13 research outputs found
HE0107-5240, A Chemically Ancient Star.I. A Detailed Abundance Analysis
We report a detailed abundance analysis for HE0107-5240, a halo giant with
[Fe/H]_NLTE=-5.3. This star was discovered in the course of follow-up
medium-resolution spectroscopy of extremely metal-poor candidates selected from
the digitized Hamburg/ESO objective-prism survey. On the basis of
high-resolution VLT/UVES spectra, we derive abundances for 8 elements (C, N,
Na, Mg, Ca, Ti, Fe, and Ni), and upper limits for another 12 elements. A
plane-parallel LTE model atmosphere has been specifically tailored for the
chemical composition of {\he}. Scenarios for the origin of the abundance
pattern observed in the star are discussed. We argue that HE0107-5240 is most
likely not a post-AGB star, and that the extremely low abundances of the
iron-peak, and other elements, are not due to selective dust depletion. The
abundance pattern of HE0107-5240 can be explained by pre-enrichment from a
zero-metallicity type-II supernova of 20-25M_Sun, plus either self-enrichment
with C and N, or production of these elements in the AGB phase of a formerly
more massive companion, which is now a white dwarf. However, significant radial
velocity variations have not been detected within the 52 days covered by our
moderate-and high-resolution spectra. Alternatively, the abundance pattern can
be explained by enrichment of the gas cloud from which HE0107-5240 formed by a
25M_Sun first-generation star exploding as a subluminous SNII, as proposed by
Umeda & Nomoto (2003). We discuss consequences of the existence of HE0107-5240
for low-mass star formation in extremely metal-poor environments, and for
currently ongoing and future searches for the most metal-poor stars in the
Galaxy.Comment: 60 pages, 16 figures. Accepted for publication in Ap
A stellar relic from the early Milky Way
The chemical composition of the most metal-deficient stars reflects the
composition of the gas from which they formed. These old stars provide crucial
clues to the star formation history and the synthesis of chemical elements in
the early Universe. They are the local relics of epochs otherwise observable
only at very high redshifts; if totally metal-free (``population III'') stars
could be found, this would allow the direct study of the pristine gas from the
Big Bang. Earlier searches for such stars found none with an iron abundance
less than 1/10,000 that of the Sun, leading to the suggestion that low-mass
stars could only form from clouds above a critical iron abundance. Here we
report the discovery of a low-mass star with an iron abundance as low as
1/200,000 of the solar value. This discovery suggests that population III stars
could still exist, that is, that the first generation of stars also contained
long-lived low-mass objects. The previous failure to find them may be an
observational selection effect.Comment: Offprint of Nature 419 (2002), 904-906 (issue 31 October 2002
TeraHertz Exploration and Zooming-in for Astrophysics (THEZA): ESA Voyage 2050 White Paper
This paper presents the ESA Voyage 2050 White Paper for a concept of
TeraHertz Exploration and Zooming-in for Astrophysics (THEZA). It addresses the
science case and some implementation issues of a space-borne radio
interferometric system for ultra-sharp imaging of celestial radio sources at
the level of angular resolution down to (sub-) microarcseconds. THEZA focuses
at millimetre and sub-millimetre wavelengths (frequencies above 300~GHz),
but allows for science operations at longer wavelengths too. The THEZA concept
science rationale is focused on the physics of spacetime in the vicinity of
supermassive black holes as the leading science driver. The main aim of the
concept is to facilitate a major leap by providing researchers with orders of
magnitude improvements in the resolution and dynamic range in direct imaging
studies of the most exotic objects in the Universe, black holes. The concept
will open up a sizeable range of hitherto unreachable parameters of
observational astrophysics. It unifies two major lines of development of
space-borne radio astronomy of the past decades: Space VLBI (Very Long Baseline
Interferometry) and mm- and sub-mm astrophysical studies with "single dish"
instruments. It also builds upon the recent success of the Earth-based Event
Horizon Telescope (EHT) -- the first-ever direct image of a shadow of the
super-massive black hole in the centre of the galaxy M87. As an amalgam of
these three major areas of modern observational astrophysics, THEZA aims at
facilitating a breakthrough in high-resolution high image quality studies in
the millimetre and sub-millimetre domain of the electromagnetic spectrum.Comment: White Paper submitted in response to the ESA Call Voyage 205
