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σ Ori (Daria Naumenko)



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SCUBA observations of the Horsehead nebula - what did the horse swallow?
We present observations taken with the Submillimetre Common-UserBolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT) ofthe Horsehead nebula in Orion (B33), at wavelengths of 450 and 850μm. We see bright emission from that part of the cloud associatedwith the photon-dominated region (PDR) at the `top' of the horse's head,which we label B33-SMM1. We characterize the physical parameters of theextended dust responsible for this emission, and find that B33-SMM1contains a more dense core than was previously suspected, with a mass of~2Msolar in a region of 0.31 × 0.13pc, and a peakvolume density of ~6 × 105cm-3. We comparethe SCUBA data with data from the Infrared Space Observatory (ISO) andfind that the emission at 6.75μm is offset towards the west,indicating that the mid-infrared emission is tracing the PDR while thesubmillimetre emission comes from the molecular cloud core behind thePDR. We calculate the virial balance of this core and find that it isnot gravitationally bound but is being confined by the external pressurefrom the HII region IC434, and that it will either be destroyed by theionizing radiation, or else may undergo triggered star formation.Furthermore, we find evidence for a lozenge-shaped clump in the `throat'of the horse, which is not seen in emission at shorter wavelengths. Welabel this source B33-SMM2 and find that it is brighter at submillimetrewavelengths than B33-SMM1. We calculate the physical parameters of SMM2and find it has a mass of ~4Msolar in a region 0.15 ×0.07pc, with a peak volume density of ~2 ×106cm-3 and peak column density of ~9 ×1022cm-2. SMM2 is seen in absorption in the6.75-μm ISO data, from which we obtain an independent estimate of thecolumn density in excellent agreement with that calculated from thesubmillimetre emission. We calculate the stability of this core againstcollapse and find that it is in approximate gravitational virialequilibrium. This is consistent with it being a pre-existing core inB33, possibly pre-stellar in nature, but that it may also eventuallyundergo collapse under the effects of the HII region.

Circumstellar discs in the young σ Orionis cluster
We present new K- and L'-band imaging observations for members of theyoung (3-5Myr) σOrionis cluster, obtained at the United KingdomInfrared Telescope (UKIRT) with the UKIRT 1-5 micron Imager Spectrometer(UIST). We determine (K - L') colour excesses with respect to thephotospheres, finding evidence for warm circumstellar dust around 27 outof 83 cluster members that have masses between 0.04 and1.0Msolar. This indicates a circumstellar disc frequency ofat least (33+/-6) per cent for this cluster, consistent with previousdeterminations from smaller samples and also consistent with the 3-Myrdisc half-life suggested by Haisch et al. There is marginal evidencethat the disc frequency declines towards lower masses, but the data arealso consistent with no mass dependence at all. There is no evidence forspatial segregation of objects with and without circumstellar discs.

Large Area Mapping at 850 μm. IV. Analysis of the Clump Distribution in the Orion B South Molecular Cloud
We present results from a survey of a 1300 arcmin2 region ofthe Orion B South molecular cloud, including NGC 2024, NGC 2023, and theHorsehead Nebula (B33), obtained using the Submillimetre Common-UserBolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT).Submillimeter continuum observations at 450 and 850 μm are discussed.Using an automated algorithm, 57 discrete emission features (``clumps'')are identified in the 850 μm map. The physical conditions withinthese clumps are investigated under the assumption that the objects arein quasi-hydrostatic equilibrium. The best-fit dust temperature for theclumps is found to be Td=18+/-4 K, with the exception ofthose associated with the few known far-infrared sources residing in NGC2024. The latter internally heated sources are found to be much warmer.In the region surrounding NGC 2023, the clump dust temperatures agreewith clump gas temperatures determined from molecular line excitationmeasurements of the CO molecule. The bounding pressure on the clumpslies in the range log(k-1P cm3K-1)=6.1+/-0.3. The cumulative mass distribution is steep atthe high-mass end, as is the stellar initial mass function. Thedistribution flattens significantly at lower masses, with a turnoveraround 3-10 Msolar.

