9 research outputs found
Binary Stars as the Source of the Far-UV Excess in Elliptical Galaxies
The discovery of an excess of light in the far-ultraviolet (UV) spectrum in
elliptical galaxies was a major surprise in 1969. While it is now clear that
this UV excess is caused by an old population of hot helium-burning stars
without large hydrogen-rich envelopes rather than young stars, their origin has
remained a mystery. Here we show that these stars most likely lost their
envelopes because of binary interactions, similar to the hot subdwarf
population in our own Galaxy. This has major implications for understanding the
evolution of the UV excess and of elliptical galaxies in general. In
particular, it implies that the UV excess is not a sign of age, as had been
postulated previously, and predicts that it should not be strongly dependent on
the metallicity of the population.Comment: Proceedings of Puerto Vallarta 07, "New Quests in Stellar
Astrophysics. II. The Ultraviolet Properties of Evolved Stellar Populations",
6 pages, 2 figures. A much better version of Figure 1 can be obtained on
reques
Hot Subdwarfs in Binaries as the Source of the Far-UV Excess in Elliptical Galaxies
The excess of far-ultraviolet (far-UV) radiation in elliptical galaxies has
remained one of their most enduring puzzles. In contrast, the origin of old
blue stars in the Milky Way, hot subdwarfs, is now reasonably well understood:
they are hot stars that have lost their hydrogen envelopes by various binary
interactions. Here, we review the main evolutionary channels that produce hot
subdwarfs in the Galaxy and present the results of binary population synthesis
simulations that reproduce the main properties of the Galactic hot-subdwarf
population. Applying the same model to elliptical galaxies, we show how this
model can explain the main observational properties of the far-UV excess,
including the far-UV spectrum, without the need to invoke ad hoc physical
processes. The model implies that the UV excess is not a sign of age, as has
been postulated previously, and predicts that it should not be strongly
dependent on the metallicity of the population.Comment: 11 pages, 6 figures (preprint version
Exomol line lists â xliv. Infrared and ultraviolet line list for silicon monoxide (28si16o)
A new silicon monoxide (
28Si16O) line list covering infrared, visible, and ultraviolet regions called SiOUVenIR is presented.
This line list extends the infrared EBJT ExoMol line list by including vibronic transitions to the A 1 and E 1+ electronic
states. Strong perturbations to the A 1 band system are accurately modelled through the treatment of six dark electronic states:
C 1â, D 1 , a 3+, b 3, e 3â, and d 3 . Along with the X 1+ ground state, these nine electronic states were used to build a
comprehensive spectroscopic model of SiO using a combination of empirical and ab initio curves, including the potential energy
(PE), spinâorbit, electronic angular momentum, and (transition) dipole moment curves. The ab initio PE and coupling curves,
computed at the multireference configuration interaction level of theory, were refined by fitting their analytical representations
to 2617 experimentally derived SiO energy levels determined from 97 vibronic bands belonging to the XâX, EâX, and AâX
electronic systems through the MARVEL (Measured Active RotationalâVibrational Energy Levels) procedure. 112 observed
forbidden transitions from the CâX, DâX, eâX, and dâX bands were assigned using our predictions, and these could be fed
back into the MARVEL procedure. The SiOUVenIR line list was computed using published ab initio transition dipole moments
for the EâX and AâX bands; the line list is suitable for temperatures up to 10 000 K and for wavelengths longer than 140 nm.
SiOUVenIR is available from www.exomol.com and the CDS data base
Werner Eissner (1930â2022): a pioneer in computational atomic physics
Werner Eissner (Figure 1), a pioneer in computational atomic physics, was born on 16 October 1930 in the city of Görlitz, Germany, to Bernhard and Frieda (nĂ©e Eckert) Eissner. He lost both parents early in life, his father in the War and later his mother at the age of 15. He attended elementary school in his hometown (1937â1941) and high school in both Görlitz (1941â1945) and LĂŒdenscheid (1946â1951). His higher education was at Göttingen University (Preliminary Diploma in physics, 1954) and TĂŒbingen University (Diploma under Professor Dr. Hubert KrĂŒger, 1959, and doctorate degree under Professor Dr. Gerhard Elwert, 1967). His doctoral thesis dissertation was entitled âRechnungen zur ElektronenstoĂanregung der M-Schale von Wasserstoff und zur Polarisation des StoĂleuchtens der H
-Linieâ (Calculations for the electron collision excitation of the M-shell of hydrogen and for the polarization of the collisionally excited H
line), which he presented in preliminary form at the Third International Conference on the Physics of Electronic and Atomic Collisions (ICPEAC, 22â26 July 1963) held at University College London (UCL) [1]. For most of Wernerâs career, he was a member of two major research groups in atomic physics led by Professor Michael J. Seaton FRS at UCL and by Professor Philip G. Burke FRS at Queenâs University Belfast (QUB) and Daresbury Laboratory (DL)
Obituary Werner Eissner (1930-2022): A Pioneer in Computational Atomic Physics
ISSN:2218-200
Anomalous Orbital Characteristics of the AQ Col (EC 05217-3914) System
AQ Col (EC 05217-3914) is one of the first detected pulsating subdwarf B (sdB) stars and has been considered to be a single star. Photometric monitoring of AQ Col reveals a pulsation timing variation with a period of 486 days, interpreted as time delay due to reflex motion in a wide binary formed with an unseen companion with expected mass larger than 1.05 Msun. The optical spectra and colorâmagnitude diagram of the system suggested that the companion is not a main-sequence star but a white dwarf or neutron star. The pulsation timing variation also shows that the system has an eccentricity of 0.424, which is much larger than any known sdB long period binary system. That might be due to the existence of another short period companion to the sdB star. Two optical spectra obtained on 1996 December 5 show a radial velocity change of 49.1 km sâ1 in 46.1 minutes, which suggests the hot subdwarf in the wide binary is itself a close binary formed with another unseen white dwarf or neutron star companion; if further observations show this interpretation to be correct, AQ Col is an interesting triple system worthy of further study