91 research outputs found
Absolute Lineshifts - A new diagnostic for stellar hydrodynamics
For hydrodynamic model atmospheres, absolute lineshifts are becoming an
observable diagnostic tool beyond the classical ones of line-strength, -width,
-shape, and -asymmetry. This is the wavelength displacement of different types
of spectral lines away from the positions naively expected from the Doppler
shift caused by stellar radial motion. Caused mainly by correlated velocity and
brightness patterns in granular convection, such absolute lineshifts could in
the past be studied only for the Sun (since the relative Sun-Earth motion, and
the ensuing Doppler shift is known). For other stars, this is now becoming
possible thanks to three separate developments: (a) Astrometric determination
of stellar radial motion; (b) High-resolution spectrometers with accurate
wavelength calibration, and (c) Accurate laboratory wavelengths for several
atomic species. Absolute lineshifts offer a tool to segregate various 2- and
3-dimensional models, and to identify non-LTE effects in line formation.Comment: 13 pages, 9 figures; to appear in "Modelling of Stellar Atmospheres",
IAU Symp.210; N.E.Piskunov, W.W.Weiss, D.F.Gray (eds.
Stellar intensity interferometry over kilometer baselines: Laboratory simulation of observations with the Cherenkov Telescope Array
A long-held astronomical vision is to realize diffraction-limited optical
aperture synthesis over kilometer baselines. This will enable imaging of
stellar surfaces and their environments, show their evolution over time, and
reveal interactions of stellar winds and gas flows in binary star systems. An
opportunity is now opening up with the large telescope arrays primarily erected
for measuring Cherenkov light in air induced by gamma rays. With suitable
software, such telescopes could be electronically connected and used also for
intensity interferometry. With no optical connection between the telescopes,
the error budget is set by the electronic time resolution of a few nanoseconds.
Corresponding light-travel distances are on the order of one meter, making the
method practically insensitive to atmospheric turbulence or optical
imperfections, permitting both very long baselines and observing at short
optical wavelengths. Theoretical modeling has shown how stellar surface images
can be retrieved from such observations and here we report on experimental
simulations. In an optical laboratory, artificial stars (single and double,
round and elliptic) are observed by an array of telescopes. Using high-speed
photon-counting solid-state detectors and real-time electronics, intensity
fluctuations are cross correlated between up to a hundred baselines between
pairs of telescopes, producing maps of the second-order spatial coherence
across the interferometric Fourier-transform plane. These experiments serve to
verify the concepts and to optimize the instrumentation and observing
procedures for future observations with (in particular) CTA, the Cherenkov
Telescope Array, aiming at order-of-magnitude improvements of the angular
resolution in optical astronomy.Comment: 18 pages, 11 figures; Presented at SPIE conference on Astronomical
Telescopes + Instrumentation in Montreal, Quebec, Canada, June 2014. To
appear in SPIE Proc.9146, Optical and Infrared Interferometry IV
(J.K.Rajagopal, M.J.Creech-Eakman, F.Malbet, eds.), 201
Spectral Fidelity. Opportunities, limitations, and future challenges
Ambitions toward spectra with maximum truthfulness are constrained by optical physics of grating spectrometers. Spectral fidelity depends on calibrating for the blaze function; segregating overlapping orders; understanding scattered light, and polarization response. No high-resolution spectra in the optical have yet been recorded of any star (or the Sun) from outside the atmosphere: all data are contaminated by telluric absorption.
Spectra with S/N = 1000 enable studies of the fine structure in interstellar absorption lines. S/N = 10,000 permits to study line shapes across stellar surfaces during exoplanet transit. S/N = 100,000 would reveal detailed line profiles from starspots temporarily covered by an exoplanet. Spectral resolutions approaching R = 1,000,000 fully resolve line asymmetries from stellar granulation and may be needed for absorption line profiles in quasars, shaped by convective motions in the intergalactic medium.
