New Pre-Main Sequence Candidates in the Taurus-Auriga Star Forming Region

International audienceWe have studied the X-ray source population of the Taurus Molecular Cloud (TMC) to search for new members of the Taurus-Auriga star forming region. Candidate members have been selected among the X-ray sources detected in 24 fields of the XMM-\emph{Newton} Extended Survey of the Taurus Molecular Cloud, having an IR counterpart in the 2MASS catalog, based on color-magnitude and color-color diagrams. Their X-ray spectral properties have been compared with those of known members and other X-ray sources in the same fields but without a NIR counterpart. A search for flare-like variability in the time series of all new candidates and the analysis of the X-ray spectra of the brightest candidates have been used to identify sources with a high probability of membership. We have found that 347 of 1909 detected X-ray sources have an infrared counterpart in the 2MASS catalogue. Among them, we have selected 57 sources that are consistent with being new pre-main sequence star candidates at the distance of the Taurus-Auriga star forming region; the X-ray spectral properties of this sample are, on the whole, similar to the properties of known TMC members and different from those of X-ray sources without an IR counterpart, most of which are likely to be of extragalactic origin. For 12 such candidates, the likelihood of membership is very high, based on the relatively high plasma temperatures derived from their X-ray spectra and/or the observation of powerful flares in their light curves

Investigating Star-disk Interactions During Late-stage Circusmtellar Disk Evolution in the Nearby Pre-MS Stars T Cha and TWA 30

We investigate, via contemporaneous X-ray and optical/IR observations, the nearby, pre-main sequence star/disk systems T Chamaeleontis (T Cha; D ~ 110 pc, age 3-5 Myr) and TWA 30A and 30B (D ~ 40 pc; age ~ 8 Myr). All three of these systems present opportunities to probe pre-main sequence (pre-MS) star-disk interactions during late-stage circumstellar disk evolution. The classical T Tauri star T Cha is the closest known example of a highly inclined, actively accreting, solar-mass star/disk system; furthermore, T Cha may be orbited by a low-mass companion or massive planet that has cleared an inner hole in its disk. We analyze near-simultaneous Chandra high-resolution X-ray and optical H-alpha spectroscopy observations of T Cha and find a correlation between X-ray and optical extinction resulting from variable photospheric obscuration from a disk warp/clump. We search for signatures of accretion and infer the X-ray absorbing properties of the T Cha circumstellar disk.We also present contemporaneous XMM-Newton X-ray and optical/IR spectroscopic observations of the nearby, actively accreting, very low-mass (mid-M) pre-MS star/disk/jet systems TWA 30A and 30B. Like T Cha, each component of this wide binary is viewed through a nearly edge-on circumstellar disk. We investigate potential X-ray accretion signatures, and compare the levels of magnetic activity in TWA 30A and 30B to those of other nearby, low-mass pre-MS stars near the H-burning limit. Both TWA 30A and 30B display large near-IR variability, suggestive of (respectively) variable obscuration of the stellar photosphere and a possible disk-rim warp. We detect only TWA 30A in X-rays and, similar to the case of T Cha, find a correlation between optical/IR and X-ray extinction associated with variable photospheric obscuration. The proximity and highly-inclined viewing geometries of the TWA 30 pair and T Cha, combined with contemporaneous optical/IR and X-ray observations, afford a unique opportunity to investigate the composition of late-stage circumstellar disks orbiting pre-MS stars

X-Ray Spectroscopy of Stars

(abridged) Non-degenerate stars of essentially all spectral classes are soft X-ray sources. Low-mass stars on the cooler part of the main sequence and their pre-main sequence predecessors define the dominant stellar population in the galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solar corona. X-ray emission from cool stars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronal plasma. Coronal structure, its thermal stratification and geometric extent can be interpreted based on various spectral diagnostics. New features have been identified in pre-main sequence stars; some of these may be related to accretion shocks on the stellar surface, fluorescence on circumstellar disks due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot stars clearly dominate the interaction with the galactic interstellar medium: they are the main sources of ionizing radiation, mechanical energy and chemical enrichment in galaxies. High-energy emission permits to probe some of the most important processes at work in these stars, and put constraints on their most peculiar feature: the stellar wind. Here, we review recent advances in our understanding of cool and hot stars through the study of X-ray spectra, in particular high-resolution spectra now available from XMM-Newton and Chandra. We address issues related to coronal structure, flares, the composition of coronal plasma, X-ray production in accretion streams and outflows, X-rays from single OB-type stars, massive binaries, magnetic hot objects and evolved WR stars.Comment: accepted for Astron. Astrophys. Rev., 98 journal pages, 30 figures (partly multiple); some corrections made after proof stag

