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An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with the Simultaneous Differential Imager at the Very Large Telescope and MMT
We present the results of a survey of 45 young (<~250 Myr), close(<~50 pc) stars with the Simultaneous Differential Imager (SDI)implemented at the VLT and the MMT for the direct detection ofextrasolar planets. As part of the survey, we observed 54 objects,consisting of 45 close, young stars; two more distant (<150 pc),extremely young (<=10 Myr) stars; three stars with known radialvelocity planets; and four older, very nearby (<=20 pc) solaranalogs. Our SDI devices use a double Wollaston prism and a quad filterto take images simultaneously at three wavelengths surrounding the 1.62μm methane absorption bandhead found in the spectrum of cool browndwarfs and gas giant planets. By differencing adaptive optics-correctedimages in these filters, speckle noise from the primary star issignificantly attenuated, resulting in photon (andflat-field)-noise-limited data. In our VLT data, we achieved H-bandcontrasts>~10 mag (5 σ) at a separation of 0.5" from theprimary star on 45% of our targets and H-band contrasts>~9 mag at aseparation of 0.5" on 80% of our targets. With these contrasts, we canimage (5 σ detection) a 7 MJ planet 15 AU from a 70 MyrK1 star at 15 pc or a 7.8 MJ planet at 2 AU from a 12 Myr Mstar at 10 pc. We detected no candidates with S/N>2 σ whichbehaved consistently like a real object. From our survey null result, wecan rule out (with 93% confidence) a model planet population whereN(a)~constant out to a distance of 45 AU.Steward Observatory, University of Arizona, Tucson, AZ 85721.
| The Trigonometric Parallax of the Brown Dwarf Planetary System 2MASSW J1207334-393254
We have measured a trigonometric parallax to the young brown dwarf2MASSW J1207334-393254. The distance (54.0+3.2-2.8pc) and space motion confirm membership in the TW Hydrae Association.The primary is a ~25 MJup brown dwarf. We discuss the``planetary mass'' secondary, which is certainly below thedeuterium-burning limit but whose colors and absolute magnitudes posechallenges to our current understanding of planetary-mass objects.
| The Nearby Young Visual Binary HIP 115147 and Its Common Proper Motion Companion LSPM J2322+7847
We report a late M-type, common proper motion companion to a nearbyyoung visual binary HIP 115147 (V368 Cep), separated by 963" from theprimary K0 dwarf. This optically dim star was identified as a candidatehigh proper motion, nearby dwarf LSPM J2322+7847 by Lépine in2005. The wide companion is one of the latest post-T Tauri low-massstars found within 20 pc. We obtain a trigonometric parallax of51.6+/-0.8 mas, in good agreement with the Hipparcos parallax of theprimary star (50.7+/-0.6 mas). Our BVRI photometric data andnear-infrared data from 2MASS are consistent with LSPM J2322+7847 beingbrighter by 1 mag in Ks than field M dwarfs atV-Ks=6.66, which indicates its pre-main-sequence status. Weconclude that the most likely age of the primary HIP 115147 and its 11"companion HIP 115147B is 20-50 Myr. The primary appears to be older thanits close analog PZ Tel (age 12-20 Myr) and members of the TWAassociation (7 Myr).