CCD detectors need attention because of interference caused by their partial transparency and induced asymmetries in the direction of electronic readout. Future-generation detectors that enable photon counting and wavelength resolution include microwave kinetic inductance detectors, transition-edge sensors, and superconducting nanowires. A grand challenge remains in designing an efficient ELT spectrometer for R = 1,000,000 and S/N = 100,000. High-fidelity spectroscopy, combined with quantum optics, remains a challenge for ELT and beyond
Intensity interferometry: Optical imaging with kilometer baselines
Optical imaging with microarcsecond resolution will reveal details across and
outside stellar surfaces but requires kilometer-scale interferometers,
challenging to realize either on the ground or in space. Intensity
interferometry, electronically connecting independent telescopes, has a noise
budget that relates to the electronic time resolution, circumventing issues of
atmospheric turbulence. Extents up to a few km are becoming realistic with
arrays of optical air Cherenkov telescopes (primarily erected for gamma-ray
studies), enabling an optical equivalent of radio interferometer arrays.
Pioneered by Hanbury Brown and Twiss, digital versions of the technique have
now been demonstrated, reconstructing diffraction-limited images from
laboratory measurements over hundreds of optical baselines. This review
outlines the method from its beginnings, describes current experiments, and
sketches prospects for future observations.Comment: 12 pages, 3 figures, 92 references. Invited keynote talk presented at
the conference 'SPIE Astronomical Telescopes + Instrumentation', Edinburgh,
Scotland (2016); to be published in SPIE Proc. 9907, 'Optical and Infrared
Interferometry and Imaging V
Astrometric radial velocities. I. Non-spectroscopic methods for measuring stellar radial velocity
High-accuracy astrometry permits the determination of not only stellar
tangential motion, but also the component along the line-of-sight. Such
non-spectroscopic (i.e. astrometric) radial velocities are independent of
stellar atmospheric dynamics, spectral complexity and variability, as well as
of gravitational redshift. Three methods are analysed: (1) changing annual
parallax, (2) changing proper motion and (3) changing angular extent of a
moving group of stars. All three have significant potential in planned
astrometric projects. Current accuracies are still inadequate for the first
method, while the second is marginally feasible and is here applied to 16
stars. The third method reaches high accuracy (<1 km/s) already with present
data, although for some clusters an accuracy limit is set by uncertainties in
the cluster expansion rate.Comment: 13 pages, 2 figures. Accepted for publication in Astronomy &
Astrophysics (main journal
Long-baseline optical intensity interferometry: Laboratory demonstration of diffraction-limited imaging
A long-held vision has been to realize diffraction-limited optical aperture
synthesis over kilometer baselines. This will enable imaging of stellar
surfaces and their environments, and reveal interacting gas flows in binary
systems. An opportunity is now opening up with the large telescope arrays
primarily erected for measuring Cherenkov light in air induced by gamma rays.
With suitable software, such telescopes could be electronically connected and
also used for intensity interferometry. Second-order spatial coherence of light
is obtained by cross correlating intensity fluctuations measured in different
pairs of telescopes. With no optical links between them, the error budget is
set by the electronic time resolution of a few nanoseconds. Corresponding
light-travel distances are approximately one meter, making the method
practically immune to atmospheric turbulence or optical imperfections,
permitting both very long baselines and observing at short optical wavelengths.
Previous theoretical modeling has shown that full images should be possible to
retrieve from observations with such telescope arrays. This project aims at
verifying diffraction-limited imaging experimentally with groups of detached
and independent optical telescopes. In a large optics laboratory, artificial
stars were observed by an array of small telescopes. Using high-speed
photon-counting solid-state detectors, intensity fluctuations were
cross-correlated over up to 180 baselines between pairs of telescopes,
producing coherence maps across the interferometric Fourier-transform plane.
