614 research outputs found
Microlensing Events from Measurements of the Deflection Angle
Microlensing events are now regularly being detected by monitoring the flux
of a large number of potential sources and measuring the combined magnification
of the images. This phenomenon could also be detected directly from the
gravitational deflection, by means of high precision astrometry using
interferometry. Relative astrometry at the level of 10\muas may become
possible in the near future. The gravitational deflection can be measured by
astrometric monitoring of a bright star having a background star within a small
angular separation. This type of monitoring program will be carried out for the
independent reasons of discovering planets from the angular motion they induce
on the nearby star around which they are orbiting, and for measuring
parallaxes, proper motions and orbits of binary stars. We discuss three
applications of the measurement of gravitational deflections by astrometric
monitoring: measuring the mass of the bright stars that are monitored,
measuring the mass of brown dwarfs or giant planets around the bright stars,
and detecting microlensing events by unrelated objects near the line of sight
to the two stars. We discuss the number of stars whose mass could be measured
by this procedure. We also give expressions for the number of expected
microlensing events by unrelated objects, which could be stars, brown dwarfs,
or other compact objects accounting for dark matter in the halo or in the disk.Comment: submitted to ApJ Letter
Fluctuations of the intergalactic UV background towards two lines of sight
We present a reanalysis of the HeII Lyman alpha absorption towards the
quasars HS1700+6416 and HE2347-4342 using new high S/N, optical observations.
An alternative analysis method is applied, which fits the high quality, optical
HI data directly to the HeII spectrum. The results are compared to those
inferred from standard line profile analyses. This new method enables us to
derive redshift scales characterizing the fluctuations of the column density
ratio eta. We find eta changing smoothly with redshift on typical scales of
Delta z ~ 0.01-0.03 corresponding to 8-24 h^-1 Mpc comoving. The real length
scales of variations of the column density ratio might be even larger, since
part of the fluctuations may be caused by noise in the HeII data and by effects
due to the applied method. However, eta variations on small scales of a few Mpc
with an amplitude of about +/- 1.5 dex cannot be ruled out completely. The data
shows an apparent correlation between low eta regions and the presence of metal
line absorbers, which corresponds to the more general correlation of low eta
and strong HI absorption. Thermal line broadening is suggested as a probable
explanation for this apparent correlation, since both fit methods would
severely underestimate eta for absorbers with log N(HI) > 13 if the line width
was dominated by thermal broadening. Indeed, lines located close to the cut-off
of the b(N) distribution yield lower column density ratios compared to the
whole sample, in particular if high density absorbers are considered. We argue
that the apparent correlation of eta with the strength of the HI absorption is
caused by insufficient consideration of thermal broadened lines by the standard
analysis. As unbiased value of the column density ratio, we find eta ~80 in
agreement with previous estimates.Comment: 15 pages, 11 figures, recommended for publication in A&
High-cadence spectroscopy of M-dwarfs – II. Searching for stellar pulsations with HARPS
Stellar oscillations appear all across the Hertzsprung–Russell diagram. Recent theoretical studies support their existence also in the atmosphere of M dwarfs. These studies predict for them short periodicities ranging from 20 min to 3 h. Our Cool Tiny Beats (CTB) programme aims at finding these oscillations for the very first time. With this goal, CTB explores the short time domain of M dwarfs using radial velocity data from the High Accuracy Radial velocity Planet Searcher (HARPS)-European Southern Observatory and HARPS-N high-precision spectrographs. Here we present the results for the two most long-term stable targets observed to date with CTB, GJ 588 and GJ 699 (i.e. Barnard's star). In the first part of this work we detail the correction of several instrumental effects. These corrections are especially relevant when searching for subnight signals. Results show no significant signals in the range where M dwarfs pulsations were predicted. However, we estimate that stellar pulsations with amplitudes larger than ∼0.5 m s−1 can be detected with a 90 per cent completeness with our observations. This result, along with the excess of power regions detected in the periodograms, opens the possibility of non-resolved very low amplitude pulsation signals. Next generation more precise instrumentation would be required to detect such oscillations. However, the possibility of detecting pulsating M-dwarf stars with larger amplitudes is feasible due to the short size of the analysed sample. This motivates the need for completeness of the CTB survey
The origin of the excess transit absorption in the HD 189733 system: planet or star?
