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Dehaes S.,Catholic University of Leuven | Bauwens E.,Catholic University of Leuven | Decin L.,Catholic University of Leuven | Decin L.,University of Amsterdam | And 6 more authors.
Astronomy and Astrophysics | Year: 2011

Context. Among late-type red giants, an interesting change occurs in the structure of the outer atmospheric layers as one moves to later spectral types in the Hertzsprung-Russell diagram: a chromosphere is always present, but the coronal emission diminishes and a cool massive wind steps in. Aims. Where most studies have focussed on short-wavelength observations, this article explores the influence of the chromosphere and the wind on long-wavelength photometric measurements. The goal of this study is to assess wether a set of standard near-infrared calibration sources are fiducial calibrators in the far-infrared, beyond 50  μm. Methods. The observational spectral energy distributions were compared with the theoretical model predictions for a sample of nine K-and M-giants. The discrepancies found are explained using basic models for flux emission originating in a chromosphere or an ionised wind. Results. For seven out of nine sample stars, a clear flux excess is detected at (sub)millimetre and/or centimetre wavelengths, while only observational upper limits are obtained for the other two. The precise start of the excess depends upon the star under consideration. For six sources the flux excess starts beyond 210 μm and they can be considered as fiducial calibrators for Herschel/PACS (60-210 μm). Out of this sample, four sources show no flux excess in the Herschel/SPIRE wavelength range (200-670 μm) and are good calibration sources for this instrument as well. The flux at wavelengths shorter than ~1mm is most likely dominated by an optically thick chromosphere, where an optically thick ionised wind is the main flux contributor at longer wavelengths. Conclusions. Although the optical to mid-infrared spectrum of the studied K-and M-type infrared standard stars is represented well by a radiative equilibrium atmospheric model, a chromosphere and/or ionised stellar wind at higher altitudes dominates the spectrum in the (sub)millimetre and centimetre wavelength ranges. The presence of a flux excess has implications on the role of the stars as fiducial spectrophotometric calibrators in these wavelength ranges. © 2011 ESO. Source

Fox O.D.,US Space Telescope Science Institute | Johansson J.,Weizmann Institute of Science | Kasliwal M.,California Institute of Technology | Andrews J.,Steward Observatory | And 13 more authors.
Astrophysical Journal Letters | Year: 2016

Supernovae Type Iax (SNe Iax) are less energetic and less luminous than typical thermonuclear explosions. A suggested explanation for the observed characteristics of this subclass is a binary progenitor system consisting of a CO white dwarf primary accreting from a helium star companion. A single-degenerate explosion channel might be expected to result in a dense circumstellar medium (CSM), although no evidence for such a CSM has yet been observed for this subclass. Here we present recent Spitzer observations of the SN Iax 2014dt obtained by the SPIRITS program nearly one year post-explosion that reveal a strong mid-IR excess over the expected fluxes of more normal SNe Ia. This excess is consistent with 10-5 of newly formed dust, which would be the first time that newly formed dust has been observed to form in a Type Ia. The excess, however, is also consistent with a dusty CSM that was likely formed in pre-explosion mass-loss, thereby suggesting a single degenerate progenitor system. Compared to other SNe Ia that show significant shock interaction (SNe Ia-CSM) and interacting core-collapse events (SNe IIn), this dust shell in SN 2014dt is less massive. We consider the implications that such a pre-existing dust shell has for the progenitor system, including a binary system with a mass donor that is a red giant, a red supergiant, or an asymptotic giant branch star. © 2016. The American Astronomical Society. All rights reserved.. Source

Folatelli G.,National University of La Plata | Folatelli G.,University of Tokyo | Van Dyk S.D.,IPAC Caltech | Kuncarayakti H.,Millennium Institute of Astrophysics MAS | And 19 more authors.
Astrophysical Journal Letters | Year: 2016