| A novel L-band imaging search for giant planets in the Tucana and β Pictoris moving groups
Context: Direct imaging using various techniques for suppressing thestellar halo nowadays can achieve the contrast levels required to detectand characterize the light of substellar companions at orbital distancesgreater than a few astronomical units from their host stars. The methodnicely complements the radial velocity surveys that provide evidencethat giant extrasolar planets in close-in orbits are relatively common. Aims: The paper presents results from a small survey of 22 young, nearbystars that was designed to detect substellar companions and ultimatelygiant extrasolar planets down to Jupiter masses. The targets are membersof the Tucana and β Pictoris moving groups, apart from the somewhatolder star HIP 71395, which has a radial velocity trend suggesting amassive planet in large orbit. Methods: The survey was carried out inthe L-band using adaptive optics-assisted imaging with NAOS-CONICA(NACO) at the VLT. The chosen observation wavelength is well-suited tosearching for close companions around young stars and it deliversunprecedented detection limits. The presented technique reaches some ofthe best sensitivities as of today and is currently the most sensitivemethod for the contrast-limited detection of substellar companions thatare cooler than about 1000 K. Results: The companion to 51 Eri, GJ3305, was found to be a very close binary on an eccentric orbit. Nosubstellar companions were found around the target stars, although themethod permitted companions to be detected down to a few Jupiter massesat orbital distances typically of 5 astronomical units. A planet with amass ≥1 M_Jup at distances ≥5 AU around AU Mic can be excluded atthe time of our observations. The absence of detected planets setsconstraints on the frequency distribution and maximum orbital distanceof giant exoplanets. For example, a radial distribution power law indexof 0.2 in combination with a maximum orbital radius exceeding 30 AU canbe rejected at a 90% confidence level.Based on observations collected at the European Southern Observatory,Chile, through the proposals 073.C-0834(A) and 074.C-0323(A). Figure 6is only available in electronic form at http://www.aanda.org
| The X-ray spectra of the flaring and quiescent states of YZ CMi observed by XMM-Newton
We analyse the X-ray spectrum of the active late-type star YZ CMi(M4.5V); for quiescent as well as active stages, we derive emissionmeasure (EM) distributions, elemental abundances, and electrontemperatures and densities, which are in turn used to estimate flareloop lengths as well as coronal magnetic field strengths.YZ CMi was observed in the wavelength range 1-40 Å by the X-raydetectors RGS, EPIC-MOS and EPIC-pn onboard XMM-Newton. Some flaresoccurred during the observation. We perform a multi-temperature fit andmodel the differential EM of both the flaring and the quiescent parts ofthe spectrum and derive the coronal temperature distribution, EMs, andelemental abundances of the flaring and quiescent states.The observed temperature covers a range from about 1.3 to 42 MK. Thetotal volume EM in this temperature interval is13.7+/-.8×1050cm-3 for the quiescent stateand 21.7+/-1.4×1050cm-3 for the activestate. The abundance pattern in the quiescent state shows some depletionof low first ionization potential (FIP) elements relative to high-FIPelements, indicating the presence of an I(nverse)FIP effect in thisactive star. No abundance differences between the quiescent and theactive states are established.Based on the X-ray light curves in combination with the temperature,density and EM, the coronal magnetic field strength at flare-site isfound to be between 50 and 100 G and the flaring loop lengths areestimated to be in the range of 5 -13 × 109 cm.
| Rotation and Activity of Pre-Main-Sequence Stars
We present a study of rotation (vsini) and chromospheric activity(Hα equivalent width) based on an extensive set of high-resolutionoptical spectra obtained with the MIKE instrument on the 6.5 m MagellanClay telescope. Our targets are 74 F-M dwarfs in four young stellarassociations, spanning ages from 6 to 30 Myr. By comparing Hα EWsin our sample to results in the literature, we see a clear evolutionarysequence: Chromospheric activity declines steadily from the T Tauriphase to the main sequence. Using activity as an age indicator, we finda plausible age range for the Tuc-Hor association of 10-40 Myr. Between5 and 30 Myr, we do not see evidence for rotational braking in the totalsample, and thus angular momentum is conserved, in contrast to youngerstars. This difference indicates a change in the rotational regulationat ~5-10 Myr, possibly because disk braking cannot operate longer thantypical disk lifetimes, allowing the objects to spin up. Therotation-activity relation is flat in our sample; in contrast tomain-sequence stars, there is no linear correlation for slow rotators.We argue that this is because young stars generate their magnetic fieldsin a fundamentally different way from main-sequence stars, and not justthe result of a saturated solar-type dynamo. By comparing our rotationalvelocities with published rotation periods for a subset of stars, wedetermine ages of 13+7-6 and9+8-2 Myr for the η Cha and TWA associations,respectively, consistent with previous estimates. Thus we conclude thatstellar radii from evolutionary models by Baraffe et al. (1998) are inagreement with the observed radii to within +/-15%.
