Observability of Debris Discs around M-stars

Debris discs are second generation dusty discs formed by collisions of planetesimals. Many debris discs have been found and resolved around hot and solar-type stars. However, only a handful have been discovered around M-stars, and the reasons for their paucity remain unclear. Here we check whether the sensitivity and wavelength coverage of present-day telescopes are simply unfavourable for detection of these discs or if they are truly rare. We approach this question by looking at the Herschel/DEBRIS survey that has searched for debris discs including M-type stars. Assuming that these cool-star discs are "similar" to those of the hotter stars in some sense (i.e., in terms of dust location, temperature, fractional luminosity, or mass), we check whether this survey should have found them. With our procedure we can reproduce the $2.1^{+4.5}_{-1.7}$% detection rate of M-star debris discs of the DEBRIS survey, which implies that these discs can indeed be similar to discs around hotter stars and just avoid detection. We then apply this procedure to IRAM NIKA-2 and ALMA bands 3, 6 and 7 to predict possible detection rates and give recommendations for future observations. We do not favour observing with IRAM, since it leads to detection rates lower than for the DEBRIS survey, with 0.6%-4.5% for a 15 min observation. ALMA observations, with detection rates 0.9%-7.2%, do not offer a significant improvement either, and so we conclude that more sensitive far-infrared and single dish sub-millimetre telescopes are necessary to discover the missing population of M-star debris discs.Comment: 11 pages, 7 figures, accepted by MNRA

An ALMA Survey of M-dwarfs in the Beta Pictoris Moving Group with Two New Debris Disc Detections

Previous surveys in the far-infrared have found very few, if any, M-dwarf debris discs among their samples. It has been questioned whether M-dwarf discs are simply less common than earlier types, or whether the low detection rate derives from the wavelengths and sensitivities available to those studies. The highly sensitive, long wavelength Atacama Large Millimetre/submillimetre Array can shed light on the problem. This paper presents a survey of M-dwarf stars in the young and nearby Beta Pictoris Moving Group with ALMA at Band 7 (880\,$\mu$m). From the observational sample we detect two new sub-mm excesses that likely constitute unresolved debris discs around GJ\,2006\,A and AT\,Mic\,A and model distributions of the disc fractional luminosities and temperatures. From the science sample of 36 M-dwarfs including AU\,Mic we find a disc detection rate of 4/36 or 11.1$^{+7.4}_{-3.3}$\% that rises to 23.1$^{+8.3}_{-5.5}$\% when adjusted for completeness. We conclude that this detection rate is consistent with the detection rate of discs around G and K type stars and that the disc properties are also likely consistent with earlier type stars. We additionally conclude that M-dwarf stars are not less likely to host debris discs, but instead their detection requires longer wavelength and higher sensitivity observations than have previously been employed.Comment: Accepted to MNRA

The clumpy structure of ϵ\epsilon Eridani's debris disc revisited by ALMA

$\epsilon$ Eridani is the closest star to our Sun known to host a debris disc. Prior observations in the (sub-)millimetre regime have potentially detected clumpy structure in the disc and attributed this to interactions with an (as yet) undetected planet. However, the prior observations were unable to distinguish between structure in the disc and background confusion. Here we present the first ALMA image of the entire disc, which has a resolution of 1.6"$\times$1.2". We clearly detect the star, the main belt and two point sources. The resolution and sensitivity of this data allow us to clearly distinguish background galaxies (that show up as point sources) from the disc emission. We show that the two point sources are consistent with background galaxies. After taking account of these, we find that resolved residuals are still present in the main belt, including two clumps with a $>3\sigma$ significance -- one to the east of the star and the other to the northwest. We perform $n$-body simulations to demonstrate that a migrating planet can form structures similar to those observed by trapping planetesimals in resonances. We find that the observed features can be reproduced by a migrating planet trapping planetesimals in the 2:1 mean motion resonance and the symmetry of the most prominent clumps means that the planet should have a position angle of either ${\sim10^\circ}$ or ${\sim190^\circ}$. Observations over multiple epochs are necessary to test whether the observed features rotate around the star.Comment: 16 pages, 10 figures, accepted for publication in MNRA

On the steady state collisional evolution of debris disks around M dwarfs

Debris disks have been found primarily around intermediate and solar mass stars (spectral types A-K), but rarely around low-mass M-type stars. This scarcity of detections in M star surveys can be confronted with the predictions of the steady state collisional evolution model. First, we determine the parameters of the disk population evolved with this model and fit to the distribution of the fractional dust luminosities measured in the surveys of A- and FGK-type stars observed by the infrared satellite Spitzer. Thus, in our approach, we stipulate that the initial disk mass distribution is bimodal and that only high-mass collisionally-dominated disks are detected. The best determined parameter is the diameter Dc of the largest planetesimals in the collisional cascade of the model, which ranges between 2 and 60 km, consistently for disks around both A- and FGK-type stars. Second, we assume that the same disk population surrounds the M dwarfs that have been the subjects of debris disk searches in the far-infrared with Spitzer and at submillimeter wavelengths with radiotelescopes. We find, in the framework of our study, that this disk population, which has been fit to the AFGK data, is still consistent with the observed lack of disks around M dwarfs with Spitzer

Stripping, dynamical excitation and structuring of debris disks undergoing gravitational interactions from neighbouring planets of stars

