arXiv subjects

J. Bouvier

Publications and source records attributed to J. Bouvier.

At least 73 records · Page 4Linked to original sources

arXiv2017

Near-Infrared Time-Series Photometry in the Field of Cygnus OB2 Association I - Rotational Scenario For Candidate Members

In the last decades, the early pre main sequence stellar rotational evolution picture has been constrained by studies targeting different young regions at a variety of ages. Observational studies suggest a mass-rotation dependence, and for some mass ranges a connection between rotation and the presence of a circumstellar disk. Not still fully explored, though, is the role of environmental conditions on the rotational regulation. We investigate the rotational properties of candidate members of the young massive association Cygnus OB2. The Stetson variability index, Lomb-Scargle periodogram, Saunders statistics, string/rope length method, and visual verification of folded light curves were applied to select 1224 periodic variable stars. Completeness and contamination of the periodic sample was derived from Monte Carlo simulations, out of which 894 periods were considered reliable. Our study was considered reasonably complete for periods from 2 to 30 days. The general rotational scenario seen in other young regions is confirmed by Cygnus OB2 period distributions, with disked stars rotating on average slower than non-disked stars. A mass-rotation dependence was also verified, but as in NGC 6530, lower mass stars are rotating on average slower than higher mass stars, with an excess of slow rotators among the lower mass population. The effect of the environment on the rotational properties of the association was investigated by re-analysing the results while taking into account the incident UV radiation arising from O stars in the association. Results compatible with the disk-locking scenario were verified for stars with low UV incidence, but no statistical significant relation between rotation and disk presence was verified for stars with high UV incidence suggesting that massive stars can have an important role on regulating the rotation of nearby low mass stars.

astro-ph.SR↗

arXiv2017

Rotation of Late-Type Stars in Praesepe with K2

We have Fourier analyzed 941 K2 light curves of likely members of Praesepe, measuring periods for 86% and increasing the number of rotation periods (P) by nearly a factor of four. The distribution of P vs. (V-K), a mass proxy, has three different regimes: (V-K)<1.3, where the rotation rate rapidly slows as mass decreases; 1.3<(V-K)<4.5, where the rotation rate slows more gradually as mass decreases; and (V-K)>4.5, where the rotation rate rapidly increases as mass decreases. In this last regime, there is a bimodal distribution of periods, with few between $\sim$2 and $\sim$10 days. We interpret this to mean that once M stars start to slow down, they do so rapidly. The K2 period-color distribution in Praesepe ($\sim$790 Myr) is much different than in the Pleiades ($\sim$125 Myr) for late F, G, K, and early-M stars; the overall distribution moves to longer periods, and is better described by 2 line segments. For mid-M stars, the relationship has similarly broad scatter, and is steeper in Praesepe. The diversity of lightcurves and of periodogram types is similar in the two clusters; about a quarter of the periodic stars in both clusters have multiple significant periods. Multi-periodic stars dominate among the higher masses, starting at a bluer color in Praesepe ((V-K)$\sim$1.5) than in the Pleiades ((V-K)$\sim$2.6). In Praesepe, there are relatively more light curves that have two widely separated periods, $ΔP >$6 days. Some of these could be examples of M star binaries where one star has spun down but the other has not.

astro-ph.SR↗

arXiv2017

A hot Jupiter around the very active weak-line T Tauri star TAP 26

We report the results of an extended spectropolarimetric and photometric monitoring of the weak-line T Tauri star TAP 26, carried out within the MaTYSSE programme with the ESPaDOnS spectropolarimeter at the 3.6 m Canada-France-Hawaii Telescope. Applying Zeeman-Doppler Imaging to our observations, concentrating in 2015 November and 2016 January and spanning 72 d in total, 16 d in 2015 November and 13 d in 2016 January, we reconstruct surface brightness and magnetic field maps for both epochs and demonstrate that both distributions exhibit temporal evolution not explained by differential rotation alone. We report the detection of a hot Jupiter (hJ) around TAP 26 using three different methods, two using Zeeman-Doppler Imaging (ZDI) and one Gaussian-Process Regression (GPR), with a false-alarm probability smaller than 6.10^-4. However, as a result of the aliasing related to the observing window, the orbital period cannot be uniquely determined; the orbital period with highest likelihood is 10.79 +/- 0.14 d followed by 8.99 +/- 0.09 d. Assuming the most likely period, and that the planet orbits in the stellar equatorial plane, we obtain that the planet has a minimum mass M.sin(i) of 1.66 +/- 0.31 M_Jup and orbits at 0.0968 +/- 0.0032 au from its host star. This new detection suggests that disc type II migration is efficient at generating newborn hJs, and that hJs may be more frequent around young T Tauri stars than around mature stars (or that the MaTYSSE sample is biased towards hJ-hosting stars).