| X-ray emission from T Tauri stars and the role of accretion: inferences from the XMM-Newton extended survey of the Taurus molecular cloud
Context: T Tau stars display different X-ray properties depending onwhether they are accreting (classical T Tau stars; CTTS) or not(weak-line T Tau stars; WTTS). X-ray properties may provide insight intothe accretion process between disk and stellar surface. Aims: We usedata from the XMM-Newton Extended Survey of the Taurus molecular cloud(XEST) to study differences in X-ray properties between CTTS and WTTS. Methods: XEST data are used to perform correlation and regressionanalysis between X-ray parameters and stellar properties. Results: Weconfirm the existence of a X-ray luminosity (L_X) vs. mass (M) relation,L_X∝ M1.69~± 0.11, but this relation is aconsequence of X-ray saturation and a mass vs. bolometric luminosity(L_*) relation for the TTS with an average age of 2.4 Myr. X-raysaturation indicates LX = const.L_*, although the constant isdifferent for the two subsamples: const. = 10-3.73~±0.05 for CTTS and const. = 10-3.39~± 0.06 forWTTS. Given a similar L* distribution of both samples, theX-ray luminosity function also reflects a real X-ray deficiency in CTTS,by a factor of ≈2 compared to WTTS. The average electron temperaturesT_av are correlated with LX in WTTS but not in CTTS; CTTSsources are on average hotter than WTTS sources. At best marginaldependencies are found between X-ray properties and mass accretion ratesor age. Conclusions: The most fundamental properties are the twosaturation laws, indicating suppressed LX for CTTS. Wespeculate that some of the accreting material in CTTS is cooling activeregions to temperatures that may not significantly emit in the X-rayband, and if they do, high-resolution spectroscopy may be required toidentify lines formed in such plasma, while CCD cameras do not detectthese components. The similarity of the LX vs. T_avdependencies in WTTS and main-sequence stars as well as their similarX-ray saturation laws suggests similar physical processes for the hotplasma, i.e., heating and radiation of a magnetic corona.
| Fast colorimetry of the flare star EV Lacertae from UBVRI observations in 2004
Aims.We report results of a quantitative colorimetric UBVRI analysis oftwo flare events on the red dwarf EV Lac. The photometric data wereobtained in September 2004, during the multi-site synchronous monitoringfrom the four observatories in Ukraine, Russia, Greece, and Bulgaria.These observations confirmed the presence of small-scale high-frequencyoscillations (HFO) initially detected by Rodonó (1974, A&A,32, 337) and recently reconfirmed by the authors. Here we discuss thecolor characteristics of flares and HFO. Methods: Colorimetric analysishad been performed with the help of the time tracks in the UBVRIcolor-color diagrams from the earliest phase of flare development.Digital filtering technique was used to evaluate the time-dependentcolor indices. Results: As can be clearly seen in the diagrams, colorindices oscillate on a time scale of seconds, far exceeding instrumentalerrors. Regarding the HFO, we conclude that the bulk of a flareoscillates during a major part of its lifetime between the states ofhydrogen plasma opaque and transparent in the Balmer continuum. We findthat at the peaks of oscillations the color tracks drift into theregions of color-color diagrams corresponding to a blackbody radiation,which provides an estimate of color temperatures from 17 000 to 22 000K. We also find that flares cover ~1% of the stellar disc.
| Unraveling the Origins of Nearby Young Stars
A systematic search for close conjunctions and clusterings in the pastof nearby stars younger than the Pleiades is undertaken, which mayreveal the time, location, and mechanism of formation of these oftenisolated, disconnected from clusters and star-forming regions, objects.The sample under investigation includes 101 T Tauri, post-TT, andmain-sequence stars and stellar systems with signs of youth, culled fromthe literature. Their Galactic orbits are traced back in time and nearapproaches are evaluated in time, distance, and relative velocity.Numerous clustering events are detected, providing clues to the originof very young, isolated stars. Each star's orbit is also matched withthose of nearby young open clusters, OB and TT associations andstar-forming molecular clouds, including the Ophiuchus, Lupus, CoronaAustralis, and Chamaeleon regions. Ejection of young stars from openclusters is ruled out for nearly all investigated objects, but thenearest OB associations in Scorpius-Centaurus, and especially, the denseclouds in Ophiuchus and Corona Australis have likely played a major rolein the generation of the local streams (TWA, Beta Pic, andTucana-Horologium) that happen to be close to the Sun today. The core ofthe Tucana-Horologium association probably originated from the vicinityof the Upper Scorpius association 28 Myr ago. A few proposed members ofthe AB Dor moving group were in conjunction with the coeval Cepheus OB6association 38 Myr ago.