Submillimeter imaging spectroscopy of the Horsehead nebula
We present 15 arcsecond resolution single-dish imaging of the Horseheadnebula in the Ci (1-0) and CO (4-3) lines, carried out using the CHAMParray at the Caltech Submillimeter Observatory (CSO). The data are usedtogether with supporting observations of the (2-1) transitions of the COisotopologues to determine the physical conditions in the atomic andmolecular gas via Photon Dominated Region (PDR) modeling. The CO(4-3)/(2-1) line ratio, which is an excellent tracer of the direction ofthe incoming UV photons, increases at the western and northern edges ofthe nebula, confirming that the illumination is provided mostly by thestars σ and ζ Orionis. The observed line intensities areconsistent with PDR models with an H nuclei volume density of ˜ 3{-} 7 × 104 cm-3. The models predict akinetic temperature of 12 K and a C18O fractional abundancewith respect to H atoms of 2.4 × 10-7 in the shieldedregion, which in turn imply a total molecular mass of 24 Mȯ in theC18O filament. The outer halo, devoid of C18O, buttraced by the Ci emission has a comparable density and contributesadditional 13 Mȯ of material, resulting in an upper limit of 37Mȯ for the total molecular mass of the nebula.

Can single O stars produce non-thermal radio emission?
We present a model for the non-thermal radio emission from presumablysingle O stars, in terms of synchrotron emission from relativisticelectrons accelerated in wind-embedded shocks. These shocks areassociated with an unstable, chaotic wind. The main improvement withrespect to earlier models is the inclusion of the radial dependence ofthe shock velocity jump and compression ratio, based on one-dimensionaltime-dependent hydrodynamical simulations. The decrease of the velocityjump and the compression ratio as a function of radius produces arapidly decreasing synchrotron emissivity. This effectively prohibitsthe models from reproducing the spectral shape of the observednon-thermal radio emission. We investigate a number of “escaperoutes” by which the hydrodynamical predictions might bereconciled with the radio observations. We find that the observedspectral shape can be reproduced by a slower decline of the compressionratio and the velocity jump, by the re-acceleration of electrons in manyshocks or by adopting a lower mass-loss rate. However, none of theseescape routes are physically plausible. In particular, re-accelerationby feeding an electron distribution through a number of shocks, is incontradiction with current hydrodynamical simulations. Thesehydrodynamical simulations have their limitations, most notably the useof one-dimensionality. At present, it is not feasible to performtwo-dimensional simulations of the wind out to the distances requiredfor synchrotron-emission models. Based on the current hydrodynamicmodels, we suspect that the observed non-thermal radio emission from Ostars cannot be explained by wind-embedded shocks associated with theinstability of the line-driving mechanism. The most likely alternativemechanism is synchrotron emission from colliding winds. That would implythat all O stars with non-thermal radio emission should be members ofbinary or multiple systems.

A high-resolution spectroscopy survey of β Cephei pulsations in bright stars
We present a study of absorption line-profile variations in early-B typenear-main-sequence stars without emission lines. We have surveyed atotal of 171 bright stars using the Nordic Optical Telescope (NOTSA),William Herschel Telescope (ING) and Coudé Auxiliary Telescope(ESO). Our sample contains 75% of all O9.5-B2.5 III-V non-emission-linestars brighter than 5.5 mag. We obtained high signal-to-noise,high-resolution spectra of the SiIII λ4560 triplet - for 125stars of our sample we obtained more than one spectrum - and examinedthese for pulsational-like line-profile variations and/or structure. Weconclude that about half of our sample stars show evidence forline-profile variations (LPV). We find evidence for LPV in about 65% ofour sample stars brighter than V=5.5. For stars with rotationalbroadening V sin i ˜100 km s-1, we find evidence for LPVin about 75% of the cases. We argue that it is likely that these LPV areof pulsational origin, and that hence more than half of thesolar-neighbourhood O9.5-B2.5 III-V stars is pulsating in modes that canbe detected with high-resolution spectroscopy. We detected LPV in 64stars previously unknown to be pulsators, and label these stars as newβ Cep candidates. We conclude that there is no obvious differencein incidence of (pulsational) LPV for early-B type near-main-sequencestars in binaries or in OB associations, with respect to single fieldstars.

