100 research outputs found
Anomalous dynamic back-action in interferometers
We analyze the dynamic optomechanical back-action in signal-recycled
Michelson and Michelson-Sagnac interferometers that are operated off dark port.
We show that in this case --- and in contrast to the well-studied canonical
form of dynamic back-action on dark port --- optical damping in a
Michelson-Sagnac interferometer acquires a non-zero value on cavity resonance,
and additional stability/instability regions on either side of the resonance,
revealing new regimes of cooling/heating of micromechanical oscillators. In a
free-mass Michelson interferometer for a certain region of parameters we
predict a stable single-carrier optical spring (positive spring and positive
damping), which can be utilized for the reduction of quantum noise in
future-generation gravitational-wave detectors.Comment: 9 pages, 5 figures. Paper reorganize
Observation of generalized optomechanical coupling and cooling on cavity resonance
Optomechanical coupling between a light field and the motion of a cavity
mirror via radiation pressure plays an important role for the exploration of
macroscopic quantum physics and for the detection of gravitational waves (GWs).
It has been used to cool mechanical oscillators into their quantum ground
states and has been considered to boost the sensitivity of GW detectors, e.g.
via the optical spring effect. Here, we present the experimental
characterization of generalized, that is, dispersive and dissipative
optomechanical coupling, with a macroscopic (1.5mm)^2-sized silicon nitride
(SiN) membrane in a cavity-enhanced Michelson-type interferometer. We report
for the first time strong optomechanical cooling based on dissipative coupling,
even on cavity resonance, in excellent agreement with theory. Our result will
allow for new experimental regimes in macroscopic quantum physics and GW
detection
Tomographic readout of an opto-mechanical interferometer
The quantum state of light changes its nature when being reflected off a
mechanical oscillator due to the latter's susceptibility to radiation pressure.
As a result, a coherent state can transform into a squeezed state and can get
entangled with the motion of the oscillator. The complete tomographic
reconstruction of the state of light requires the ability to readout arbitrary
quadratures. Here we demonstrate such a readout by applying a balanced homodyne
detector to an interferometric position measurement of a thermally excited
high-Q silicon nitride membrane in a Michelson-Sagnac interferometer. A readout
noise of \unit{1.9 \cdot 10^{-16}}{\metre/\sqrt{\hertz}} around the
membrane's fundamental oscillation mode at \unit{133}{\kilo\hertz} has been
achieved, going below the peak value of the standard quantum limit by a factor
of 8.2 (9 dB). The readout noise was entirely dominated by shot noise in a
rather broad frequency range around the mechanical resonance.Comment: 7 pages, 5 figure
Interferometer readout-noise below the Standard Quantum Limit of a membrane
Here we report on the realization of a Michelson-Sagnac interferometer whose
purpose is the precise characterization of the motion of membranes showing
significant light transmission. Our interferometer has a readout noise spectral
density (imprecision) of 3E-16 m/sqrt(Hz) at frequencies around the fundamental
resonance of a SiN_x membrane at about 100 kHz, without using optical cavities.
The readout noise demonstrated is more than 16 dB below the peak value of the
membrane's standard quantum limit (SQL). This reduction is significantly higher
than those of previous works with nano-wires [Teufel et al., Nature Nano. 4,
820 (2009); Anetsberger et al., Nature Phys. 5, 909 (2009)]. We discuss the
meaning of the SQL for force measurements and its relation to the readout
performance and conclude that neither our nor previous experiments achieved a
total noise spectral density as low as the SQL
Optical absorption of silicon nitride membranes at 1064 nm and at 1550 nm
Because of a low mechanical loss, thin films made of silicon nitride (Si3N4) are interesting for
fundamental research and development in the field of gravitational-wave detection. Si3N4 membranes
allow for the characterization of quantum radiation pressure noise (RPN), which will be a limiting noise
source in gravitational-wave detectors of the second and third generations. Furthermore, Si3N4 is an
interesting material for possible thermal noise reduction in highly reflective mirror coatings. For both
applications, the optical absorption of Si3N4 needs to be low. This paper presents absorption measurements
on low-stress Si3N4 membranes showing an absorption a factor of 7 lower at 1550 nm than at 1064 nm
resulting in an estimated 2 times higher sensitivity in RPN experiments at the higher wavelength and
making Si3N4 an interesting material for highly reflective multimaterial mirror coatings at 1550 nm