| The Solar Neighborhood. XVII. Parallax Results from the CTIOPI 0.9 m Program: 20 New Members of the RECONS 10 Parsec Sample
Astrometric measurements for 25 red dwarf systems are presented,including the first definitive trigonometric parallaxes for 20 systemswithin 10 pc of the Sun, the horizon of the RECONS sample. The threenearest systems that had no previous trigonometric parallaxes (otherthan perhaps rough preliminary efforts) are SO 0253+1652 (3.84+/-0.04pc, the 23rd nearest system), SCR 1845-6357 AB (3.85+/-0.02 pc, 24thnearest), and LHS 1723 (5.32+/-0.04 pc, 56th nearest). In total, sevenof the systems reported here rank among the nearest 100 stellar systems.Supporting photometric and spectroscopic observations have been made toprovide full characterization of the systems, including completeVRIJHKs photometry and spectral types. A study of thevariability of 27 targets reveals six obvious variable stars, includingGJ 1207, for which we observed a flare event in the V band that causedit to brighten by 1.7 mag. Improved parallaxes for GJ 54 AB and GJ 1061,both important members of the 10 pc sample, are also reported.Definitive parallaxes for GJ 1001 A, GJ 633, and GJ 2130 ABC, all ofwhich have been reported to be within 10 pc, indicate that they arebeyond 10 pc. From the analysis of systems with (previously) hightrigonometric parallax errors, we conclude that parallaxes with errorsin excess of 10 mas are insufficiently reliable for inclusion in theRECONS sample. The cumulative total of new additions to the 10 pc samplesince 2000 is now 34 systems: 28 by the RECONS team and six by othergroups. This total represents a net increase of 16% in the number ofstellar systems reliably known to be nearer than 10 pc.
| Search for cold debris disks around M-dwarfs
Debris disks are believed to be related to planetesimals left overaround stars after planet formation has ceased. The frequency of debrisdisks around M-dwarfs which account for 70% of the stars in the Galaxyis unknown while constrains have already been found for A- to K-typestars. We have searched for cold debris disks around 32 field M-dwarfsby conducting observations at λ = 850~μm with the SCUBAbolometer array camera at the JCMT and at λ = 1.2 mm with theMAMBO array at the IRAM 30-m telescopes. This is the first survey of alarge sample of M-dwarfs conducted to provide statistical constraints ondebris disks around this type of stars. We have detected a new debrisdisk around the M0.5 dwarf GJ 842.2 at λ = 850~μm, providingevidence for cold dust at large distance from this star (~300 AU). Bycombining the results of our survey with the ones of Liu et al. (2004),we estimate for the first time the detection rate of cold debris disksaround field M-dwarfs with ages between 20 and 200 Myr. This detectionrate is 13+6-8% and is consistent with thedetection rate of cold debris disks (9-23%) around A- to K-type mainsequence stars of the same age. This is an indication that cold disksmay be equally prevalent across stellar spectral types.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| Accretion Disks around Young Stars: Lifetimes, Disk Locking, and Variability
We report the findings of a comprehensive study of disk accretion andrelated phenomena in four of the nearest young stellar associationsspanning 6-30 million years in age, an epoch that may coincide with thelate stages of planet formation. We have obtained ~650 multiepochhigh-resolution optical spectra of 100 low-mass stars that are likelymembers of the η Chamaeleontis (~6 Myr), TW Hydrae (~8 Myr), βPictoris (~12 Myr), and Tucanae-Horologium (~30 Myr) groups. Our datawere collected over 12 nights between 2004 December and 2005 July on theMagellan Clay 6.5 m telescope. Based on Hα line profiles, alongwith a variety of other emission lines, we find clear evidence ofongoing accretion in 3 out of 11 η Cha stars and 2 out of 32 TWHydrae members. None of the 57 β Pic or Tuc-Hor members showsmeasurable signs of accretion. Together, these results imply significantevolution of the disk-accretion process within the first several Myr ofa low-mass star's life. While a few disks can continue to accrete for upto ~10 Myr, our findings suggest that disks accreting for beyond thattimescale are rather rare. This result provides an indirect constrainton the timescale for gas dissipation in inner disks and, in turn, ongas-giant planet formation. All accretors in our sample are slowrotators, whereas nonaccretors cover a large range in rotationalvelocities. This may hint at rotational braking by disks at ages up to~8 Myr. Our multiepoch spectra confirm that emission-line variability iscommon even in somewhat older T Tauri stars, among which accretors tendto show particularly strong variations. Thus, our results indicate thataccretion and wind activity undergo significant and sustained variationsthroughout the lifetime of accretion disks.
| Soft X-ray oscillations from AT Mic: Flare plasma diagnostics
We consider the flare oscillations from the active red dwarf AT Micdetected with the XMM-Newton space observatory in the soft X-ray energyrange (0.2 12 keV). Following Mitra-Kraev et al. (2005a), we associatethe observed oscillations with a period of ≈750 s with the excitationof a standing slow magnetoacoustic (SMA) wave in a coronal loop. Thedamping of flare loop SMA oscillations is shown to be governed byelectron thermal conduction. We have estimated the plasma density (≈3× 1010 cm-3) and the minimum magnetic field strength(≈100 G) in the region of flare energy release. The adopted model isconsistent with the results of a spectral analysis of the soft X-rayemission. The piston mechanism is assumed to be responsible for theexcitation of loop SMA oscillations.