XMM-Newton observations of the σ Orionis cluster. II. Spatial and spectral analysis of the full EPIC field
We present the results of an XMM-Newton observation of the young (˜2{-}4 Myr) cluster around the hot star σ Orionis. In a previouspaper we presented the analysis of the RGS spectrum of the central hotstar; here we discuss the results of the analysis of the full EPICfield. We have detected 175 X-ray sources, 88 of which have beenidentified with cluster members, including very low-mass stars down tothe substellar limit. We detected eleven new possible candidate membersfrom the 2MASS catalogue. We find that late-type stars have a median logL_X/L_bol ˜ -3.3, i.e. very close to the saturation limit. Wedetected significant variability in ˜ 40% of late-type members orcandidates, including 10 flaring sources; rotational modulation isdetected in one K-type star and possibly in another 3 or 4 stars.Spectral analysis of the brightest sources shows typical quiescenttemperatures in the range T_1˜ 0.3{-}0.8 keV and T_2˜ 1{-}3 keV,with subsolar abundances Z˜ 0.1{-}0.3 Z_ȯ, similar to what isfound in other star-forming regions and associations. We find nosignificant difference in the spectral properties of classical andweak-lined T Tauri stars, although classical T Tauri stars tend to beless X-ray luminous than weak-lined T Tauri stars.

A Revised Geometry for the Magnetic Wind of θ1 Orionis C
The star θ1 Orionis C (O6-7 V) is often cited as a hotanalog of Bp variables, because its optical and UV line and X-raycontinuum fluxes modulate over the magnetic/rotational period. In thiscircumstance, one expects emission and absorption components of the UVresonance lines to vary as a flattened magnetosphere corotates with thestar. In this paper, we reexamine the detailed velocity behavior ofseveral strong UV lines. Whereas past work has focused on variations ofthe full profiles, we find that the blue and red wings of the C IV and NV resonance lines exhibit anticorrelated modulations. These appear asabsorption excesses at large blueshifts and as flux elevations atmoderate redshifts at the edge-on phase φ=0.5. No rest-frameabsorption features, which are the typical signatures of cool, staticdisks surrounding Bp stars, can be detected at any phase. We suggestthat this behavior is caused by two geometrically distinct components ofthe wind, which are defined by the relationship between the extent of amagnetic loop and the local Alfvén radius. Streams on field linesopening outside this radius are first channeled toward the magneticequator, but after reaching the Alfven radius, they are forced outwardby radiative forces, eventually to become an expanding radial outflow.This wind component causes blueshifted absorption as the corotatingmagnetic equatorial plane crosses the observer's line of sight(φ=0.5). The geometry of the inner component requires a morecomplicated interpretation. Wind streams first follow closed loops andcollide at the magnetic equator with counterpart streams from theopposite pole. There they coalesce and fall back to the star along theiroriginal field lines. The high temperatures in these fallingcondensations cause the redshifted emission. The rapid circulation ofthese flows is likely the reason for the absence of signatures of a cooldisk (e.g., zero-velocity absorptions at φ~0.5) in the strong UVlines.

The Horsehead nebula: A beautiful case
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Membership, binarity and accretion among very low-mass stars and brown dwarfs of the σ Orionis cluster
Intermediate-resolution (R~ 7000) spectroscopy is presented for 76photometrically selected very low-mass (0.04 < M < 0.3Msolar) candidate members of the young cluster around σOrionis (σ Ori). More than two-thirds appear to be genuine clustermembers on the basis that they exhibit LiI 6708-Åabsorption, weakNaI 8183/8195 Åfeatures and a radial velocity consistent with thecluster mean. Photometric selection alone therefore appears to be veryeffective in identifying cluster members in this mass range. Only sixobjects appear to be certain non-members; however, a substantial subsetof 13 candidates have ambiguous or contradictory indications ofmembership and lack Li absorption. Together with an observed spread inthe equivalent width of the Li absorption feature in the cooler stars ofour sample, this indicates that there may be deficiencies in ourunderstanding of the formation of this line in cool, low-gravityobjects.Four candidate binary cluster members are identified. Consideration ofsampling and radial velocity measurement precision leads us to concludethat either the fraction of very low-mass stars and brown dwarfs insmall separation (a < 1 au) binary systems is larger than in fieldM-dwarfs, or the distribution of separations is much less skewed towardslarge separations. This conclusion hinges critically on the correctidentification of the small number of binary candidates, although itremains significant even when only the candidate members displaying Liabsorption are considered.Broadened Hα emission, indicative of circum(sub)stellar accretiondiscs is found in five or six of the candidate cluster members, three ofwhich probably have substellar masses. The fraction of accretors (10 +/-5 per cent) is similar to that found in stars of higher mass in theσ Ori cluster using Hα emission as a diagnostic, but muchlower than found for very low-mass stars and brown dwarfs of youngerclusters. The time-scale for accretion rates to drop to<~10-11 Msolar yr-1 is hence lessthan the age of the σ Ori cluster (3-7 Myr) for most low-massobjects.

