16 research outputs found
Noise Sources in Photometry and Radial Velocities
The quest for Earth-like, extrasolar planets (exoplanets), especially those
located inside the habitable zone of their host stars, requires techniques
sensitive enough to detect the faint signals produced by those planets. The
radial velocity (RV) and photometric transit methods are the most widely used
and also the most efficient methods for detecting and characterizing
exoplanets. However, presence of astrophysical "noise" makes it difficult to
detect and accurately characterize exoplanets. It is important to note that the
amplitude of such astrophysical noise is larger than both the signal of
Earth-like exoplanets and state-of-the-art instrumentation limit precision,
making this a pressing topic that needs to be addressed. In this chapter, I
present a general review of the main sources of noise in photometric and RV
observations, namely, stellar oscillations, granulation, and magnetic activity.
Moreover, for each noise source I discuss the techniques and observational
strategies which allow us to mitigate their impact.Comment: 11 pages, 2 tables, Lecture presented at the IVth Azores
International Advanced School in Space Sciences on "Asteroseismology and
Exoplanets: Listening to the Stars and Searching for New Worlds"
(arXiv:1709.00645), which took place in Horta, Azores Islands, Portugal in
July 201
Astrobiologically Interesting Stars within 10 parsecs of the Sun
The existence of life based on carbon chemistry and water oceans relies upon
planetary properties, chiefly climate stability, and stellar properties, such
as mass, age, metallicity and Galactic orbits. The latter can be well
constrained with present knowledge. We present a detailed, up-to-date
compilation of the atmospheric parameters, chemical composition, multiplicity
and degree of chromospheric activity for the astrobiologically interesting
solar-type stars within 10 parsecs of the Sun. We determine their state of
evolution, masses, ages and space velocities, and produce an optimized list of
candidates that merit serious scientific consideration by the future
space-based interferometry probes aimed at directly detecting Earth-sized
extrasolar planets and seeking spectroscopic infrared biomarkers as evidence of
photosynthetic life. The initially selected stars number 33 solar-type within
the population of 182 stars (excluding late M-dwarfs) closer than 10 pc. A
comprehensive and detailed data compilation for these objects is still
essentially lacking: a considerable amount of recent data has so far gone
unexplored in this context. We present 13 objects as the nearest "biostars",
after eliminating multiple stars, young, chromospherically active, hard X-ray
emitting stars, and low metallicity objects. Three of these "biostars", HD
1581, 109358 and 115617, closely reproduce most of the solar properties and are
considered as premier targets. We show that approximately 7% of the nearby
stars are optimally interesting targets for exobiology.Comment: 36 pages, recommended for publication in Astrobiolog
When Do Stalled Stars Resume Spinning Down? Advancing Gyrochronology with Ruprecht 147
Recent measurements of rotation periods () in the benchmark open clusters Praesepe (670 Myr), NGC 6811 (1 Gyr), and NGC 752 (1.4 Gyr) demonstrate that, after converging onto a tight sequence of slowly rotating stars in mass-period space, stars temporarily stop spinning down. These data also show that the duration of this epoch of stalled spin-down increases toward lower masses. To determine when stalled stars resume spinning down, we use data from the K2 mission and the Palomar Transient Factory to measure for 58 dwarf members of the 2.7 Gyr old cluster Ruprecht 147, 39 of which satisfy our criteria designed to remove short-period or near-equal-mass binaries. Combined with the Kepler data for the approximately coeval cluster NGC 6819 (30 stars with M ∗ > 0.85, our new measurements more than double the number of ≈2.5 Gyr benchmark rotators and extend this sample down to ≈0.55. The slowly rotating sequence for this joint sample appears relatively flat (22 ± 2 days) compared to sequences for younger clusters. This sequence also intersects the Kepler intermediate-period gap, demonstrating that this gap was not created by a lull in star formation. We calculate the time at which stars resume spinning down and find that 0.55 stars remain stalled for at least 1.3 Gyr. To accurately age-date low-mass stars in the field, gyrochronology formulae must be modified to account for this stalling timescale. Empirically tuning a core-envelope coupling model with open cluster data can account for most of the apparent stalling effect. However, alternative explanations, e.g., a temporary reduction in the magnetic braking torque, cannot yet be ruled out
Stellar activity cycles and contribution of the deep layers knowledge
It is believed that magnetic activity on the Sun and solar-type stars are
tightly related to the dynamo process driven by the interaction between
rotation, convection, and magnetic field. However, the detailed mechanisms of
this process are still incompletely understood. Many questions remain