HD 144432: a young triple system
We present new imaging and spectroscopic data of the young Herbig star HD
144432 A, which was known to be a binary star with a separation of 1.47 arcsec.
High-resolution NIR imaging data obtained with NACO at the VLT reveal that HD
144432 B itself is a close binary pair with a separation of 0.1 arcsec.
High-resolution optical spectra, acquired with FEROS at the 2.2m MPG/ESO
telescope in La Silla, of the primary star and its co-moving companions were
used to determine their main stellar parameters such as effective temperature,
surface gravity, radial velocity, and projected rotational velocity by fitting
synthetic spectra to the observed stellar spectra. The two companions, HD
144432 B and HD 144432 C, are identified as low-mass T Tauri stars of spectral
type K7V and M1V, respectively. From the position in the HRD the triple system
appears to be co-eval with a system age of 6+/-3 Myr.Comment: Accepted for publication in Astronomy & Astrophysics, 4 pages, 4
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Rotational velocities of nearby young stars
Stellar rotation is a crucial parameter driving stellar magnetism, activity
and mixing of chemical elements. Furthermore, the evolution of stellar rotation
is coupled to the evolution of circumstellar disks. Disk-braking mechanisms are
believed to be responsible for rotational deceleration during the accretion
phase, and rotational spin-up during the contraction phase after decoupling
from the disk for fast rotators arriving at the ZAMS. We investigate the
projected rotational velocities vsini of a sample of young stars with respect
to the stellar mass and disk evolutionary state to search for possible
indications of disk-braking mechanisms. We analyse the stellar spectra of 220
nearby (mostly <100pc) young (2-600 Myr) stars for their vsini, stellar age,
Halpha emission, and accretion rates. The stars have been observed with FEROS
and HARPS in La Silla, Chile. The spectra have been cross-correlated with
appropriate theoretical templates. We build a new calibration to be able to
derive vsini values from the cross-correlated spectra. Stellar ages are
estimated from the LiI equivalent width at 6708 Ang. The equivalent width and
width at 10% height of the Halpha emission are measured to identify accretors
and used to estimate accretion rates. The vsini is then analysed with respect
to the evolutionary state of the circumstellar disks to search for indications
of disk-braking mechanisms in accretors. We find that the broad vsini
distribution of our targets extends to rotation velocities of up to more than
100 km/s and peaks at a value of 7.8+-1.2 km/s, and that ~70% of our stars show
vsini<30 km/s. Furthermore, we can find indications for disk-braking in
accretors and rotational spin-up of stars which are decoupled from their disks.
In addition, we show that a number of young stars are suitable for precise
radial-velocity measurements for planet-search surveys.Comment: 16 pages, 6 figures, accepted for publication in A&
Optical spectroscopy of EX Lupi during quiescence and outburst: Infall, wind, and dynamics in the accretion flow
We explore the accretion mechanisms in EX Lupi, prototype of EXor variables,
during its quiescence and outburst phases. We analyse high-resolution optical
spectra taken before, during, and after its 2008 outburst. In quiescence and
outburst, the star presents many permitted emission lines, including typical
CTTS lines and numerous neutral and ionized metallic lines. During the
outburst, the number of emission lines increases to over a thousand, with
narrow plus broad component structure (NC+BC). The BC profile is highly
variable on short timescales (24-72h). An active chromosphere can explain the
metallic lines in quiescence and the outburst NC. The dynamics of the BC line
profiles suggest an origin in a hot, dense, non-axisymmetric, and non-uniform
accretion column that suffers velocity variations along the line-of-sight on
timescales of days. Assuming Keplerian rotation, the emitting region would be
located at ~0.1-0.2 AU, consistent with the inner disk rim, but the velocity
profiles of the lines reveal a combination of rotation and infall. Line ratios
of ions and neutrals can be reproduced with a temperature of T~6500 K for
electron densities of a few times 10cm in the line-emitting
region. The data confirm that the 2008 outburst was an episode of increased
accretion, albeit much stronger than previous EX Lupi and typical EXors
outbursts. The line profiles are consistent with the infall/rotation of a
non-axisymmetric structure that could be produced by clumpy accretion during
the outburst phase. A strong inner disk wind appears in the epochs of higher
accretion. The rapid recovery of the system after the outburst and the
similarity between the pre-outburst and post-outburst states suggest that the
accretion channels are similar during the whole period, and only the accretion
rate varies, providing a superb environment for studying the accretion
processes.Comment: 15 pages plus 26 pages online material, accepted by A&
EX Lupi in quiescence
EX Lup is the prototype of EXors, a subclass of low-mass pre-main sequence
stars whose episodic eruptions are attributed to temporarily increased
accretion. In quiescence the optical and near-infrared properties of EX Lup
cannot be distinguished from those of normal T Tau stars. Here we investigate
whether it is the circumstellar disk structure which makes EX Lup an atypical
Class II object. During outburst the disk might undergo structural changes. Our
characterization of the quiescent disk is intended to serve as a reference to
study the physical changes related to one of EX Lupi's strongest known
eruptions in 2008 Jan-Sep. We searched the literature for photometric and
spectroscopic observations including ground-based, IRAS, ISO and Spitzer data.
After constructing the optical-infrared spectral energy distribution (SED), we
compared it with the typical SEDs of other young stellar objects and modeled it
using the Monte Carlo radiative transfer code RADMC. A mineralogical
decomposition of the 10 micron silicate emission feature and also the
description of the optical and near-infrared spectra were performed. The SED is
in general similar to that of a typical T Tauri star, though above 7 micron EX
Lup emits higher flux. The quiescent phase data suggest low level variability
in the optical-mid-infrared domain. Integrating the optical and infrared fluxes
we derived a bolometric luminosity of 0.7 L_Sun. The 10 micron silicate profile
could be fitted by a mixture consisting of amorphous silicates, no crystalline
silicates were found. A modestly flaring disk model with a total mass of 0.025
M_Sun and an outer radius of 150 AU was able to reproduce the observed SED. The
derived inner radius of 0.2 AU is larger than the sublimation radius, and this
inner gap sets EX Lup aside from typical T Tauri stars.Comment: Accepted for publication in Astronomy and Astrophysics, 10 pages, 6
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