These measurements were used to extract parameters about the simulated stars,
and to reconstruct their two-dimensional images. As far as we are aware, these
are the first diffraction-limited images obtained from an optical array only
linked by electronic software, with no optical connections between the
telescopes.Comment: 13 pages, 9 figures, Astronomy & Astrophysics, in press. arXiv admin
note: substantial text overlap with arXiv:1407.599
Stellar intensity interferometry: Optimizing air Cherenkov telescope array layouts
Kilometric-scale optical imagers seem feasible to realize by intensity
interferometry, using telescopes primarily erected for measuring Cherenkov
light induced by gamma rays. Planned arrays envision 50--100 telescopes,
distributed over some 1--4 km. Although array layouts and telescope sizes
will primarily be chosen for gamma-ray observations, also their interferometric
performance may be optimized. Observations of stellar objects were numerically
simulated for different array geometries, yielding signal-to-noise ratios for
different Fourier components of the source images in the interferometric
-plane. Simulations were made for layouts actually proposed for future
Cherenkov telescope arrays, and for subsets with only a fraction of the
telescopes. All large arrays provide dense sampling of the -plane due to
the sheer number of telescopes, irrespective of their geographic orientation or
stellar coordinates. However, for improved coverage of the -plane and a
wider variety of baselines (enabling better image reconstruction), an exact
east-west grid should be avoided for the numerous smaller telescopes, and
repetitive geometric patterns avoided for the few large ones. Sparse arrays
become severely limited by a lack of short baselines, and to cover
astrophysically relevant dimensions between 0.1--3 milliarcseconds in visible
wavelengths, baselines between pairs of telescopes should cover the whole
interval 30--2000 m.Comment: 12 pages, 10 figures; presented at the SPIE conference "Optical and
Infrared Interferometry II", San Diego, CA, USA (June 2010
Solar Photospheric Spectrum Microvariability I. Theoretical searches for proxies of radial-velocity jittering
Extreme precision radial-velocity spectrometers enable extreme precision
stellar spectroscopy. Searches for low-mass exoplanets around solar-type stars
are limited by the physical variability in stellar spectra, such as the
short-term jittering of apparent radial velocities. To understand the physical
origins of such jittering, the solar spectrum is assembled, as far as possible,
from basic principles. Surface convection is modeled with time-dependent 3D
hydrodynamics, followed by the computation of hyper-high resolution spectra
during numerous instances of the simulation sequences. The behavior of
different classes of photospheric absorption lines is monitored to identify
commonalities or differences between different classes of lines: weak or
strong, neutral or ionized, high- or low-excitation, atomic or molecular. For
Fe I and Fe II lines, the radial-velocity jittering over the small simulation
area typically amounts to +-150 m/s, scaling to about 2 m/s for the full solar
disk. Most photospheric lines vary in phase but with different amplitudes among
different classes of lines. Radial-velocity excursions are greater for stronger
and for ionized lines, decreasing at longer wavelengths. The differences
between various line-groups are about one order of magnitude less than the full
jittering amplitudes. By matching very precisely measured radial velocities to
the characteristic jittering patterns between different line-groups should
enable to identify and to remove a significant component of the stellar noise
originating in granulation. To verify the modeling toward such a filter,
predictions of solar center-to-limb dependences of jittering amplitudes are
presented for different classes of lines, testable with spatially resolving
solar telescopes connected to existing radial-velocity instruments.Comment: 18 pages, 20 figures, accepted for publication in Astronomy &
Astrophysic
Stellar Intensity Interferometry: Astrophysical targets for sub-milliarcsecond imaging
Intensity interferometry permits very long optical baselines and the
observation of sub-milliarcsecond structures. Using planned kilometric arrays
of air Cherenkov telescopes at short wavelengths, intensity interferometry may
increase the spatial resolution achieved in optical astronomy by an order of
magnitude, inviting detailed studies of the shapes of rapidly rotating hot
stars with structures in their circumstellar disks and winds, or mapping out
patterns of nonradial pulsations across stellar surfaces. Signal-to-noise in
intensity interferometry favors high-temperature sources and emission-line
structures, and is independent of the optical passband, be it a single spectral
line or the broad spectral continuum. Prime candidate sources have been
identified among classes of bright and hot stars. Observations are simulated
for telescope configurations envisioned for large Cherenkov facilities,
synthesizing numerous optical baselines in software, confirming that
resolutions of tens of microarcseconds are feasible for numerous astrophysical
targets.Comment: 12 pages, 4 figures; presented at the SPIE conference "Optical and
Infrared Interferometry II", San Diego, CA, USA (June 2010
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