astro-ph.SR↗

arXiv2016

Investigating the rotational evolution of very low-mass stars and brown dwarfs in young clusters using Monte Carlo simulations

Context. Very low-mass (VLM) stars and brown dwarfs (BDs) present a different rotational behaviour from their solar mass counter-parts. Aims. We investigate the rotational evolution of young VLM stars and BDs using Monte Carlo simulations under the hypothesis of disk locking and stellar angular momentum conservation. Methods. We built a set of objects with masses ranging from 0.01 Mo to 0.4 Mo and considered models with single- and double- peaked initial period distributions with and without disk locking. An object is considered to be diskless when its mass accretion rate is below a given threshold. Results. Models with initial single-peaked period distributions reproduce the observations well given that BDs rotate faster than VLM stars. We observe a correlation between rotational period and mass when we relax the disk locking hypothesis, but with a shallower slope compared to some observational results. The angular momentum evolution of diskless stars is flatter than it is for stars with a disk which occurs because the moment of inertia of objects less massive than 0.2 Mo remains pratically constant for a time scale that increases with decreasing stellar mass. Conclusions. Comparing our results with the available observational data we see that disk locking is not as important in the low-mass regime and that the rotational behaviour of VLM stars and BDs is different from what is seen in their solar mass counterparts.

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arXiv2016

The hot Jupiter of the magnetically-active weak-line T Tauri star V830 Tau

We report results of an extended spectropolarimetric and photometric monitoring of the weak-line T Tauri star V830 Tau and its recently-detected newborn close-in giant planet. Our observations, carried out within the MaTYSSE programme, were spread over 91d, and involved the ESPaDOnS and Narval spectropolarimeters linked to the 3.6m Canada-France-Hawaii, the 2m Bernard Lyot and the 8-m Gemini-North Telescopes. Using Zeeman-Doppler Imaging, we characterize the surface brightness distributions, magnetic topologies and surface differential rotation of V830 Tau at the time of our observations, and demonstrate that both distributions evolve with time beyond what is expected from differential rotation. We also report that near the end of our observations, V830 Tau triggered one major flare and two weaker precursors, showing up as enhanced red-shifted emission in multiple spectral activity proxies. With 3 different filtering techniques, we model the radial velocity (RV) activity jitter (of semi-amplitude 1.2km/s) that V830 Tau generates, successfully retrieve the 68m/s RV planet signal hiding behind the jitter, further confirm the existence of V830 Tau b and better characterize its orbital parameters. We find that the method based on Gaussian-process regression performs best thanks to its higher ability at modelling not only the activity jitter, but also its temporal evolution over the course of our observations, and succeeds at reproducing our RV data down to a rms precision of 35m/s. Our result provides new observational constraints on scenarios of star / planet formation and demonstrates the scientific potential of large-scale searches for close-in giant planets around T Tauri stars.