Accretion and outflow-related X-rays in T Tauri stars

We report on accretion- and outflow-related X-rays from T Tauri stars, based on results from the "XMM-Newton Extended Survey of the Taurus Molecular Cloud.” X-rays potentially form in shocks of accretion streams near the stellar surface, although we hypothesize that direct interactions between the streams and magnetic coronae may occur as well. We report on the discovery of a "soft excess” in accreting T Tauri stars supporting these scenarios. We further discuss a new type of X-ray source in jet-driving T Tauri stars. It shows a strongly absorbed coronal component and a very soft, weakly absorbed component probably related to shocks in microjets. The excessive coronal absorption points to dust-depletion in the accretion stream

X-shooter spectroscopy of young stars with disks. The TW Hydrae association as a probe of the final stages of disk accretion

Context. Measurements of the fraction of disk-bearing stars in clusters as a function of age indicate protoplanetary disk lifetimes ≲10 Myr. However, our knowledge of the time evolution of mass accretion in young stars over the disk lifespans is subject to many uncertainties, especially at the lowest stellar masses (M⋆). Aims: We investigate ongoing accretion activity in young stars in the TW Hydrae association (TWA). The age of the association (∼8-10 Myr) renders it an ideal target for probing the final stages of disk accretion, and its proximity (∼50 pc) enables a detailed assessment of stellar and accretion properties down to brown dwarf masses. Methods: Our sample comprises eleven TWA members with infrared excess, amounting to 85% of the total TWA population with disks. Our targets span spectral types between M0 and M9, and masses between 0.58 M☉ and 0.02 M☉. We employed homogeneous spectroscopic data from 300 nm to 2500 nm, obtained synoptically with the X-shooter spectrograph, to derive the individual extinction, stellar parameters, and accretion parameters for each object simultaneously. We then examined the luminosity of Balmer lines and forbidden emission lines to probe the physics of the star-disk interaction environment. Results: Disk-bearing stars represent around 24% of the total TWA population. We detected signatures of ongoing accretion for 70% of our TWA targets for which accurate measurements of the stellar parameters could be derived. This implies a fraction of accretors between 13-17% across the entire TWA (that accounts for the disk-bearing and potentially accreting members not included in our survey). The spectral emission associated with these stars reveals a more evolved stage of these accretors compared to younger PMS populations studied with the same instrument and analysis techniques (e.g., Lupus): first, a large fraction (∼50%) exhibit nearly symmetric, narrow Hα line profiles; second, over 80% of them exhibit Balmer decrements that are consistent with moderate accretion activity and optically thin emission; third, less than a third exhibit forbidden line emission in [O I] 6300 Å, which is indicative of winds and outflows activity; and fourth, only one sixth exhibit signatures of collimated jets. However, the distribution in accretion rates (Ṁacc) derived for the TWA sample closely follows that of younger regions (Lupus, Chamaeleon I, σ Orionis) over the mass range of overlap (M⋆ ∼ 0.1-0.3 M☉). An overall correlation between Ṁacc and M⋆ is detected and best reproduced by the function Ṁacc ∝ M∝2.1±0.5. Conclusion. At least in the lowest M⋆ regimes, stars that still retain a disk at ages ∼8-10 Myr are found to exhibit statistically similar, albeit moderate, accretion levels as those measured around younger objects. This "slow" Ṁacc evolution that is apparent at the lowest masses may be associated with longer evolutionary timescales of disks around low-mass stars, which is suggested by the mass-dependent disk fractions reported in the literature within individual clusters. Reduced spectra are also available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/632/A46 Based on observations collected at the European Southern Observatory under ESO programs 084.C-0269(B), 085.C-0238(A), 086.C-0173(A), 087.C-0244(A), 287.C-5039(A), 089.C-0143(A), 093.C-0097(A), and 095.C-0147(A)

Life-Cycle Switching and Coexistence of Species with No Niche Differentiation

The increasing evidence of coexistence of cryptic species with no recognized niche differentiation has called attention to mechanisms reducing competition that are not based on niche-differentiation. Only sex-based mechanisms have been shown to create the negative feedback needed for stable coexistence of competitors with completely overlapping niches. Here we show that density-dependent sexual and diapause investment can mediate coexistence of facultative sexual species having identical niches. We modelled the dynamics of two competing cyclical parthenogens with species-specific density-dependent sexual and diapause investment and either equal or different competitive abilities. We show that investment in sexual reproduction creates an opportunity for other species to invade and become established. This may happen even if the invading species is an inferior competitor. Our results suggests a previously unnoticed mechanism for species coexistence and can be extended to other facultative sexual species and species investing in diapause where similar density-dependent life-history switches could act to promote coexistence

Search for Third Generation Vector Leptoquarks in p anti-p Collisions at sqrt(s) = 1.96 TeV

