435 research outputs found
Constraining the evolution of young radio-loud AGN
GPS and CSS radio sources are the objects of choice to investigate the
evolution of young radio-loud AGN. Previous investigations, mainly based on
number counts and source size distributions, indicate that GPS/CSS sources
decrease significantly in radio power when evolving into old, extended objects.
We suggest this is preceded by a period of increase in radio luminosity, which
lasts as long as the radio source is confined within the core-radius of its
host galaxy. We have selected a sample of nearby compact radio sources,
unbiased by radio spectrum, to determine their radio luminosity function, size
distribution, dynamical ages, and emission line properties in a complete and
homogeneous way. First results indicate that the large majority of objects
(>80%) exhibit classical GPS/CSS radio spectra, and show structures consistent
with the being compact double, or compact symmetric objects. This sample
provides and ideal basis to further test and constrain possible evolution
scenarios, and to investigate the relation between radio spectra and
morphologies, orientation and Doppler boosting in samples of young radio-loud
AGN, in an unbiased way.Comment: LaTeX, 8 pages, 3 figs: Accepted by Publications of the Astronomical
Society of Australia, as part of the proceedings of the 3rd GPS/CSS workshop,
eds. T. Tzioumis et a
Optical spectroscopy of faint gigahertz peaked spectrum sources
We present spectroscopic observations of a sample of faint Gigahertz Peaked
Spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey
(WENSS). Redshifts have been determined for 19 (40%) of the objects. The
optical spectra of the GPS sources identified with low redshift galaxies show
deep stellar absorption features. This confirms previous suggestions that their
optical light is not significantly contaminated by AGN-related emission, but is
dominated by a population of old (>9 Gyr) and metal-rich (>0.2 [Fe/H]) stars,
justifying the use of these (probably) young radio sources as probes of galaxy
evolution. The optical spectra of GPS sources identified with quasars are
indistinguishable from those of flat spectrum quasars, and clearly different
from the spectra of Compact Steep Spectrum (CSS) quasars. The redshift
distribution of the GPS quasars in our radio-faint sample is comparable to that
of the bright samples presented in the literature, peaking at z ~ 2-3. It is
unlikely that a significant population of low redshift GPS quasars is missed
due to selection effects in our sample. We therefore claim that there is a
genuine difference between the redshift distributions of GPS galaxies and
quasars, which, because it is present in both the radio-faint and bright
samples, can not be due to a redshift-luminosity degeneracy. It is therefore
unlikely that the GPS quasars and galaxies are unified by orientation, unless
the quasar opening angle is a strong function of redshift. We suggest that the
GPS quasars and galaxies are unrelated populations and just happen to have
identical observed radio-spectral properties, and hypothesise that GPS quasars
are a sub-class of flat spectrum quasars.Comment: LaTeX, 13 pages. Accepted by MNRAS. For related papers see
http://www.ast.cam.ac.uk/~snelle
Using the chromatic Rossiter-McLaughlin effect to probe the broadband signature in the optical transmission spectrum of HD 189733b
Transmission spectroscopy is a powerful technique for probing exoplanetary
atmospheres. A successful ground-based observational method uses a differential
technique based on high-dispersion spectroscopy, but that only preserves narrow
features in transmission spectra. Here we use the chromatic Rossiter-McLaughlin
(RM) effect to measure the Rayleigh-scattering slope in the transmission
spectrum of HD 189733b with the aim to show that it can be effectively used to
measure broadband transmission features. The amplitude of the RM effects
depends on the effective size of the planet, and in the case of an atmospheric
contribution therefore depends on the observed wavelength. We analysed archival
HARPS data of three transits of HD 189733b, covering a wavelength range of 400
to 700 nm. We measured the slope in the transmission spectrum of HD 189733b at
a significance. Assuming it is due to Rayleigh scattering and not
caused by stellar activity, it would correspond to an atmospheric temperature,
as set by the scale height, of , well in line with
previously obtained results. This shows that ground-based high-dispersion
spectral observations can be used to probe broad-band features in the
transmission spectra of extrasolar planets, by using the chromatic RM effect.
This method will be particularly interesting in conjunction with the new
echelle spectrograph ESPRESSO, which currently is under construction for ESOs
Very Large Telescope and will provide a gain in signal-to-noise ratio of about
a factor 4 compared to HARPS. This will be of great value because of the
limited and uncertain future of the Hubble Space Telescope and because the
future James Webb Space Telescope will not cover this wavelength regime.Comment: 8 pages, 7 figures, accepted for publication on Astronomy and
Astrophysic
A stringent upper limit to 18cm radio emission from the extrasolar planet system tau Bootis
Context: It has been speculated for many years that some extrasolar planets
may emit strong cyclotron emission at low radio frequencies in the range 10-100
MHz. Despite several attempts no such emission has yet been seen.