unanswered, e.g.: why some stars are more active than others?; why some stars
have a flat activity?; why is there a Maunder minimum?; are all the cycles
regular? A large number of prox- ies are typically used to study the magnetic
activity of stars as we cannot resolve stellar discs. Recently, it was shown
that asteroseismology can also be used to study stellar activity, making it an
even more powerful tool. If short cycles are not so un- common, we expect to
detect many of them with missions such as CoRoT, Kepler, and possibly the PLATO
mission. We will review some of the latest results obtained with spectroscopic
measurements. We will show how asteroseismology can help us to better
understand the complex process of dynamo and illustrate how the CoRoT and
Kepler missions are revolutionizing our knowledge on stellar activity. A new
window is being opened over our understanding of the magnetic variability of
stars.Comment: 7 pages. To appear in Astrophysics and Space Science Proceedings
series of the 20th Stellar pulsation conference held in Granada (Spain) from
6 to 10 September 2011
Cooler and Hotter X-ray Bright Points from Hinode/XRT Observations
International audienceWe use a 7-hour (17:00 UT - 24:00 UT) time sequence of soft X-ray images observed almost simultaneously in two filters (Ti_poly and Al_mesh) on April 14, 2007 with X-Ray Telescope (XRT) on-board the Hinode mission to determine the temperature of X-ray bright points (XBPs). A sample of 14 XBPs and 2 background coronal regions have been identified and selected on both the images for detailed analysis. The temperature of XBPs is determined by filter ratio method. We find that the XBPs show temperature fluctuations and that the average temperature ranges from 1.1 MK to 3.4 MK which may correspond to different X-ray fluxes. These results suggest the existence of cooler and hotter XBPs and that the heating rate of XBPs is highly variable on short time scales
Cooler and Hotter X-ray Bright Points from Hinode/XRT Observations
International audienceWe use a 7-hour (17:00 UT - 24:00 UT) time sequence of soft X-ray images observed almost simultaneously in two filters (Ti_poly and Al_mesh) on April 14, 2007 with X-Ray Telescope (XRT) on-board the Hinode mission to determine the temperature of X-ray bright points (XBPs). A sample of 14 XBPs and 2 background coronal regions have been identified and selected on both the images for detailed analysis. The temperature of XBPs is determined by filter ratio method. We find that the XBPs show temperature fluctuations and that the average temperature ranges from 1.1 MK to 3.4 MK which may correspond to different X-ray fluxes. These results suggest the existence of cooler and hotter XBPs and that the heating rate of XBPs is highly variable on short time scales
Ultra-luminous X-Ray Sources in HARO II and the Role of X-Ray Binaries in Feedback in Lyα Emitting Galaxies
Lyman Break Analogs (LBAs) are local proxies of high-redshift Lyman Break Galaxies. Spatially resolved studies of nearby starbursts have shown that Lyman continuum and line emission are absorbed by dust and that the Lyα is resonantly scattered by neutral hydrogen. In order to observe Lyα emission from star-forming regions, some source of feedback is required to blow the neutral gas away from the starburst to prevent scattering and allow the Lyα emission to escape. We show that there are two X-ray point sources embedded in the diffuse emission of the LBA galaxy Haro 11. CXOU J003652.4-333316 (abbreviated to Haro 11 X-1) is an extremely luminous (L erg s−1), spatially compact source with a hard-X-ray spectrum. We suggest that the X-ray emission from Haro 11 X-1 is dominated by a single accretion source. This might be an active galactic nucleus or a source similar to the extreme black hole binary (BHB) M82 X-1. The hard X-ray spectrum indicates that Haro 11 X-1 may be a BHB in a low accretion state. In this case, the very high X-ray luminosity suggests an intermediate mass black hole that could be the seed for formation of a supermassive black hole. Source CXOU J003652.7-33331619.5 (abbreviated Haro 11 X-2) has an X-ray luminosity of erg s−1 and a soft X-ray spectrum (power-law photon index Γ ~ 2.2). This strongly suggests that Haro 11 X-2 is an X-ray binary in the ultra luminous state (i.e., an Ultra Luminous X-ray source, ULX). Haro 11 X-2 is coincident with the star-forming knot that is the source of the Lyα emission. The association of a ULX with Lyα emission raises the possibility that strong winds from X-ray binaries play an important role in injecting mechanical power into the interstellar medium, thus blowing away neutral material from the starburst region and allowing the Lyα to escape. We suggest that feedback from X-ray binaries may play a significant role in allowing Lyα emission to escape from galaxies in the early universe
The Influence of Initial Conditions on Stellar Rotation History
International audienceThe question of whether the effects of different initial states persist during the main sequence is important, since it could shed light on the pre-main sequence phase
Asteroseismology and spectropolarimetry of the exoplanet host star Lambda Serpentis
Stars and planetary system