The science case and challenges of space-borne sub-millimeter interferometry
Interstellar matter and star formatio
The science case and challenges of space-borne sub-millimeter interferometry
Ultra-high angular resolution in astronomy has always been an important vehicle for making fundamental discoveries. Recent results in direct imaging of the vicinity of the supermassive black hole in the nucleus of the radio galaxy M87 by the millimeter VLBI system Event Horizon Telescope and various pioneering results of the Space VLBI mission RadioAstron provided new momentum in high angular resolution astrophysics. In both mentioned cases, the angular resolution reached the values of about 10–20 microarcseconds (0.05–0.1 nanoradian). Further developments towards at least an order of magnitude “sharper” values, at the level of 1 microarcsecond are dictated by the needs of advanced astrophysical studies. The paper emphasis that these higher values can only be achieved by placing millimeter and submillimeter wavelength interferometric systems in space. A concept of such the system, called Terahertz Exploration and Zooming-in for Astrophysics, has been proposed in the framework of the ESA Call for White Papers for the Voyage 2050 long term plan in 2019. In the current paper we present new science objectives for such the concept based on recent results in studies of active galactic nuclei and supermassive black holes. We also discuss several approaches for addressing technological challenges of creating a millimeter/sub-millimeter wavelength interferometric system in space. In particular, we consider a novel configuration of a space-borne millimeter/sub-millimeter antenna which might resolve several bottlenecks in creating large precise mechanical structures. The paper also presents an overview of prospective space-qualified technologies of low-noise analogue front-end instrumentation for millimeter/sub-millimeter telescopes. Data handling and processing instrumentation is another key technological component of a sub-millimeter Space VLBI system. Requirements and possible implementation options for this instrumentation are described as an extrapolation of the current state-of-the-art Earth-based VLBI data transport and processing instrumentation. The paper also briefly discusses approaches to the interferometric baseline state vector determination and synchronisation and heterodyning system. The technology-oriented sections of the paper do not aim at presenting a complete set of technological solutions for sub-millimeter (terahertz) space-borne interferometers. Rather, in combination with the original ESA Voyage 2050 White Paper, it sharpens the case for the next generation microarcsecond-level imaging instruments and provides starting points for further in-depth technology trade-off studies.</p
TeraHertz Exploration and Zooming-in for Astrophysics (THEZA): ESA Voyage 2050 White Paper
The astrophysical agenda of the 21st century requires a very sharp view of celestial objects. High angular resolution studies are essential for fundamental studies of a broad variety of astrophysical phenomena ranging from relativistic physics of black holes, their gravitational and electromagnetic imprints, violent transient processes, including those producing detectable gravitational waves, birth and evolution of planetary systems. Over the past decades, radio astronomy made huge leap in achieving ground-breaking angular resolution measured in tens of microarcseconds (one tenth of nanoradian and better). Recently a global Event Horizon Telescope (EHT) collaboration obtained first direct images of the shadow of a super-massive black hole in the nucleus of the active galaxy M87. These observations were conducted at 230 GHz. The two first generation Space Very Long Baseline Interferometry (VLBI) missions, VSOP/HALCA led by the Japan Aerospace Exploration Agency (JAXA) and RadioAstron led by the Russia Roscosmos State Corporation and Russia Academy of Sciences, achieved the highest angular resolution at frequencies from 0.3 to 22 GHz in observations conducted in the period 1997 - 2019. The next step in advancing high angular resolution radio astronomy is in combining high frequency (millimeter and sub-millimeter wavelengths) and interferometric baselines exceeding the Earth diameter. The present THEZA White Paper describes a combination which would unify technology developments in giga-/tera-hertz instrumentation and space-borne radio astronomy. The current preprint version of the THEZA White Paper is slightly re-formatted and edited comparing to the official submitted version
The science case and challenges of space-borne sub-millimeter interferometry
Interstellar matter and star formatio