Gadallah K.A.K.,Al - Azhar University of Egypt |
Gadallah K.A.K.,University Observatory |
Mutschke H.,University Observatory |
Jager C.,Friedrich - Schiller University of Jena
EAS Publications Series | Year: 2012
Hydrogenated amorphous carbons (HACs) are considered as laboratory analogues to cosmic carbonaceous nanoparticles in the interstellar medium (ISM). The optical properties of nano-sized HACs may be influenced by the UV processing. The variation of the internal structure leads to dramatic changes in the spectral properties in the FUV-VIS range. This scenario can explain some astronomical features such as the interstellar UV bump at 4.6 μm -1. The spectrum of HACs, irradiated by a dose of UV irradiation that corresponds to 21-33% of the average dose of the UV radiation in diffuse ISM, exhibits a new band centered at 4.6 μm-1. This result confirms, for the first time, the suggestion by Mennella et al. (1996) that irradiated HACs might be considered as the carriers of the interstellar UV bump at 4.6 μm-1. However, the amount of carbon needed to reproduce this band is higher than that available for interstellar carbon dust grains. So the ideal structure of irradiated HACs that would produce a band of sufficient strength has still to be searched for. © The Author(s) 2013.
Kramm U.,University of Rostock |
Nettelmann N.,University of Rostock |
Fortney J.J.,University of California at Santa Cruz |
Neuhauser R.,University Observatory |
Redmer R.,University of Rostock
Astronomy and Astrophysics | Year: 2012
Context. Transit and radial velocity observations continuously discover an increasing number of exoplanets. However, when it comes to the composition of the observed planets the data are compatible with several interior structure models. Thus, a planetary parameter sensitive to the planet's density distribution could help constrain this large number of possible models even further. Aims. We aim to investigate to what extent an exoplanet's interior can be constrained in terms of core mass and envelope metallicity by taking the tidal Love number k 2 into account as an additional, possibly observable parameter. Methods. Because it is the only planet with an observationally determined k 2, we constructed interior models for the Hot Jupiter exoplanet HAT-P-13b by solving the equations of hydrostatic equilibrium and mass conservation for different boundary conditions. In particular, we varied the surface temperature and the outer temperature profile, as well as the envelope metallicity within the widest possible parameter range. We also considered atmospheric conditions that are consistent with nongray atmosphere models. For all these models we calculated the Love number k 2 and compared it to the allowed range of k 2 values that could be obtained from eccentricity measurements of HAT-P-13b. Results. We use the example of HAT-P-13b to show the general relationships between the quantities temperature, envelope metallicity, core mass, and Love number of a planet. For any given k 2 value a maximum possible core mass can be determined. For HAT-P-13b we find M core < 27 M ⊕, based on the latest eccentricity measurement. We favor models that are consistent with our model atmosphere, which gives us the temperature of the isothermal region as ∼2100 K. With this external boundary condition and our new k 2-interval we are able to constrain both the envelope and bulk metallicity of HAT-P-13b to 1-11 times stellar metallicity and the extension of the isothermal layer in the planet's atmosphere to 3-44 bar. Assuming equilibrium tidal theory, we find lower limits on the tidal Q consistent with 10 3-10 5. Conclusions. Our analysis shows that the tidal Love number k 2 is a very useful parameter for studying the interior of exoplanets. It allows one to place limits on the core mass and estimate the metallicity of a planet's envelope. © 2012 ESO.
News Article | August 18, 2016
Astronomers have found the first direct evidence that prove the nova hibernation hypothesis, a theory that posits binary star systems undergo cyclical explosion that repeat itself over a period of time. In a new study published in the journal Nature on Aug. 17, Przemek Mróz, from the Warsaw University Observatory in Poland, and colleagues reported the explosion of a hibernating star. They observed mini outbursts leading up to the classical nova, the final explosion of a white dwarf, the remains of sun-like star that has exhausted its nuclear fuel. Compared with more powerful supernova explosion which means the death of stars, nova eruptions do not necessarily result in the destruction of the stellar parents. The nova that astronomers call V1213 Cen, or Nova Centauri 2009 erupted in 2009 but they have been observing its source star since 2003. Using data that were collected years before and after the eruption, researchers were able to learn about the evolution of this particular type of nova. Classical novas such as the Nova Centauri 2009 happen in binary systems, which consist of two stars orbiting each other. The nova occurs when a white dwarf stars gains matter from the other star over a period of time. The cycle starts with the hibernation phase, wherein the companion star sends mass in the form of hydrogen to the white dwarf star. The white dwarf cyclically gets brighter, dims and then awakes erupting in a big explosion, an event that creates significant increase in brightness. The process is slow but eventually starts all over again. Although astronomers observe several classical novae in the Milky Way per year, most of them are faint due to interstellar gas and dust that hide them. Long term observations with this particular nova, however, showed that the white dwarf star experienced periodic brightening since 2003 before the explosion. The fluctuation in brightness means that during this time, a low mass-transfer occurred between the two stars. Mass transfer rate also increased after the explosion. The system is currently bright but is slow fading. This will continue for a while before the process starts all over again. "Within the six years before the explosion, the system revealed dwarf nova outbursts indicative of a low mass-transfer rate," the researchers wrote. "The post-nova is two orders of magnitude brighter than the pre-nova at minimum light with no trace of dwarf nova behaviour, implying that the mass-transfer rate increased considerably as a result of the nova explosion." © 2016 Tech Times, All rights reserved. Do not reproduce without permission.
News Article | August 22, 2016
(Phys.org)—A team of researchers affiliated with the Warsaw University Observatory has captured for the first time the events that led to a classical nova exploding, the explosion itself and then what happened afterwards. In their paper published in the journal Nature, the team describes how they happened to capture the star activity and why they believe it may help bolster the theory of star hibernation.
Adam C.,University Observatory |
Mugrauer M.,University Observatory
Monthly Notices of the Royal Astronomical Society | Year: 2014
We report the detection of a new low-mass stellar companion to the white dwarf HIP 3678 A, the central star of the planetary nebula NGC 246. The newly found companion is located about 1 arcsec (at projected separation of about 500 au) north-east of HIP 3678 A, and shares a common proper motion with the white dwarf and its known comoving companion HIP 3678 B. The hypothesis that the newly detected companion is a non-moving background object can be rejected on a significance level of more than 8σ, by combining astrometricmeasurements from the literature with follow-up astrometry, obtained withWide Field Planetary Camera 2/Hubble Space Telescope andNACO/Very Large Telescope. From our deepNACOimaging data, we can rule out additional stellar companions of the white dwarf with projected separations between 130 up to 5500 au. In the deepest high-contrast NACO observation, we achieve a detection limit in the Ks band of about 20 mag, which allows the detection of brown dwarf companions with masses down to 36Mjup at an assumed age of the system of 260 Myr. To approximate the masses of the companions HIP 3678 B and C, we use the evolutionary Baraffe et al. models and obtain about 0.85 M⊙ for HIP 3678 B and about 0.1 M⊙ for HIP 3678 C. According to the derived absolute photometry, HIP 3678 B should be a early to mid-K dwarf (K2-K5), while HIP 3678 C should be a mid M dwarf with a spectral type in the range between M5 and M6. © 2014 The Authors.