Aims: The hot Jupiter system tau Bootis is one of the nearest (d=15 pc)
exoplanets known to date. The gravitational influence of this massive hot
Jupiter (M=6 M_jup) has locked the star-planet system, making the star rotate
in P~3.3 days, similar to the orbital period of the planet. From the well
established correlation between stellar rotation and radio luminosity, it is
conceivable that the tau Bootis system emits strong radio emission at
significantly higher frequencies than currently probed, which we aimed to
investigate with this work.
Methods: We observed tau Bootis with the Westerbork Synthesis Radio Telescope
(WSRT) at a frequency of 1.7 GHz. for 12 hours in spectral line mode, reaching
a noise level of 42 microJy/beam at the position of the target.
Results: No 18cm radio emission is detected from tau Bootis, resulting in a 3
sigma upper limit of 0.13 mJy, corresponding to a 18cm radio luminosity of
<3.7e13 erg/s/Hz. We observe tau Bootis to be two orders of magnitude fainter
than expected from the stellar relation between radio luminosity and rotation
velocity.
Conclusions: This implies that either the tau Bootis system is underluminous
in the radio compared to similar fast-rotating stars, or that we happened to
observe the target during a low state of radio emission.Comment: 4 pages, 1 figure: Accepted for publication in A&
Searching for reflected light from Bootis b with high-resolution ground-based spectroscopy: Approaching the contrast barrier
It is challenging to measure the starlight reflected from exoplanets because
of the extreme contrast with their host stars. For hot Jupiters, this contrast
is in the range of to , depending on their albedo, radius
and orbital distance. Searches for reflected light have been performed since
the first hot Jupiters were discovered, but with very limited success because
hot Jupiters tend to have low albedo values due to the general absence of
reflective cloud decks. The aim of this study is to search for reflected light
from Boo b, a hot Jupiter with one of the brightest host stars. Since
its discovery in 1997, it has been the subject of several reflected-light
searches using high-dispersion spectroscopy. Here we aim to combine these data
in to a single meta-analysis. We analysed more than 2,000 archival
high-dispersion spectra obtained with the UVES, ESPaDOnS, NARVAL UES and
HARPS-N spectrographs during various epochs between 1998 and 2013. Each
spectrum was first cleaned of the stellar spectrum and subsequently
cross-correlated with a PHOENIX model spectrum. These were then Doppler shifted
to the planet rest-frame and co-added in time, weighted according to the
expected signal-to-noise of the planet signal. We reach a 3 upper limit
of the planet to star contrast of . Assuming a planet
radius of 1.15 , this corresponds to an optical albedo of 0.12 between
400-700 nm. This low albedo is in line with secondary eclipse and phase curve
observations of other hot Jupiters using space-based observatories, as well as
theoretical predictions of their reflective properties.Comment: 15 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
A Parkes half-Jansky sample of GPS galaxies
This paper describes the selection of a new southern/equatorial sample of
Gigahertz Peaked Spectrum (GPS) radio galaxies, and subsequent optical CCD
imaging and spectroscopic observations using the ESO 3.6m telescope. The sample
consists of 49 sources with -4020 degrees, and
S(2.7GHz)>0.5 Jy, selected from the Parkes PKSCAT90 survey. About 80% of the
sources are optically identified, and about half of the identifications have
available redshifts. The R-band Hubble diagram and evolution of the host
galaxies of GPS sources are reviewed.Comment: Latex, 12 pages, 8 figures, accepted for publication in MNRA
Detecting isotopologues in exoplanet atmospheres using ground-based high-dispersion spectroscopy
Cross-correlation is a well-tested method for exoplanet characterization. A
new, potentially powerful application is the measurement of atmospheric isotope
ratios. In particular D/H can give unique insights into a planet's formation
and evolution. Here we aim to study the detectability of isotopologues in the
high-dispersion spectra of exoplanets, to identify the optimal wavelengths
ranges, and to predict the required observational efforts with current and
future ground-based instruments. High-dispersion (R=10) thermal emission
(and sometimes reflection) spectra were simulated by self-consistently modeling
exoplanet atmospheres over a wide range of temperatures. These were
synthetically observed with telescopes equivalent to the VLT or ELT, and
analyzed with cross-correlation, resulting in S/N predictions for the detection
of CO, HDO, and CHD. For the best observable exoplanets, CO
is in range of current telescopes. It will be most favorably detected at 2.4
microns, just longward of the spectral range probed by several high-dispersion
observations in the literature. CHD can best be seen at 4.7 microns, using
40m-class telescopes for planets with below 600 K. In this case,
sky emission is often dominating the noise. HDO can be targeted at 3.7 microns,
where sky emission is smaller. 40m-class telescopes may detect it in planets
with below 900~K, potentially even 8m-class telescopes in the
case of methane quenching. If Proxima Cen b is water-rich, HDO could be
detected with the ELT in 1 night in reflected light. Isotopologues will soon
belong to the exoplanet characterisation tools. Measuring D/H, and ratios of
other isotopes, could be a prime science case for the METIS instrument on the
ELT, especially for nearby rocky and ice giant planets. This can give unique
insights in their history of ice enrichment and atmospheric evaporation.Comment: 22 pages, 12 figures, updated version, accepted for publication in A
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