| Astrophysics in 2005
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
| Analysis and modeling of high temporal resolution spectroscopic observations of flares on AD Leonis
We report the results of a high temporal resolution spectroscopicmonitoring of the flare star AD Leo. During 4 nights,more than 600 spectra were taken in the optical range using the IsaacNewton Telescope (INT) and the Intermediate Dispersion Spectrograph(IDS). We observed a large number of short and weak flares occurringvery frequently (flare activity > 0.71 h-1). This isconsistent with the very important role that flares can play in stellarcoronal heating. The detected flares are non white-light flares and,although most solar flares are of this kind, very few such events havebeen observed previously in stars. The behaviour of differentchromospheric lines (Balmer series from Hα to H{11}, Ca II H &K, Na I D1 and D2, He I 4026 Å and He I D3) was studied in detailfor a total of 14 flares. We estimated the physical parameters of theflaring plasma by using a procedure that assumes a simplified slab modelof flares. All the obtained physical parameters are consistent withpreviously derived values for stellar flares, and the areas - less than2.3% of the stellar surface - are comparable with the size inferred forother solar and stellar flares. We studied the relationships between thephysical parameters and the area, duration, maximum flux and energyreleased during the detected flares.
| A compact flare eclipsed in the corona of SV Camelopardalis
The eclipsing active binary SV Cam (G0V/K6V, Porb=0.593071 d)was observed with XMM-Newton during two campaigns in 2001 and 2003. Noeclipses in the quiescent emission are clearly identified, but a flarewas eclipsed during the 2001 campaign, allowing us to stronglyconstrain, from purely geometrical considerations, the position and sizeof the event: the flare is compact and it is formed at a latitude below65°. The size, temperature and Emission Measure of the flare implyan electron density of log ne (cm-3)10.6{-}13.3 and a magnetic field of 65-1400 G in order to confine theplasma, consistent with the measurements that are obtained fromdensity-sensitive line ratios in other similar active stars. Averageemission seems to come from either extended or polar regions because oflack of eclipses. The Emission Measure Distribution, coronal abundancesand characteristics of variability are very similar to other activestars such as AB Dor (K1V).
| A Spitzer Study of Dusty Disks around Nearby, Young Stars
We have obtained Spitzer Space Telescope MIPS (Multiband ImagingPhotometer for Spitzer) observations of 39 A- through M-type dwarfs,with estimated ages between 12 and 600 Myr; IRAC observations for asubset of 11 stars; and follow-up CSO SHARC II 350 μm observationsfor a subset of two stars. None of the objects observed with IRACpossess infrared excesses at 3.6-8.0 μm however, seven objectsobserved with MIPS possess 24 and/or 70 μm excesses. Four objects(κ Phe, HD 92945, HD 119124, and AU Mic), with estimated ages12-200 Myr, possess strong 70 μm excesses, >=100% larger thantheir predicted photospheres, and no 24 μm excesses, suggesting thatthe dust grains in these systems are cold. One object (HD 112429)possesses moderate 24 and 70 μm excesses with a color temperature,Tgr=100 K. Two objects (α1 Lib and HD177724) possess such strong 24 μm excesses that their 12, 24, and 70μm fluxes cannot be self-consistently modeled using a modifiedblackbody despite a 70 μm excess >2 times greater than thephotosphere around α1 Lib. The strong 24 μm excessesmay be the result of emission in spectral features, as observed towardthe Hale-Bopp star HD 69830.
| Oscillations of Optical Emission from Flare Stars and Coronal Loop Diagnostics
Based on an analogy between stellar and solar flares, we investigate theten-second oscillations detected in the U and B bands on the star EVLac. The emission pulsations are associated with fast magnetoacousticoscillations in coronal loops. We have estimated the magnetic field, B≈ 320 G; the temperature, T ≈ 3.7 × 107 K; and the plasmadensity, n ≈ 1.6 × 1011 cm-3, in the region of energy release.We provide evidence suggesting that the optical emission source islocalized at the loop footpoints.
| PHOENIX model chromospheres of mid- to late-type M dwarfs
We present semi-empirical model chromospheres computed with theatmosphere code PHOENIX. The models are designed to fit the observedspectra of five mid- to late-type M dwarfs. Next to hydrogen lines fromthe Balmer series we used various metal lines, e.g. from Fe i, for thecomparison between data and models. Our computations show that an NLTEtreatment of C, N, O impacts on the hydrogen line formation, while NLTEtreatment of less abundant metals such as nickel influences the lines ofthe considered species itself. For our coolest models we investigatedalso the influence of dust on the chromospheres and found that dustincreases the emission line flux. Moreover we present an (electronicallypublished) emission line list for the spectral range of 3100 to 3900 and4700 to 6800 Å for a set of 21 M dwarfs and brown dwarfs. The linelist includes the detection of the Na i D lines in emission for a L3dwarf.