We have detected excess absorption in the emission cores of Ca II H&K during transits of HD 189733b for the first time. Using observations of three transits, we investigate the origin of the absorption, which is also seen in Hα and the Na I D lines. Applying differential spectrophotometry methods to the Ca II H and Ca II K lines combined, using respective passband widths of Δλ = 0.4 and 0.6 Å yields excess absorption of td = 0.0074 ± 0.0044 (1.7σ; Transit 1) and 0.0214 ± 0.0022 (9.8σ; Transit 2). Similarly, we detect excess Hα absorption in a passband of width Δλ = 0.7 Å, with td = 0.0084 ± 0.0016 (5.2σ) and 0.0121 ± 0.0012 (9.9σ). For both lines, Transit 2 is thus significantly deeper. Combining all three transits for the Na I D lines yields excess absorption of td = 0.0041 ± 0.0006 (6.5σ). By considering the time series observations of each line, we find that the excess apparent absorption is best recovered in the stellar reference frame. These findings lead us to postulate that the main contribution to the excess transit absorption in the differential light curves arises because the normalizing continuum bands form in the photosphere, whereas the line cores contain a chromospheric component. We cannot rule out that part of the excess absorption signature arises from the planetary atmosphere, but we present evidence which casts doubt on recent claims to have detected wind motions in the planet's atmosphere in these data
A capture approach for supercoiled plasmid DNA using a triplex-forming oligonucleotide
This is the final version of the article. Available from the publisher via the DOI in this record.Proteins that recognize and bind specific sites in DNA are essential for regulation of numerous biological functions. Such proteins often require a negative supercoiled DNA topology to function correctly. In current research, short linear DNA is often used to study DNA-protein interactions. Although linear DNA can easily be modified, for capture on a surface, its relaxed topology does not accurately resemble the natural situation in which DNA is generally negatively supercoiled. Moreover, specific binding sequences are flanked by large stretches of non-target sequence in vivo. Here, we present a straightforward method for capturing negatively supercoiled plasmid DNA on a streptavidin surface. It relies on the formation of a temporary parallel triplex, using a triple helix forming oligonucleotide containing locked nucleic acid nucleotides. All materials required for this method are commercially available. Lac repressor binding to its operator was used as model system. Although the dissociation constants for both the linear and plasmid-based operator are in the range of 4 nM, the association and dissociation rates of Lac repressor binding to the plasmid-based operator are ~18 times slower than on a linear fragment. This difference underscores the importance of using a physiologically relevant DNA topology for studying DNA-protein interactions.Netherlands Organisation for Scientific Research and the
Netherlands Institute for Space Research [ALW-GO-PL/
08-08]; NWO Vidi grant [864.11.005 to S.J.J.B.]. Funding
for open access charge: Microbiology department/
Wageningen UR library
AD Leonis: Radial Velocity Signal of Stellar Rotation or Spin–Orbit Resonance?
AD Leonis is a nearby magnetically active M dwarf. We find Doppler variability with a period of 2.23 days, as well as photometric signals: (1) a short-period signal, which is similar to the radial velocity signal, albeit with considerable variability; and (2) a long-term activity cycle of 4070 ± 120 days. We examine the short-term photometric signal in the available All-Sky Automated Survey and Microvariability and Oscillations of STars (MOST) photometry and find that the signal is not consistently present and varies considerably as a function of time. This signal undergoes a phase change of roughly 0.8 rad when considering the first and second halves of the MOST data set, which are separated in median time by 3.38 days. In contrast, the Doppler signal is stable in the combined High-Accuracy Radial velocity Planet Searcher and High Resolution Echelle Spectrometer radial velocities for over 4700 days and does not appear to vary in time in amplitude, phase, period, or as a function of extracted wavelength. We consider a variety of starspot scenarios and find it challenging to simultaneously explain the rapidly varying photometric signal and the stable radial velocity signal as being caused by starspots corotating on the stellar surface. This suggests that the origin of the Doppler periodicity might be the gravitational tug of a planet orbiting the star in spin–orbit resonance. For such a scenario and no spin–orbit misalignment, the measured v sin i indicates an inclination angle of 15°̣5 ± 2°̣5 and a planetary companion mass of 0.237 ± 0.047 M Jup
Analysis of apsidal motion in eclipsing binaries using TESS data: I. A test of gravitational theories
The change in the argument of periastron of eclipsing binaries, i.e., the
apsidal motion caused by classical and relativistic effects, can be measured
from variations in the difference between the time of minimum light of the
primary and secondary eclipses. Poor apsidal motion rate determinations and
large uncertainties in the classical term have hampered previous attempts to
determine the general relativistic term with sufficient precision to test
General Relativity predictions.
