On the N=2 Supersymmetric Camassa-Holm and Hunter-Saxton Equations

We consider N=2 supersymmetric extensions of the Camassa-Holm and Hunter-Saxton equations. We show that they admit geometric interpretations as Euler equations on the superconformal algebra of contact vector fields on the 1|2-dimensional supercircle. We use the bi-Hamiltonian formulation to derive Lax pairs. Moreover, we present some simple examples of explicit solutions. As a by-product of our analysis we obtain a description of the bounded traveling-wave solutions for the two-component Hunter-Saxton equation.Comment: 1+19 pages, 3 figures; v2: reference added; v3: more references added, published in LM

The LBV HR Car has a partner: Discovery of a companion with the VLTI

Luminous Blue Variables (LBVs) are massive stars caught in a post-main sequence phase, during which they are losing a significant amount of mass. As, on one hand, it is thought that the majority of massive stars are close binaries that will interact during their lifetime, and on the other, the most dramatic example of an LBV, Eta Car, is a binary, it would be useful to find other binary LBVs. We present here interferometric observations of the LBV HR Car done with the AMBER and PIONIER instruments attached to ESO's Very Large Telescope Interferometer (VLTI). Our observations, spanning two years, clearly reveal that HR Car is a binary star. It is not yet possible to constrain fully the orbit, and the orbital period may lie between a few years and several hundred years. We derive a radius for the primary in the system and possibly resolve as well the companion. The luminosity ratio in the H-band between the two components is changing with time, going from about 6 to 9. We also tentatively detect the presence of some background flux which remained at the 2% level until January 2016, but then increased to 6% in April 2016. Our AMBER results show that the emission line forming region of Br gamma is more extended than the continuum emitting region as seen by PIONIER and may indicate some wind-wind interaction. Most importantly, we constrain the total masses of both components, with the most likely range being 33.6 and 45 solar masses. Our results show that the LBV HR Car is possibly an Eta Car analog binary system with smaller masses, with variable components, and further monitoring of this object is definitively called for.Comment: A&A, in pres

The large-scale disk fraction of brown dwarfs in the Taurus cloud as measured with Spitzer

Aims. The brown dwarf (BD) formation process has not yet been completely understood. To shed more light on the differences and similarities between star and BD formation processes, we study and compare the disk fraction among both kinds of objects over a large angular region in the Taurus cloud. In addition, we examine the spatial distribution of stars and BD relative to the underlying molecular gas Methods. In this paper, we present new and updated photometry data from the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope on 43 BDs in the Taurus cloud, and recalculate of the BD disk fraction in this region. We also useed recently available CO mm data to study the spatial distribution of stars and BDs relative to the cloud's molecular gas. Results. We find that the disk fraction among BDs in the Taurus cloud is 41 \pm 12%, a value statistically consistent with the one among TTS (58 \pm 9%). We find that BDs in transition from a state where they have a disk to a diskless state are rare, and we study one isolated example of a transitional disk with an inner radius of \approx 0.1 AU (CFHT BD Tau 12, found via its relatively small mid-IR excess compared to most members of Taurus that have disks. We find that BDs are statistically found in regions of similar molecular gas surface density to those associated with stars. Furthermore, we find that the gas column density distribution is almost identical for stellar and substellar objects with and without disks.Comment: 8 page, 6 figures, Accepted in Astronomy & Astrophysics

Contrasted Saharan dust events in LNLC environments: impact on nutrient dynamics and primary production