| A Search for Hot Massive Extrasolar Planets around Nearby Young Stars with the Adaptive Optics System NACO
We report on a survey devoted to the search of exoplanets around youngand nearby stars carried out with NACO at the VLT. The detection limitfor 28 among the best available targets versus the angular separationfrom the star is presented. The nondetection of any planetary masscompanion in our survey is used to derive, for the first time, thefrequency of the upper limit of the projected planet-star separation. Inparticular, we find that in 50% of the cases, no 5MJ (or moremassive) planet has been detected at projected separations larger than14 AU, and no 10MJ (or more massive) planet has been detectedat projected separations larger than 8.5 AU. In 100% of the cases, thesevalues increase to 36 and 65 AU, respectively. The excellent sensitivityreached by our study leads to a much lower upper limit of the projectedplanet-star separation compared with previous studies. For example, forthe β Pictoris group (~12 Myr), we did not detect any10MJ planet at distances larger than 15 AU. A previous studycarried out with 4 m class telescopes put an upper limit for10MJ planets at ~60 AU. For our closest target (V2306 Oph;d=4.3 pc), it is shown that it would be possible to detect a10MJ planet at a minimum projected separation from the starof 1 AU and a 5MJ planet at a minimum projected separation of3.7 AU. Our results are discussed with respect to mechanisms explainingplanet formation and migration and forthcoming observational strategiesand future planet-finder observations from the ground.Based on observations collected at the European Southern Observatory,Chile. Program 70.C-0777D, 70.C-0777E, and 71.C-0029A.
| The first observed stellar X-ray flare oscillation: Constraints on the flare loop length and the magnetic field
We present the first X-ray observation of an oscillation during astellar flare. The flare occurred on the active M-type dwarf ATMic and was observed with XMM-Newton. The soft X-ray lightcurve (0.2-12 keV) is investigated with wavelet analysis. The flare'sextended, flat peak shows clear evidence for a damped oscillation with aperiod of around 750 s, an exponential damping time of around 2000 s,and an initial, relative peak-to-peak amplitude of around 15%. Wesuggest that the oscillation is a standing magneto-acoustic wave tied tothe flare loop, and find that the most likely interpretation is alongitudinal, slow-mode wave, with a resulting loop length of (2.5± 0.2)×1010 cm. The local magnetic fieldstrength is found to be 105 ± 50 G. These values are consistentwith (oscillation-independent) flare cooling time models and pressurebalance scaling laws. Such a flare oscillation provides an excellentopportunity to obtain coronal properties like the size of a flare loopor the local magnetic field strength for the otherwisespatially-unresolved star.
| Detection of red line asymmetries in LHS 2034
We report very pronounced line asymmetries during a long duration flareon the dM6 star LHS 2034 (AZ Cnc). While all lines of the Balmer seriesand all strong He i lines show these asymmetries, the metal lines donot. This can be explained with the help of PHOENIX model chromospheresconsidering the formation depth of the lines involved. Moreover, theasymmetries persist over about one hour changing shape and amplitude.Fitting the asymmetries with an additional broad Gaussian componentleads us to the scenario of a series of downward propagatingcondensations that decelerate due to the higher density of the lowerchromosphere. In addition, similar but weaker line asymmetries werefound in LHS 2397a.
| Flares observed with XMM-Newton and the VLA
We present lightcurves obtained in X-ray by the XMM-Newton EPIC camerasand simultaneous radio lightcurves obtained with the VLA for five activeM-type flare stars. A number of flare events were observed, and bycomparing radio with X-ray data, we consider various possible flaremechanisms. In cases where there seems to be a clear correlation betweenradio and X-ray activity, we use an energy budget argument to show thatthe heating which leads to the X-ray emission could be due to the sameparticles emitting in the radio. In cases where there is radio activitywithout corresponding X-ray activity, we argue that the radio emissionis likely to arise from coherent processes involving comparatively fewparticles. In one case, we are able to show from polarization of theradio emission that this is almost certainly the case. Cases for whichX-ray activity is seen without corresponding radio activity are moredifficult to explain. We suggest that the heating particles may beaccelerated to very high energy, and the resulting synchrotron radioemission may be beamed in directions other than the line of sight.
