidal disruption event of a minor body around a Galactic isolated neutron star. This result would indicate either that minor bodies can be captured by compact stellar remnants more frequently than it occurs in our Solar System or that minor body formation is relatively easy around millisecond radio pulsars. A peculiar Supernova associated to a GRB may provide an alternative explanation.

astro-ph.HE↗

arXiv2011

Photometric determination of the mass accretion rates of pre-main sequence stars. III. Results in the Large Magellanic Cloud

We present a multi-wavelength study of three star forming regions, spanning the age range 1-14 Myr, located between the 30 Doradus complex and supernova SN1987A in the Large Magellanic Cloud (LMC). We reliably identify about 1000 pre-main sequence (PMS) star candidates actively undergoing mass accretion and estimate their stellar properties and mass accretion rate (Macc). Our measurements represent the largest Macc dataset of low-metallicity stars presented so far. As such, they offer a unique opportunity to study on a statistical basis the mass accretion process in the LMC and, more in general, the evolution of the mass accretion process around low-metallicity stars. We find that the typical \dot{M} of PMS stars in the LMC is higher than for galactic PMS stars of the same mass, independently of their age. Taking into account the caveats of isochronal age and \dot{M} estimates, the difference in Macc between the LMC and our Galaxy appears to be about an order of magnitude. We review the main mechanisms of disk dispersal and find indications that typically higher Macc are to be expected in low-metallicity environments. However, many issues of this scenario need to be clarified by future observations and modeling. We also find that, in the mass range 1-2 M_sun, the Macc of PMS stars in the LMC increases with stellar mass as \dot{M}acc\proptoM^b, with b \approx1, i.e. slower than the second power low found for galactic PMS stars in the same mass regime.

astro-ph.SR↗

arXiv2011

The Hubble Space Telescope Cluster Supernova Survey: II. The Type Ia Supernova Rate in High-Redshift Galaxy Clusters

We report a measurement of the Type Ia supernova (SN Ia) rate in galaxy clusters at 0.9 < z < 1.45 from the Hubble Space Telescope (HST) Cluster Supernova Survey. This is the first cluster SN Ia rate measurement with detected z > 0.9 SNe. Finding 8 +/- 1 cluster SNe Ia, we determine a SN Ia rate of 0.50 +0.23-0.19 (stat) +0.10-0.09 (sys) SNuB (SNuB = 10^-12 SNe L_{sun,B}^-1 yr^-1). In units of stellar mass, this translates to 0.36 +0.16-0.13 (stat) +0.07-0.06 (sys) SNuM (SNuM = 10^-12 SNe M_sun^-1 yr^-1). This represents a factor of approximately 5 +/- 2 increase over measurements of the cluster rate at z < 0.2. We parameterize the late-time SN Ia delay time distribution with a power law (proportional to t^s). Under the assumption of a cluster formation redshift of z_f = 3, our rate measurement in combination with lower-redshift cluster SN Ia rates constrains s = -1.41 +0.47/-0.40, consistent with measurements of the delay time distribution in the field. This measurement is generally consistent with expectations for the "double degenerate" scenario and inconsistent with some models for the "single degenerate" scenario predicting a steeper delay time distribution at large delay times. We check for environmental dependence and the influence of younger stellar populations by calculating the rate specifically in cluster red-sequence galaxies and in morphologically early-type galaxies, finding results similar to the full cluster rate. Finally, the upper limit of one host-less cluster SN Ia detected in the survey implies that the fraction of stars in the intra-cluster medium is less than 0.47 (95% confidence), consistent with measurements at lower redshifts.

astro-ph.CO↗

arXiv2011

The Hubble Space Telescope Cluster Supernova Survey: VI. The Volumetric Type Ia Supernova Rate

We present a measurement of the volumetric Type Ia supernova (SN Ia) rate out to z ~ 1.6 from the Hubble Space Telescope Cluster Supernova Survey. In observations spanning 189 orbits with the Advanced Camera for Surveys we discovered 29 SNe, of which approximately 20 are SNe Ia. Twelve of these SNe Ia are located in the foregrounds and backgrounds of the clusters targeted in the survey. Using these new data, we derive the volumetric SN Ia rate in four broad redshift bins, finding results consistent with previous measurements at z > 1 and strengthening the case for a SN Ia rate that is equal to or greater than ~0.6 x 10^-4/yr/Mpc^3 at z ~ 1 and flattening out at higher redshift. We provide SN candidates and efficiency calculations in a form that makes it easy to rebin and combine these results with other measurements for increased statistics. Finally, we compare the assumptions about host-galaxy dust extinction used in different high-redshift rate measurements, finding that different assumptions may induce significant systematic differences between measurements.

astro-ph.CO↗

arXiv2011

Probing the ambient medium of GRB 090618 with XMM-Newton observations

Long Gamma-ray Bursts (GRBs) signal the death of massive stars. The afterglow emission can be used to probe the progenitor ambient through a detailed study of the absorption pattern imprinted by the circumburst material as well as the host galaxy interstellar medium on the continuum spectrum. This has been done at optical wavelengths with impressive results. Similar studies can in principle be carried out in the X-ray band, allowing us to shed light on the material metallicity, composition and distance of the absorber. We start exploiting this route through high resolution spectroscopy XMM-Newton observations of GRB 090618. We find a high metallicity absorbing medium (Z> 0.2 Zsun) with possible enhancements of S and Ne with respect to the other elements (improving the fit at a level of >3.4 sigma). Including the metallicity effects on the X-ray column density determination, the X-ray and optical evaluations of the absorption are in agreement for a Small Magellanic Cloud extinction curve.

astro-ph.HE↗