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arXiv2016

Violent environment of the inner disk of RW Aur A probed by the 2010 and 2015 dimming events

RW Aur is a young binary system showing strong signatures of a recent tidal encounter between the circumprimary disk and the secondary star. The primary star has recently undergone two major dimming events ($Δ$mag $\approx$ 2 in V-band), whose origin is still under debate. To shed light on the mechanism leading to the dimming events, we study the extinction properties, accretion variability, and gas kinematics using absorption lines from the material obscuring star RW Aur A. We compare our moderate resolution X-Shooter spectra of the dim state of RW Aur A with other spectral observations. In particular, we analyse archival high resolution UVES spectra obtained during the bright state of the system, in order to track the evolution of the spectral properties across the second dimming event. The spectrum obtained during the dim state shows narrow absorption lines in the Na and K optical doublets, where the former is saturated. With a velocity of -60 km/s these lines indicate that during the dim state the disk wind is either enhanced, or significantly displaced into the line of sight. The photometric evolution across the dimming event shows a gray extinction, and is correlated with a significant reduction of the EW of all photospheric lines. Emission lines tracing accretion do not vary significantly across the dimming. We conclude that the dimming event is related to a major perturbation on the inner disk. We suggest that the inner disk is occulting (most of) the star, and thus its photosphere, but is not occulting the accretion regions within a few stellar radii. Since observations of the outer disk indicate that the disk is modestly inclined (45 - 60 deg), we propose that the inner disk might be warped by a yet unseen (sub-)stellar companion, which may also explain the 2.77 day periodic variability of the spectral lines.

astro-ph.SR↗

arXiv2016

CSI 2264: Investigating rotation and its connection with disk accretion in the young open cluster NGC 2264

The low spin rates measured for solar-type stars at an age of a few Myr (~10% of the break-up velocity) indicate that some mechanism of angular momentum regulation must be at play in the early pre-main sequence. We characterize the rotation properties for members of the region NGC 2264 (~3 Myr), and investigate the accretion-rotation connection at an age where about 50% of the stars have already lost their disks. We examined a sample of 500 cluster members whose photometric variations were monitored in the optical for 38 consecutive days with CoRoT. Light curves were analyzed for periodicity using the Lomb-Scargle periodogram, the autocorrelation function and the string-length method. The period distribution obtained for the cluster consists of a smooth distribution centered around P=5.2 d with two peaks at P=1-2 d and 3-4 d. A separate analysis of CTTS and WTTS indicates that the P=1-2 d peak is associated with the latter, while both groups contribute to the P=3-4 d peak. The comparison between CTTS and WTTS supports the idea of a rotation-accretion connection: their respective rotational properties are statistically different, and CTTS rotate on average more slowly than WTTS. We also observe a clear dearth of fast rotators with strong accretion signatures (large UV flux excess). This is consistent with earlier findings that fast rotators in young star clusters are typically devoid of dusty disks. Our sample shows some evidence of a mass dependence in the rotation properties of NGC 2264 members, lower-mass stars spinning on average faster. This study confirms that disks influence the rotational evolution of young stars. The idea of disk-locking may be consistent with the picture of rotation and rotation-accretion connection that we observe for the NGC 2264 cluster. However, the origin of the several substructures that we observe in the period distribution deserves further investigation.

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arXiv2016

NIKEL_AMC: Readout electronics for the NIKA2 experiment

The New Iram Kid Arrays-2 (NIKA2) instrument has recently been installed at the IRAM 30 m telescope. NIKA2 is a state-of-art instrument dedicated to mm-wave astronomy using microwave kinetic inductance detectors (KID) as sensors. The three arrays installed in the camera, two at 1.25 mm and one at 2.05 mm, feature a total of 3300 KIDs. To instrument these large array of detectors, a specifically designed electronics, composed of 20 readout boards and hosted in three microTCA crates, has been developed. The implemented solution and the achieved performances are presented in this paper. We find that multiplexing factors of up to 400 detectors per board can be achieved with homogeneous performance across boards in real observing conditions, and a factor of more than 3 decrease in volume with respect to previous generations.