| X-ray properties of active M dwarfs as observed by XMM-Newton
We present a comparative study of X-ray emission from a sample of activeM dwarfs with spectral types using XMM-Newton observations of twosingle stars, AD Leonis and EV Lacertae, and two unresolved binarysystems, AT Microscopii and EQ Pegasi. The light curves reveal frequentflaring during all four observations. We perform a uniform spectralanalysis and determine plasma temperatures, abundances and emissionmeasures in different states of activity. Applying multi-temperaturemodels with variable abundances separately to data obtained with theEPIC and RGS detectors we are able to investigate the consistency of theresults obtained by the different instruments onboard XMM-Newton. Wefind that the X-ray properties of the sample M dwarfs are very similar,with the coronal abundances of all sample stars following a trend ofincreasing abundance with increasing first ionization potential, theinverse FIP effect. The overall metallicities are below solarphotospheric ones but appear consistent with the measured photosphericabundances of M dwarfs like these. A significant increase in theprominence of the hotter plasma components is observed during flareswhile the cool plasma component is only marginally affected by flaring,pointing to different coronal structures. AT Mic, probably a youngpre-main-sequence system, has the highest X-ray luminosity and exhibitsalso the hottest corona. While results of EQ Peg and EV Lac arepresented here for the first time, AT Mic and AD Leo have beeninvestigated before with different analysis approaches, allowing acomparison of the results.
| Resolving the Inner Cavity of the HD 100546 Disk: A Candidate Young Planetary System?
The inner 100 AU of HD 100546 is studied via far-ultraviolet long-slitspectroscopy with the Hubble Space Telescope Space Telescope ImagingSpectrograph (STIS). The star is surrounded by reflection nebulosity,which can be traced 100 AU in the continuum, and by emission from H ILyα, N I, Si II, and fluorescent H2 transitions. TheLyα emission can be traced up to 200 AU along the system semimajoraxis and 300 AU along the semiminor axis. The radial surface brightnessprofile and the presence of both reflection nebulosity and molecular gassuggest that we have detected the flared surface of the disk predictedfrom analysis of the IR spectral energy distribution. When corrected forthe r-2 falloff in illumination from the Herbig Be star, thereflection nebulosity, neutral atomic gas, and H2 emissionall reveal the presence of a central cavity extending 0.13"+/-0.025" (13AU) from the star, more than 20 times larger in radius than would beexpected from dust sublimation alone. The reflection nebulosity withinthe cavity is centered on a location 0.05" (5 AU) to the southeast ofthe star along the system semimajor axis. The pericenter asymmetry inthe cavity is inconsistent with cavity formation by the combined effectsof ice sublimation, radiation pressure blowout on small grains, or otherdisk chemistry that should produce azimuthally symmetric features. TheSTIS data are also consistent with a current accretion rate onto theHerbig Be star no higher than a few times 10-9Msolar yr-1, an order of magnitude lower than seenin 5-8 Myr old Herbig Ae stars. The low accretion rate, large cavity,pericenter asymmetry, and deficit of warm dust grain emission observedover 2-8 μm are all consistent with dynamical sculpting of the diskby one or more bodies within the disk. An upper limit to the flux fromany chromospherically active, low-mass companion is a factor of 5-10fainter than AU Mic (M1 Ve; t=12 Myr) at the distance of HD 100546. Thisupper limit firmly excludes a low-mass stellar companion within thecavity but does not exclude a less active and/or luminous brown dwarf.The absence of similar central clearing in younger Herbig Ae starssuggests that any companion must become externally detectable late inthe evolution of the disk, favoring a giant planet as the source of thestructure in the HD 100546 disk.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| Relationship between X-ray and ultraviolet emission of flares from dMe stars observed by XMM-Newton
We present simultaneous ultraviolet and X-ray observations of thedMe-type flaring stars AT Mic, AUMic, EV Lac, UV Cet andYZ CMi obtained with the XMM-Newton observatory.During 40 h of simultaneous observation we identify 13 flares whichoccurred in both wave bands. For the first time, a correlation betweenX-ray and ultraviolet flux for stellar flares has been observed. We findpower-law relationships between these two wavelength bands for the flareluminosity increase, as well as for flare energies, with power-lawexponents between 1 and 2. We also observe a correlation between theultraviolet flare energy and the X-ray luminosity increase, which is inagreement with the Neupert effect and demonstrates that chromosphericevaporation is taking place.