We describe a search for a third generation vector leptoquark (VLQ3) that decays to a b quark and tau lepton using the CDF II detector and 322 pb^(-1) of integrated luminosity from the Fermilab Tevatron. Vector leptoquarks have been proposed in many extensions of the standard model (SM). Observing a number of events in agreement with SM expectations, assuming Yang-Mills (minimal) couplings, we obtain the most stringent upper limit on the VLQ3 pair production cross section of 344 fb (493 fb) and lower limit on the VLQ3 mass of 317 GeV/c^2 (251 GeV/c^2) at 95% C.L.Comment: 7 pages, 2 figures, submitted to PR

X-shooter spectroscopy of young stellar objects in Lupus. Accretion properties of class II and transitional objects

The mass accretion rate, Ṁacc, is a key quantity for the understanding of the physical processes governing the evolution of accretion discs around young low-mass (M⋆ ≲ 2.0 M☉) stars and substellar objects (YSOs). We present here the results of a study of the stellar and accretion properties of the (almost) complete sample of class II and transitional YSOs in the Lupus I, II, III and IV clouds, based on spectroscopic data acquired with the VLT/X-shooter spectrograph. Our study combines the dataset from our previous work with new observations of 55 additional objects. We have investigated 92 YSO candidates in total, 11 of which have been definitely identified with giant stars unrelated to Lupus. The stellar and accretion properties of the 81 bona fide YSOs, which represent more than 90% of the whole class II and transition disc YSO population in the aforementioned Lupus clouds, have been homogeneously and self-consistently derived, allowing for an unbiased study of accretion and its relationship with stellar parameters. The accretion luminosity, Lacc, increases with the stellar luminosity, L⋆, with an overall slope of 1.6, similar but with a smaller scatter than in previous studies. There is a significant lack of strong accretors below L⋆ ≈ 0.1 L☉, where Lacc is always lower than 0.01 L⋆. We argue that the Lacc - L⋆ slope is not due to observational biases, but is a true property of the Lupus YSOs. The log Ṁacc - log M⋆ correlation shows a statistically significant evidence of a break, with a steeper relation for M⋆ ≲ 0.2 M☉ and a flatter slope for higher masses. The bimodality of the Ṁacc - M⋆ relation is confirmed with four different evolutionary models used to derive the stellar mass. The bimodal behaviour of the observed relationship supports the importance of modelling self-gravity in the early evolution of the more massive discs, but other processes, such as photo-evaporation and planet formation during the YSO's lifetime, may also lead to disc dispersal on different timescales depending on the stellar mass. The sample studied here more than doubles the number of YSOs with homogeneously and simultaneously determined Lacc and luminosity, Lline, of many permitted emission lines. Hence, we also refined the empirical relationships between Lacc and Lline on a more solid statistical basis. Based on observations collected at the European Southern Observatory at Paranal, under programs 084.C-0269(A), 085.C-0238(A), 086.C-0173(A), 087.C-0244(A), 089.C-0143(A), 095.C-0134(A), 097.C-0349(A), and archive data of programmes 085.C-0764(A) and 093.C-0506(A)

Diapause as escape strategy to exposure to toxicants: response of Brachionus calyciforus to arsenic

Invertebrate organisms commonly respond to environmental fluctuation by entering diapause. Production of diapause in monogonont rotifers involves a previous switch from asexual to partial sexual reproduction. Although zooplankton have been used in ecotoxicological assays, often their true vulnerability to toxicants is underestimated by not incorporating the sexual phase. We experimentally analyzed traits involved in sexual reproduction and diapause in the cyclically parthenogenetic freshwater rotifer, Brachionus calyciflorus, exposed to arsenic, a metalloid naturally found in high concentrations in desert zones, focusing on the effectiveness of diapause as an escape response in the face of an adverse condition. Addition of sublethal concentrations of arsenic modified the pattern of diapause observed in the rotifer: investment in diapause with arsenic addition peaked earlier and higher than in non-toxicant conditions, which suggests that sexual investment could be enhanced in highly stressed environmental conditions by increased responsiveness to stimulation. Nevertheless, eggs produced in large amount with arsenic, were mostly low quality, and healthy-looking eggs had lower hatching success, therefore it is unclear whether this pattern is optimum in an environment with arsenic, or if rather arsenic presence in water bodies disturbs the optimal allocation of offspring entering diapause. We observed high accumulation of arsenic in organisms exposed to constant concentration after several generations, which suggests that arsenic may be accumulated transgenerationally. The sexual phase in rotifers may be more sensitive to environmental conditions than the asexual one, therefore diapause attributes should be considered in ecotoxicological assessment because of its ecological and evolutionary implications on lakes biodiversity