astro-ph.IM↗

arXiv2016

A revised moving cluster distance to the Pleiades open cluster

Aims: We provide a new distance estimate for the Pleiades based on the moving cluster method, which will be useful to further discuss the so-called Pleiades distance controversy and compare it with the very precise parallaxes from the Gaia space mission. Methods: We apply a refurbished implementation of the convergent point search method to an updated census of Pleiades stars to calculate the convergent point position of the cluster from stellar proper motions. Then, we derive individual parallaxes for 64 cluster members using radial velocities compiled from the literature, and approximate parallaxes for another 1146 stars based on the spatial velocity of the cluster. This represents the largest sample of Pleiades stars with individual distances to date. Results: The parallaxes derived in this work are in good agreement with previous results obtained in different studies (excluding Hipparcos) for individual stars in the cluster. We report a mean parallax of $7.44\pm 0.08$~mas and distance of $134.4^{+2.9}_{-2.8}$pc that is consistent with the weighted mean of $135.0\pm 0.6$pc obtained from the non-Hipparcos results in the literature. Conclusions: Our result for the distance to the Pleiades open cluster is not consistent with the Hipparcos catalog, but favors the recent and more precise distance determination of $136.2\pm 1.2$pc obtained from Very Long Baseline Interferometry observations. It is also in good agreement with the mean distance of $133\pm 5$pc obtained from the first trigonometric parallaxes delivered by the Gaia satellite for the brightest cluster members in common with our sample.

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arXiv2016

The connection between stellar activity cycles and magnetic field topology

Zeeman Doppler imaging has successfully mapped the large-scale magnetic fields of stars over a large range of spectral types, rotation periods and ages. When observed over multiple epochs, some stars show polarity reversals in their global magnetic fields. On the Sun, polarity reversals are a feature of its activity cycle. In this paper, we examine the magnetic properties of stars with existing chromospherically determined cycle periods. Previous authors have suggested that cycle periods lie on multiple branches, either in the cycle period-Rossby number plane or the cycle period-rotation period plane. We find some evidence that stars along the active branch show significant average toroidal fields that exhibit large temporal variations while stars exclusively on the inactive branch remain dominantly poloidal throughout their entire cycle. This lends credence to the idea that different shear layers are in operation along each branch. There is also evidence that the short magnetic polarity switches observed on some stars are characteristic of the inactive branch while the longer chromospherically determined periods are characteristic of the active branch. This may explain the discrepancy between the magnetic and chromospheric cycle periods found on some stars. These results represent a first attempt at linking global magnetic field properties obtained form ZDI and activity cycles.

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arXiv2016

Readout technologies for directional WIMP Dark Matter detection

The measurement of the direction of WIMP-induced nuclear recoils is a compelling but technologically challenging strategy to provide an unambiguous signature of the detection of Galactic dark matter. Most directional detectors aim to reconstruct the dark-matter-induced nuclear recoil tracks, either in gas or solid targets. The main challenge with directional detection is the need for high spatial resolution over large volumes, which puts strong requirements on the readout technologies. In this paper we review the various detector readout technologies used by directional detectors. In particular, we summarize the challenges, advantages and drawbacks of each approach, and discuss future prospects for these technologies.

physics.ins-det↗

arXiv2016

The 2008 outburst in the Young Stellar System Z CMa. III - Multi-epoch high-angular resolution images and spectra of the components in near-infrared

Z CMa is a complex pre-main sequence binary with a current separation of 100 mas, known to consist of an FU Orionis star (SE component) and an embedded Herbig Be star (NW component). Immediately when the late-2008 outburst of Z CMa was announced to the community, we initiated a high angular resolution imaging campaign with VLT/NaCo, Keck/NIRC2, VLT/SINFONI, and Keck/OSIRIS which aimed at characterizing the outburst of both components of the system in the near-infrared. We confirm that the NW star dominates the system flux in the 1.1-3.8 microns range and is responsible for the photometric outburst. We extract the first medium-resolution (R=2000-4000) near-infrared (1.1-2.4 microns) spectra of the individual components during and after the outburst. The SE component has a spectrum typical of FU Orionis objects. The NW component spectrum is characteristic of embedded outbursting protostars and EX Or objects. It displays numerous emission lines during the outburst whose intensity correlates with the system activity. In particular, we find a correlation between the Brackett gamma equivalent width and the system brightness. The bluing of the continuum of the NW component along with the absolute flux and color-variation of the system during the outburst suggests that the outburst was caused by a complex interplay between a variation of the extinction in the line of sight of the NW component on one hand, and the emission of shocked regions close to the NW component on the other. We confirm the recently reported wiggling of the SE component jet from [Fe II] line emission. We find a point-like structure associated with a peak emission at 2.098 microns coincidental with the clump or arm seen in broadband polarization differential imaging as well as additional diffuse emission along a PA=214 degrees. The origin of these two structures is unclear and deserves further investigation.