The response of the phytoplanktonic community (primary production and algal biomass) to contrasted Saharan dust events (wet and dry deposition) was studied in the framework of the DUNE ("a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem") project. We simulated realistic dust deposition events (10 gm(-2)) into large mesocosms (52m(3)). Three distinct dust addition experiments were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition; DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1 and DUNE-2-R2: simulation of two successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentrations (Chl a) were observed after a dry deposition event, while a wet deposition event resulted in a rapid (24 h after dust addition), strong (up to 2.4-fold) and long (at least a week in duration) increase in PP and Chl a. We show that, in addition to being a source of dissolved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of nitrate (NO3-) (net increases up to +9.8 mu M NO3- at 0.1m in depth) to the nutrient-depleted surface waters, due to cloud processes and mixing with anthropogenic species such as HNO3. The dry deposition event was shown to be a negligible source of NO3-. By transiently increasing DIP and NO3- concentrations in N-P starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phytoplanktonic activity. Due to the higher input of NO3- relative to DIP, and taking into account the stimulation of the biological activity, a wet deposition event resulted in a strong increase in the NO3-/DIP ratio, from initially less than 6, to over 150 at the end of the DUNE-2-R1 experiment, suggesting a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP strongly increased after wet dust deposition events, from initially 15% to 60-70% 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE experiments show that wet and dry dust deposition events induce contrasting responses of the phytoplanktonic community due to differences in the atmospheric supply of bioavailable new nutrients. Our results from original mesocosm experiments demonstrate that atmospheric dust wet deposition greatly influences primary productivity and algal biomass in LNLC environments through changes in the nutrient stocks, and alters the NO3-/DIP ratio, leading to a switch in the nutrient limitation of the phytoplanktonic activity

On the circum(sub)stellar environment of brown dwarfs in Taurus

Aims : We want to investigate whether brown dwarfs (BDs) form like stars or are ejected embryos. We study the presence of disks around BDs in the Taurus cloud, and discuss implications for substellar formation models. Methods : We use photometric measurements from the visible to the far infrared to determine the spectral energy distributions (SEDs) of Taurus BDs. Results: We use Spitzer color indices, Halpha as an accretion indicator, and models fit to the SEDs in order to estimate physical parameters of the disks around these BDs. We study the spatial distribution of BDs with and without disks across the Taurus aggregates, and we find that BDs with and without disks are not distributed regularly across the Taurus cloud. Conclusions: We find that 48%+/- 14% of Taurus BDs have a circumstellar disk signature, a ratio similar to recent results from previous authors in other regions. We fit the SEDs and find that none of the disks around BDs in Taurus can be fitted convincingly with a flaring index beta = 0, indicating that heating by the central object is efficient and that the disks we observe retain a significant amount of gas. We find that BDs with disks are proportionally more numerous in the northern Taurus filament, possibly the youngest filament. We do not find such a clear segregation for classical T Tauri stars (CTTS) and weak-lined T Tauri stars (WTTS), suggesting that, in addition to the effects of evolution, any segregation effects could be related to the mass of the object. A by-product of our study is to propose a recalibration of the Barrado y Navascues & Martin (2003) accretion limit in the substellar domain. The global shape of the limit fits our data points if it is raised by a factor 1.25-1.30.Comment: 11 pages, 5 figures, A&A accepte

Contrasted Saharan dust events in LNLC environments: impact on nutrient dynamics and primary production

International audienceThe response of the phytoplanktonic community (primary production and algal biomass) to contrasted Sa-haran dust events (wet and dry deposition) was studied in the framework of the DUNE ("a DUst experiment in a low-Nutrient, low-chlorophyll Ecosystem") project. We simu-lated realistic dust deposition events (10 g m −2) into large mesocosms (52 m 3). Three distinct dust addition experiments were conducted in June 2008 (DUNE-1-P: simulation of a wet deposition; DUNE-1-Q: simulation of a dry deposition) and 2010 (DUNE-2-R1 and DUNE-2-R2: simulation of two successive wet depositions) in the northwestern oligotrophic Mediterranean Sea. No changes in primary production (PP) and chlorophyll a concentrations (Chl a) were observed after a dry deposition event, while a wet deposition event resulted in a rapid (24 h after dust addition), strong (up to 2.4-fold) and long (at least a week in duration) increase in PP and Chl a. We show that, in addition to being a source of dis-solved inorganic phosphorus (DIP), simulated wet deposition events were also a significant source of nitrate (NO − 3) (net in-creases up to +9.8 µM NO − 3 at 0.1 m in depth) to the nutrient-depleted surface waters, due to cloud processes and mixing with anthropogenic species such as HNO 3 . The dry deposi-tion event was shown to be a negligible source of NO − 3 . By transiently increasing DIP and NO − 3 concentrations in N–P starved surface waters, wet deposition of Saharan dust was able to relieve the potential N or NP co-limitation of the phy-toplanktonic activity. Due to the higher input of NO − 3 relative to DIP, and taking into account the stimulation of the bio-logical activity, a wet deposition event resulted in a strong increase in the NO − 3 /DIP ratio, from initially less than 6, to over 150 at the end of the DUNE-2-R1 experiment, suggest-ing a switch from an initial N or NP co-limitation towards a severe P limitation. We also show that the contribution of new production to PP strongly increased after wet dust de-position events, from initially 15 % to 60–70 % 24 h after seeding, indicating a switch from a regenerated-production based system to a new-production based system. DUNE ex-periments show that wet and dry dust deposition events in-duce contrasting responses of the phytoplanktonic commu-nity due to differences in the atmospheric supply of bioavail-able new nutrients. Our results from original mesocosm ex-periments demonstrate that atmospheric dust wet deposition Published by Copernicus Publications on behalf of the European Geosciences Union. 4784 C. Ridame et al.: Phytoplanktonic response to Saharan dust events greatly influences primary productivity and algal biomass in LNLC environments through changes in the nutrient stocks, and alters the NO − 3 /DIP ratio, leading to a switch in the nu-trient limitation of the phytoplanktonic activity