Supernova (SN) iPTF13bvn in NGC 5806 was the first Type Ib SN to have been tentatively associated with a progenitor in pre-explosion images. We performed deep ultraviolet (UV) and optical Hubble Space Telescope observations of the SN site ∼740 days after explosion. We detect an object in the optical bands that is fainter than the pre-explosion object. This dimming is likely not produced by dust absorption in the ejecta; thus, our finding confirms the connection of the progenitor candidate with the SN. The object in our data is likely dominated by the fading SN, implying that the pre-SN flux is mostly due to the progenitor. We compare our revised pre-SN photometry with previously proposed models. Although binary progenitors are favored, models need to be refined. In particular, to comply with our deep UV detection limit, any companion star must be less luminous than a late-O star or substantially obscured by newly formed dust. A definitive progenitor characterization will require further observations to disentangle the contribution of a much fainter SN and its environment. © 2016. The American Astronomical Society. All rights reserved. Source

Foley R.J.,University of Illinois at Urbana - Champaign | Van Dyk S.D.,IPAC Caltech | Jha S.W.,Rutgers University | Clubb K.I.,University of California at Berkeley | And 5 more authors.
Astrophysical Journal Letters | Year: 2015

We present pre-explosion and post-explosion Hubble Space Telescope images of the Type Iax supernova (SN Iax) 2014dt in M61. After astrometrically aligning these images, we do not detect any stellar sources at the position of the SN in the pre-explosion images to relatively deep limits (3σ limits of MF438W > -5.0 mag and MF814W > -5.9 mag). These limits are similar to the luminosity of SN 2012Z's progenitor system (MF435W = -5.43 ± 0.15 and MF814W = -5.24 ± 0.16 mag), the only probable detected progenitor system in pre-explosion images of a SN Iax, and indeed, of any white-dwarf supernova. SN 2014dt is consistent with having a C/O white-dwarf primary/helium-star companion progenitor system, as was suggested for SN 2012Z, although perhaps with a slightly smaller or hotter donor. The data are also consistent with SN 2014dt having a low-mass red giant or main-sequence star companion. The data rule out main-sequence stars with Minit ≳ 16 M⊙ and most evolved stars with Minit ≳ 8 M⊙ as being the progenitor of SN 2014dt. Hot Wolf-Rayet stars are also allowed, but the lack of nearby bright sources makes this scenario unlikely. Because of its proximity (D = 12 Mpc), SN 2014dt is ideal for long-term monitoring, where images in ∼2 yr may detect the companion star or the luminous bound remnant of the progenitor white dwarf. © 2015. The American Astronomical Society. All rights reserved. Source

Weiler K.W.,U.S. Navy | Panagia N.,US Space Telescope Science Institute | Panagia N.,National institute for astrophysics | Panagia N.,Supernova Ltd | And 4 more authors.
Memorie della Societa Astronomica Italiana, Supplementi - Journal of the Italian Astronomical Society, Supplement | Year: 2010

Study of radio supernovae over the past 30 years includes more than three dozen detected objects and more than 150 upper limits. From this work it is possible 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. Along with reviewing these general properties of the radio emission from supernovae, we present our extensive observations of the radio emission from supernova (SN) 1993J in M 81 (NGC 3031) made with the Very Large Array and other radio telescopes. The SN 1993J radio emission evolves regularly in both time and frequency, and the usual interpretation in terms of shock interaction with a circumstellar medium (CSM) formed by a pre-supernova stellar wind describes the observations rather well considering the complexity of the phenomenon. However: 1) The highest frequency measurements at 85-110 GHz at early times (< 40 days) are not well fitted by the parameterization which describes the cm wavelength measurements. 2) At a time ∼ 3100 days after shock breakout, the decline rate of the radio emission steepens from (t+β) β ∼ 0.7 to β∼ -2.7 without change in the spectral index (v+α; α ∼ -0.81). This decline is best described not as a power-law, but as an exponential decay with an e-folding time of ∼ 1100 days. 3) The best overall fit to all of the data is a model including both non-thermal synchrotron self-absorption (SSA) and a thermal free-free absorbing (FFA) components at early times, evolving to a constant spectral index, optically thin decline rate, until a break in that decline rate at day ∼ 3100, as mentioned above. © SAIt 2010. Source

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