astro-ph.SR↗

arXiv2016

The seven sisters DANCe.II. Proper motions and the lithium-rotation-activity connection for G and K Pleiads

Stellar clusters are open windows to understand stellar evolution. Specifically, the change with time and the dependence on mass of different stellar properties. As such, they are our laboratories where different theories can be tested. We try to understand the origin of the connection between lithium depletion in F, G and K stars, rotation and activity, in particular in the Pleiades open cluster. We have collected all the relevant data in the literature, including information regarding rotation period, binarity and activity, and cross-matched with proper motions, multi-wavelength photometry and membership probability from the DANCe database. In order to avoid biases, only Pleiades single members with probabilities larger than p=0.75 have been included in the discussion. Results. The analysis confirms that there is a strong link between activity, rotation and the lithium equivalent width excess, specially for the range Lum(bol) = 0.5-0.2 Lsun (about K2-K7 spectral types or 0.75-0.95 Msun). It is not possible to disentangle these effects but we cannot exclude that the observed lithium overabundance is partially an observational effect due to enhanced activity, due to a large coverage by stellar spots induced by high rotation rates. Since a bona fide lithium enhancement is present in young, fast rotators, both activity and rotation should play a role in the lithium problem.

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arXiv2016

MIMAC low energy electron-recoil discrimination measured with fast neutrons

MIMAC (MIcro-TPC MAtrix of Chambers) is a directional WIMP Dark Matter detector project. Direct dark matter experiments need a high level of electron/recoil discrimination to search for nuclear recoils produced by WIMP-nucleus elastic scattering. In this paper, we proposed an original method for electron event rejection based on a multivariate analysis applied to experimental data acquired using monochromatic neutron fields. This analysis shows that a $10^5$ rejection power is reachable for electron/recoil discrimination. Moreover, the efficiency was estimated by a Monte-Carlo simulation showing that a 105 electron rejection power is reached with a $86.49\pm 0.17$\% nuclear recoil efficiency considering the full energy range and $94.67\pm0.19$\% considering a 5~keV lower threshold.

astro-ph.IM↗

arXiv2016

Rotation in the Pleiades With K2: III. Speculations on Origins and Evolution

We use high quality K2 light curves for hundreds of stars in the Pleiades to understand better the angular momentum evolution and magnetic dynamos of young, low mass stars. The K2 light curves provide not only rotational periods but also detailed information from the shape of the phased light curve not available in previous studies. A slowly rotating sequence begins at $(V-K_{\rm s})_0\sim$1.1 (spectral type F5) and ends at $(V-K_{\rm s})_0\sim$ 3.7 (spectral type K8), with periods rising from $\sim$2 to $\sim$11 days in that interval. Fifty-two percent of the Pleiades members in that color interval have periods within 30\% of a curve defining the slow sequence; the slowly rotating fraction decreases significantly redward of $(V-K_{\rm s})_0$=2.6. Nearly all of the slow-sequence stars show light curves that evolve significantly on timescales less than the K2 campaign duration. The majority of the FGK Pleiades members identified as photometric binaries are relatively rapidly rotating, perhaps because binarity inhibits star-disk angular momentum loss mechanisms during pre-main sequence evolution. The fully convective, late M dwarf Pleiades members (5.0 $<(V-K_{\rm s})_0<$ 6.0) nearly always show stable light curves, with little spot evolution or evidence of differential rotation. During PMS evolution from $\sim$3 Myr (NGC2264 age) to $\sim$125 Myr (Pleiades age), stars of 0.3 $M_{\odot}$ shed about half their angular momentum, with the fractional change in period between 3 and 125 Myr being nearly independent of mass for fully convective stars. Our data also suggest that very low mass binaries form with rotation periods more similar to each other and faster than would be true if drawn at random from the parent population of single stars.