| Searching for massive extrasolar planets around young and nearby stars: from NACO to CHEOPS
We report on a survey devoted to the search of exo-planets around youngand nearby stars carried out with NACO at the VLT. The detection limitfor 28 among the best available targets vs. the angular separation fromthe star is presented. The non-detection of any planetary mass companionin our survey is used to derive, for the first time, the frequency ofthe upper limit of the projected separation planet/stars. In particular,we find that in 50% of cases, no 5MJ planet (or more massive)has been detected at projected separations larger than 14 AU and no10MJ planet (or more massive) has been detected at projectedseparations larger than 8.5 AU. The excellent sensitivity reached by ourstudy leads to a much lower upper limit of the projected separationplanet-star compared with previous studies. For our closest target(V2306 Oph - d = 4.3 pc) it is shown that it would be possible to detecta 10MJ planet at a minimum projected separation from the starof 1 AU and a 5MJ planet at a minimum projected separation of3.7 AU. Our results are discussed with respect to forthcomingobservational strategies (Simultaneous Differential Imaging technique)and future planet finder observations from the ground.
| On the sizes of stellar X-ray coronae
Spatial information from stellar X-ray coronae cannot be assesseddirectly, but scaling laws from the solar corona make it possible toestimate sizes of stellar coronae from the physical parameterstemperature and density. While coronal plasma temperatures have longbeen available, we concentrate on the newly available densitymeasurements from line fluxes of X-ray lines measured for a large sampleof stellar coronae with the Chandra and XMM-Newton gratings. We compileda set of 64 grating spectra of 42 stellar coronae. Line counts of strongH-like and He-like ions and Fe XXI lines were measured with the CORAsingle-purpose line fitting tool by \cite{newi02}. Densities areestimated from He-like f/i flux ratios of O VII and Ne IX representingthe cooler (1-6 MK) plasma components. The densities scatter between logne ≈ 9.5-11 from the O VII triplet and between logne ≈ 10.5-12 from the Ne IX triplet, but we caution thatthe latter triplet may be biased by contamination from Fe XIX and Fe XXIlines. We find that low-activity stars (as parameterized by thecharacteristic temperature derived from H- and He-like line flux ratios)tend to show densities derived from O VII of no more than a few times1010 cm-3, whereas no definitive trend is foundfor the more active stars. Investigating the densities of the hotterplasma with various Fe XXI line ratios, we found that none of thespectra consistently indicates the presence of very high densities. Weargue that our measurements are compatible with the low-density limitfor the respective ratios (≈ 5× 1012cm-3). These upper limits are in line with constant pressurein the emitting active regions. We focus on the commonly used \cite{rtv}scaling law to derive loop lengths from temperatures and densitiesassuming loop-like structures as identical building blocks. We derivethe emitting volumes from direct measurements of ion-specific emissionmeasures and densities. Available volumes are calculated from theloop-lengths and stellar radii, and are compared with the emittingvolumes to infer filling factors. For all stages of activity we findsimilar filling factors up to 0.1.Appendix A is only available in electronic form athttp://www.edpsciences.org
| A Unified Picture of the First Ionization Potential and Inverse First Ionization Potential Effects
We discuss models for coronal abundance anomalies observed in thecoronae of the sun and other late-type stars following a scenario firstintroduced by Schwadron, Fisk, & Zurbuchen of the interaction ofwaves at loop footpoints with the partially neutral gas. Instead ofconsidering wave heating of ions in this location, we explore theeffects on the upper chromospheric plasma of the wave pondermotiveforces. These can arise when upward-propagating waves from thechromosphere transmit or reflect upon reaching the chromosphere-coronaboundary, and are in large part determined by the properties of thecoronal loop above. Our scenario has the advantage that for realisticwave energy densities both positive and negative changes in theabundance of ionized species compared to neutrals can result, allowingboth first ionization potential (FIP) and inverse FIP effects to comeout of the model. We discuss how variations in model parameters canaccount for essentially all of the abundance anomalies observed in solarspectra. Expected variations with stellar spectral type are alsoqualitatively consistent with observations of the FIP effect in stellarcoronae.
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Datos observacionales y astrométricos
| Constelación: | Microscopio |
| Ascensión Recta: | 20h41m51.15s |
| Declinación: | -32°26'06.7" |
| Magnitud Aparente: | 11.243 |
| Distancia: | 10.225 parsecs |
| Movimiento Propio en Ascensión Recta: | 261.3 |
| Movimiento Propio en Declinación: | -344.8 |
| B-T magnitude: | 12.666 |
| V-T magnitude: | 11.361 |
Catálogos y designaciones:
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