New Young Star Candidates in CG4 and Sa101

The CG4 and Sa101 regions together cover a region of ~0.5 square degree in the vicinity of a "cometary globule" that is part of the Gum Nebula. There are seven previously identified young stars in this region; we have searched for new young stars using mid- and far-infrared data (3.6 to 70 microns) from the Spitzer Space Telescope, combined with ground-based optical data and near-infrared data from the Two-Micron All-Sky Survey (2MASS). We find infrared excesses in all 6 of the previously identified young stars in our maps, and we identify 16 more candidate young stars based on apparent infrared excesses. Most (73%) of the new young stars are Class II objects. There is a tighter grouping of young stars and young star candidates in the Sa101 region, in contrast to the CG4 region, where there are fewer young stars and young star candidates, and they are more dispersed. Few likely young objects are found in the "fingers" of the dust being disturbed by the ionization front from the heart of the Gum Nebula.Comment: Accepted for publication in A

On the mass segregation of stars and brown dwarfs in Taurus

We use the new minimum spanning tree (MST) method to look for mass segregation in the Taurus association. The method computes the ratio of MST lengths of any chosen subset of objects, including the most massive stars and brown dwarfs, to the MST lengths of random sets of stars and brown dwarfs in the cluster. This mass segregation ratio (ΛMSR) enables a quantitative measure of the spatial distribution of high- and low-mass stars, and brown dwarfs to be made in Taurus. We find that the most massive stars in Taurus are inversely mass segregated with ΛMSR= 0.70 ± 0.10 (ΛMSR= 1 corresponds to no mass segregation), which differs from the strong mass segregation signatures found in more dense and massive clusters such as Orion. The brown dwarfs in Taurus are not mass segregated, although we find evidence that some low-mass stars are, with an ΛMSR= 1.25 ± 0.15. Finally, we compare our results to previous measures of the spatial distribution of stars and brown dwarfs in Taurus, and briefly discuss their implication

On the mass segregation of stars and brown dwarfs in Taurus

We use the new minimum spanning tree (MST) method to look for mass segregation in the Taurus association. The method computes the ratio of MST lengths of any chosen subset of objects, including the most massive stars and brown dwarfs, to the MST lengths of random sets of stars and brown dwarfs in the cluster. This mass segregation ratio (Lambda_MSR) enables a quantitative measure of the spatial distribution of high-mass and low-mass stars, and brown dwarfs to be made in Taurus. We find that the most massive stars in Taurus are inversely mass segregated, with Lambda_MSR = 0.70 +/- 0.10 (Lambda_MSR = 1 corresponds to no mass segregation), which differs from the strong mass segregation signatures found in more dense and massive clusters such as Orion. The brown dwarfs in Taurus are not mass segregated, although we find evidence that some low-mass stars are, with an Lambda_MSR = 1.25 +/- 0.15. Finally, we compare our results to previous measures of the spatial distribution of stars and brown dwarfs in Taurus, and briefly discuss their implications.Comment: 10 pages, 8 figures, accepted for publication in MNRA