astro-ph.SR↗

arXiv2016

Rotation in the Pleiades with K2: I. Data and First Results

Young (125 Myr), populous ($>$1000 members), and relatively nearby, the Pleiades has provided an anchor for stellar angular momentum models for both younger and older stars. We used K2 to explore the distribution of rotation periods in the Pleiades. With more than 500 new periods for Pleiades members, we are vastly expanding the number of Pleiads with periods, particularly at the low mass end. About 92\% of the members in our sample have at least one measured spot-modulated rotation period. For the $\sim$8\% of the members without periods, non-astrophysical effects often dominate (saturation, etc.), such that periodic signals might have been detectable, all other things being equal. We now have an unusually complete view of the rotation distribution in the Pleiades. The relationship between $P$ and $(V-K_{\rm s})_0$ follows the overall trends found in other Pleiades studies. There is a slowly rotating sequence for $1.1\lesssim(V-K_{\rm s})_0\lesssim 3.7$, and a primarily rapidly rotating population for $(V-K_{\rm s})_0\gtrsim 5.0$. There is a region in which there seems to be a disorganized relationship between $P$ and $(V-K_{\rm s})_0$ for $3.7 \lesssim(V-K_{\rm s})_0\lesssim 5.0$. Paper II continues the discussion, focusing on multi-period structures, and Paper III speculates about the origin and evolution of the period distribution in the Pleiades.

astro-ph.SR↗

arXiv2016

Rotation in the Pleiades with K2: II. Multi-Period Stars

We use K2 to continue the exploration of the distribution of rotation periods in Pleiades that we began in Paper I. We have discovered complicated multi-period behavior in Pleiades stars using these K2 data, and we have grouped them into categories, which are the focal part of this paper. About 24% of the sample has multiple, real frequencies in the periodogram, sometimes manifesting as obvious beating in the light curves. Those having complex and/or structured periodogram peaks, unresolved multiple periods, and resolved close multiple periods are likely due to spot/spot group evolution and/or latitudinal differential rotation; these largely compose the slowly rotating sequence in $P$ vs.~$(V-K_{\rm s})_0$ identified in Paper I. The fast sequence in $P$ vs.~$(V-K_{\rm s})_0$ is dominated by single-period stars; these are likely to be rotating as solid bodies. Paper III continues the discussion, speculating about the origin and evolution of the period distribution in the Pleiades.

astro-ph.SR↗

arXiv2016

The Gaia-ESO Survey: A lithium-rotation connection at 5 Myr?

We investigate here the lithium content of young low-mass stars in the 5~Myr-old star forming region NGC~2264 and its relationship with rotation. We combine lithium equivalent width measurements, EW(Li), from the Gaia-ESO Survey with the determination of rotational periods from the CSI~2264 survey. We consider only bona fide non accreting cluster members in order to minimize uncertainties on EW(Li). We report the existence of a relationship between lithium content and rotation in NGC~2264 at an age of 5~Myr. The Li-rotation connection is seen over a restricted temperature range, Teff=3800--4400K, where fast rotators are Li-rich compared to slow ones. This correlation is similar to, albeit of lower amplitude than, the Li-rotation connection previously reported for K dwarfs in the 125 Myr-old Pleiades cluster. We investigate whether the non-standard pre-main sequence models developed so far to explain the Pleiades results, which are based on episodic accretion, pre-main sequence core-envelope decoupling, and/or radius inflation due to enhanced magnetic activity, can account for an early development of the Li-rotation connection. While radius inflation appears to be the most promising possibility, each of these models has issues. We therefore also discuss external causes that might operate during the first few Myr of pre-main sequence evolution, such as planet engulfment and/or steady disk accretion, as possible candidates for the common origin for Li-excess and fast rotation in young low-mass pre-main sequence stars. The emergence of a connection between lithium content and rotation rate at such an early age as 5~Myr suggests a complex link between accretion processes, early angular momentum evolution, and possibly planet formation, which likely impacts early stellar evolution and still is to be fully deciphered.

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