As a product of the TESS mission, thousands of high-precision light curves
from eclipsing binaries are now available. Using a selection of suitable
well-studied eccentric eclipsing binary systems, we aim to determine their
apsidal motion rates and place constraints on key gravitational parameters.
We compute the time of minimum light from the TESS light curves of 15
eclipsing binaries with precise absolute parameters and with an expected
general relativistic contribution to the total apsidal motion rate greater than
60%. We use the changing primary and secondary eclipse timing differences over
time to compute the apsidal motion rate, when possible, or the difference
between the linear periods as computed from primary and secondary eclipses. For
a greater time baseline we carefully combine the high-precision TESS timings
with archival reliable timings.
We determine the apsidal motion rate of 9 eclipsing binaries, 5 of which are
reported for the first time. From these, we are able to measure the general
relativistic apsidal motion rate of 6 systems with sufficient precision to test
General Relativity for the first time using this method. This test explores a
regime of gravitational forces and potentials that had not been probed earlier.
We find perfect agreement with the theoretical predictions, and we are able to
set stringent constraints on two parameters of the parametrised post-Newtonian
formalism.Comment: Accepted for publication in A&
High-cadence spectroscopy of M dwarfs – I. Analysis of systematic effects in HARPS-N line profile measurements on the bright binary GJ 725A+B
Understanding the sources of instrumental systematic noise is a must to improve the design of future spectrographs. In this study, we alternated observations of the well-suited pair of M-stars GJ 725A+B to delve into the sub-night High Accuracy Radial Velocity Planet Searcher for the Northern hemisphere (HARPS-N) response. Besides the possible presence of a low-mass planet orbiting GJ 725B, our observations reveal changes in the spectral energy distribution (SED) correlated with measurements of the width of the instrumental line profile and, to a lower degree, with the Doppler measurements. To study the origin of these effects, we searched for correlations among several quantities defined and measured on the spectra and on the acquisition images. We find that the changes in apparent SED are very likely related to flux losses at the fibre input. Further tests indicate that such flux losses do not seriously affect the shape of the instrumental point spread function of HARPS-N, but identify an inefficient fitting of the continuum as the most likely source of the systematic variability observed in the full width at half-maximum. This index, accounting for the HARPS-N cross-correlation profiles width, is often used to decorrelate Doppler time series. We show that the Doppler measurement obtained by a parametric least-squares fitting of the spectrum accounting for continuum variability is insensitive to changes in the slope of the SED, suggesting that forward modelling techniques to measure moments of the line profile are the optimal way to achieve higher accuracy. Remaining residual variability at ∼1 m s−1 suggests that for M-stars Doppler surveys the current noise floor still has an instrumental origin
Formation of the Black Holes in the Highest Redshift Quasars
The recent discovery of luminous quasars up to a redshift z=6.43 has renewed
interest in the formation of black holes massive enough to power quasars. If
black holes grow by Eddington-limited gas accretion with a radiative efficiency
of at least 10%, the time required to grow from a stellar black hole to ~10^9
msun is ~10^9 years, close to the age of the universe at z=6. Black hole
mergers may accelerate the rate of mass growth, but can also completely eject
black holes from halo centers owing to the gravitational wave recoil effect.
Recently, Haiman concluded that black hole ejections likely do not allow black
holes to grow to ~10^9 msun by z=6.43. We reexamine this problem and show that,
by using a different halo escape velocity, accounting for the dependence of the
recoil velocity on the black hole binary mass ratio and spins, and allowing
seed black holes to form in all halos down to virial temperatures of 2000 K,
black hole masses may reach ~10^9 msun as early as z=9 starting from stellar
seeds, without super-Eddington accretion. In this particular case, we find that
these massive black holes form from the merger of ~10^4 stellar black holes
formed in low-mass halos at z~20, which must all grow close to the maximum
Eddington rate over most of the time available from their birth to z~6. The
alternative is that black holes can grow more rapidly by super-Eddington
accretion.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
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