A debris disk around a main sequence star is made of planetesimals, which are the remnant of the planet formation process according to the core-accretion theory. In the Solar system, the main asteroid belt and the Kuiper belt are examples of debris disks. Around other stars, debris disks are observable if they are massive enough for collisions between planetesimals to produce continuously enough dust to be detected, by their thermal emission in the far infrared, or by scattered light in the visible spectrum. In this work, we have studied the stripping, the dynamical excitation and the structuring of debris disksundergoing the gravitational interaction with a planet inside a system, a stellar companion in a binary system, and a passing star in the dense environment of an open cluster during the first 100 millions years after the birth of the star. We have addressed these problems by the numerical simulation of the dynamics of a disk of planetesimals in these various conditions. We have finally carried out a study to determine the characteristics of the debris disk population around stars of different types, with the standard collisional evolution model, our results about dynamical excitation of disks and the data of the Spitzer surveys. We show that the lack of debris disks detected around low mass M type stars can be explained by planetesimals 10 times smaller than around solar type or more massive stars.Un disque de débris autour d'une étoile de la séquence principale est composé de planétésimaux, reste de la formation des planètes selon la théorie core-accretion. Dans le Système solaire, il s'agit de la ceinture d'astéroïdes et de la ceinture de Kuiper. Autour des autres étoiles, les disques de débris sont observables s'ils sont assez massifs pour que les collisions entre planétésimaux produisent continûment assez de poussière détectable en émission thermique dans l'infrarouge lointain ou en lumière diffusée dans le visible. Dans cette thèse, nous étudions la purge (stripping), l'excitation dynamique, et la structuration d'un disque soumis à une interaction gravitationnelle avec une planète à l'intérieur du système, un compagnon stellaire dans un système binaire, et une étoile de passage dans l'environnement dense d'un amas ouvert pendant 100 millions d'années après la naissance de l'étoile. Nous avons abordé ces problèmes par la simulation de la dynamique d'un disque de planétésimaux dans ces différentes conditions. Enfin, nous avons mené une étude pour déterminer les caractéristiques de la population de disques de débris autour des étoiles de différents types stellaires à l'aide du modèle d'évolution collisionnelle standard, de nos résultats sur l'excitation dynamique des disques et des données des relevés Spitzer. Ainsi, nous montrons que la quasi-absence des disques de débris observée autour des étoiles de faibles masses de type stellaire M peut être expliquée par des planétésimaux au moins 10 fois plus petits en taille que ceux autour des étoiles de type solaire ou plus massives.PARIS-Observatoire (751142302) / SudocSudocFranceF

Simulation de la scintillation interstellaire des pulsars (caractérisation des Extreme scattering events observés en direction de B1937+21)

ORLEANS-BU Sciences (452342104) / SudocMEUDON-Observatoire (920482302) / SudocSudocFranceF

OGT-1 is required for the <i>C. elegans</i> response to <i>S. aureus</i>.

<p><i>O-</i>GlcNAc cycling null nematodes are similarly susceptible to <i>P. aeruginosa</i> exposure as N2 animals: (A) <i>ogt-1(ok1474)</i>, (B) <i>oga-1(tm3642)</i>, (C) <i>ogt-1(ok430)</i>, and (D) <i>oga-1(ok1207)</i>. (A–H) <i>pmk-1(km25)</i> animals are hypersensitive to <i>P. aeruginosa</i> and <i>S. aureus</i>. (A, C) <i>ogt-1; pmk-1</i> and (B, D) <i>oga-1; pmk-1</i> are equally susceptible to <i>P. aeruginosa</i> as <i>pmk-1(km25)</i> single mutants. (E, G) <i>ogt-1</i> animals are hypersensitive to <i>S. aureus</i> while (F, H) <i>oga-1</i> mutants maintain survival similar to N2. (E, G) <i>ogt-1; pmk-1</i> are more susceptible to <i>S. aureus</i> than either <i>ogt-1</i> or <i>pmk-1</i> single mutants and (F, H) <i>pmk-1; oga-1</i> are similarly susceptible to <i>S. aureus</i> as <i>pmk-1</i> single mutants. Results are representative of at least two independent assays and are represented by comprehensive plots. <i>n</i>≧162. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0113231#pone.0113231.s008" target="_blank">Table S1</a> for individual assay statistical analysis.</p

Probing the subsurface of the two faces of Iapetus

Saturn’s moon Iapetus, which is in synchronous rotation, is covered by an optically dark material mainly on its leading side, while its trailing side is significantly brighter. Because longer wavelengths probe deeper into the subsurface, observing both sides at a variety of wavelengths brings to light possible changes in thermal, compositional, and physical properties with depth. We have observed Iapetus’s leading and trailing hemispheres at 1.2 and 2.0 mm, using the NIKA2 camera mounted on the IRAM 30-m telescope, and compared our observations to others performed at mm to cm wavelengths. We calibrate our observations on Titan, which is simultaneously observed within the field of view. Due to the proximity of Saturn, it is sometimes difficult to separate Iapetus’s and Titan’s flux from that of Saturn, detected in the telescope’s side lobes. Preliminary results show that the trailing hemisphere brightness temperatures at the two wavelengths are equal within error bars, unlike the prediction made by Ries (2012)[1]. On the leading side, we report a steep spectral slope of increasing brightness temperature (by 10 K) from 1.2 to 2.0 mm, which may indicate rapidly varying emissivities within the top few centimeters of the surface. Comparison to a diffuse scattering model and a thermal model will be necessary to further constrain the thermophysical properties of the subsurface of Iapetus’s two faces