Observational evidence for gravitationally trapped massive axion(-like) particles

Unexpected astrophysical observations can be explained by gravitationally captured massive particles, which are produced inside the Sun or other Stars and are accumulated over cosmic times. Their radiative decay in solar outer space would give rise to a `self-irradiation' of the whole star, providing the time-independent component of the corona heating source. In analogy with the Sun-irradiated Earth atmosphere, the temperature and density gradient in the corona - chromosphere transition region is suggestive for an omnipresent irradiation of the Sun. The same scenario fits other astrophysical X-ray observations. The radiative decay of a population of such elusive particles mimics a hot gas. X-ray observatories, with an unrivalled sensitivity below ~10 keV, can search for such particles. The elongation angle relative to the Sun is the relevant new parameter.Comment: 35 pages, LaTeX, 9 figures. Accepted by Astroparticle Physic

Spectrophotometric properties of pre-main sequence stars: the Epsilon Chamaeleontis cluster

We present a study of flux-calibrated low-resolution optical spectroscopy of ten stars belonging to eight systems in the ~ 5 Myr-old Epsilon Chamaeleontis (Eps Cha) pre-main-sequence (PMS) star cluster. Using synthetic broadband colours, narrow-band continuum, atomic and molecular lines derived from the spectra, we compare the Eps Cha stars to a slightly older PMS cluster, the ~ 8 Myr-old Eta Cha cluster, and to main-sequence dwarfs. Using synthetic VRI colours and other indices, we find that the relationship between broadband colours and spectroscopic temperature indicators for Eps Cha cluster members is indistinguishable from that of Gyr-old dwarfs. This result is identical to that found earlier in Eta Cha. Gravity-sensitive line indices place the cluster between the giant and dwarf sequences, and there is clear evidence that Eps Cha stars have lower surface gravity than Eta Cha stars. This result is consistent with Eps Cha being the slightly younger PMS association, a few Myr younger according to the Hertzsprung-Russell (HR) diagram placement of these two clusters and comparison with PMS evolutionary grids. Late M-type Eps Cha cluster members show a B-band flux excess of ~ 0.2 mag compared to observations of standard dwarfs, which might be related to enhanced magnetic activity. A similar level of excess B-band emission appears to be a ubiquitous feature of low mass members of young stellar populations with ages less than a few hundred Myr, a very similar timescale to the PMS phase of elevated relative X-ray luminosity.Comment: 8 figures and 3 tables It accepted for publication in Monthly Notices of the Royal Astronomical Societ

On the origin of the X-ray emission from Herbig Ae/Be stars

We performed a systematic search for Chandra archival observations of Herbig Ae/Be stars. These stars are fully radiative and not expected to support dynamo action analogous to their convective lower-mass counterparts, the T Tauri stars. Their X-ray emission has remained unexplained. The superior spatial resolution of Chandra with respect to previous X-ray instrumentation has allowed us to examine the possible role of late-type companions in generating the observed X-rays. In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-ray emitting faint companions or other unrelated X-ray bright objects within 10". The detection fraction of Herbig Ae/Be stars is 76 %, but lowers to 35 % if all emission is attributed to further known and unresolved companions. The spectral analysis confirms the high X-ray temperatures (~ 20 MK) and large range of fractional X-ray luminosities (log L_x/L_star) of this class derived from earlier studies of individual objects. Radiative winds are ruled out as emission mechanism on basis of the high temperatures. The X-ray properties of Herbig Ae/Be stars are not vastly different from those of their late-type companion stars (if such are known), nor from other young late-type stars used for comparison. Therefore, either a similar kind of process takes place on both classes of objects, or there must be as yet undiscovered companion stars.Comment: accepted for publication by Astronomy & Astrophysic

Habitable Zones in the Universe

Habitability varies dramatically with location and time in the universe. This was recognized centuries ago, but it was only in the last few decades that astronomers began to systematize the study of habitability. The introduction of the concept of the habitable zone was key to progress in this area. The habitable zone concept was first applied to the space around a star, now called the Circumstellar Habitable Zone. Recently, other, vastly broader, habitable zones have been proposed. We review the historical development of the concept of habitable zones and the present state of the research. We also suggest ways to make progress on each of the habitable zones and to unify them into a single concept encompassing the entire universe.Comment: 71 pages, 3 figures, 1 table; to be published in Origins of Life and Evolution of Biospheres; table slightly revise

Pressure dependence of the exchange interaction in the dimeric single-molecule magnet [Mn₄O₃Cl₄(O₂CEt₃)(py₃)₃]₂ from inelastic neutron scatttering

The low-lying magnetic excitations in the dimer of single-molecule magnets (Mn4)2 are studied by inelastic neutron scattering as a function of hydrostatic pressure. The anisotropy parameters D and B04, which describe each Mn4 subunit, are essentially pressure independent, while the antiferromagnetic exchange coupling J between the two Mn4 subunits strongly depends on pressure, with an increase of 42% at 17 kbar. Additional pressure dependent powder X-ray measurements allow a structural interpretation of the findings.Comment: 4 pages, 4 figures, REVTEX4, to be published in PR

Inelastic neutron scattering studies on the odd-membered antiferromagnetic wheel Cr8Ni

International audience[(iC3H7)2NH2][Cr8NiF9(O2CCMe)18], or Cr8Ni, is a prominent example of an odd-membered antiferromagnetic "wheel." A detailed characterization of the magnetic properties of Cr8Ni has been conducted. Inelastic neutron scattering (INS) is used to investigate the energy and momentum transfer dependence of the low-lying spin excitations, including excited states inaccessible by other experimental techniques. The richness of the INS data, in conjunction with microscopic spin Hamiltonian simulations, enables an accurate characterization of the magnetic properties of Cr8Ni. Nearest-neighbor exchange constants of JCrCr = 1.31 meV and JCrNi = 3.22 meV are determined, and clear evidence of axial single-ion anisotropy is found. The parameters determined by INS are shown to fit magnetic susceptibility. The spectroscopic identification of several successive S=1 excited total spin states and lowest spin band excitations show that the rotational band picture, valid for bipartite AFM wheels, breaks down for this odd-numbered wheel. The exchange constants determined here differ from previous efforts based on bulk measurements, and possible reasons are discussed. The large JCrNi/JCrCr ratio in Cr8Ni puts this wheel into a regime with strong quantum fluctuations in which the ground state can be described with a valence bond solid state picture