Variability of Stars in the Pulkovo Spectrophotometric Catalog
We present the results of a statistical study of brightness variabilityfor 693 stars of the Pulkovo spectrophotometric database in fivespectral bands in the range λλ 320 1080 nm. Significantbrightness variations were detected in at least one spectral bandagainst the background of the random noise for one-third of the starsnot earlier believed to be variable. A comparison of the distributionsof these variations in amplitude and spectral band for the normal andvariable stars shows that variability is inherent to most stars to someextent and is often wavelength dependent.

A Medium Resolution Near-Infrared Spectral Atlas of O and Early-B Stars
We present intermediate-resolution (R~8000-12,000) high signal-to-noise(S/N) H- and K-band spectroscopy of a sample of 37 optically visiblestars, ranging in spectral type from O3 to B3 and representing mostluminosity classes. Spectra of this quality can be used to constrain thetemperature, luminosity, and general wind properties of OB stars, whenused in conjunction with sophisticated atmospheric model codes. Mostimportant is the need for moderately high resolutions (R>=5000) andvery high signal-to-noise (S/N>=150) spectra for a meaningful profileanalysis. When using near-infrared spectra for a classification system,moderately high signal-to-noise (S/N~100) is still required, though theresolution can be relaxed to just a thousand or two. In the Appendix weprovide a set of very high-quality near-infrared spectra of Brackettlines in six early-A dwarfs. These can be used to aid in the modelingand removal of such lines when early-A dwarfs are used for telluricspectroscopic standards.

A Parsec-Scale Outflow in the Rosette Molecular Cloud?
We report on new observations of a purported parsec-scale outflow in thehostile environment at the boundary of the Rosette Nebula, a well-knownH II region driven by several O stars in the open cluster NGC 2244, andthe Rosette Molecular Cloud (RMC). Several emission features were firstreported by Ybarra & Phelps in 2004 , but with only optical [S II]imaging, their nature could not then be established. Our newnear-infrared (NIR) H2 imaging observations reveal bulletedNIR emission between a Two Micron All Sky Survey (2MASS) NIR excesspoint source and the optical [S II] emission for RMC C of Ybarra &Phelps. Analysis of the 2MASS colors indicates that the point source islikely to be a Class I young stellar object (YSO). Together theseresults imply that RMC C is in fact part of a true YSO outflow, nowdesignated HH 871. The presence of additional [S II] emission,apparently connected to the HH 871 outflow but separated by ~1.2 pc,assuming a distance of 1600 pc for the RMC, also lends support to theYbarra & Phelps hypothesis that a parsec-scale outflow is present inthis harsh ionizing environment.

The Substellar Mass Function: A Bayesian Approach
We report our efforts to constrain the form of the low-mass star andbrown dwarf mass function via Bayesian inference. Recent surveys of M,L, and T dwarfs in the local solar neighborhood are an essentialcomponent of our study. Uncertainties in the age distribution of localfield stars make reliable inference complicated. We adopt a wide rangeof plausible assumptions about the rate of Galactic star formation andshow that their deviations from a uniform rate produce little effect onthe resulting luminosity function for a given mass function. As anancillary result, we calculate the age distribution for M, L, and Tspectral types. We demonstrate that late L dwarfs, in particular, aresystematically younger than objects with earlier or later spectraltypes, with a mean age of 3 Gyr. Finally, we use a Bayesian statisticalformalism to evaluate the probability of commonly used mass functions inthe light of recent discoveries. We consider three functional forms ofthe mass function, including a two-segment power law, a single power lawwith a low-mass cutoff, and a lognormal distribution. Our results showthat at a 60% confidence level the power-law index α for thelow-mass arm of a two-segment power law has a value between -0.6 and 0.6for objects with masses between 0.04 and 0.10 Msolar. Thebest-fit index is α=0.3+/-0.6 at the 60% confidence level for asingle-segment mass function. Current data require this function toextend to at least 0.05 Msolar with no restrictions placed ona lower mass cutoff. Inferences of the parameter values for a lognormalmass function are virtually unaffected by recent estimates of the localspace density of L and T dwarfs. We find that we have no preferenceamong these three forms using this method. We discuss current and futurecapabilities that may eventually discriminate between mass functionmodels and refine estimates of their associated parameter values.

Rotation and variability of young very low-mass objects
Variability studies are an important tool to investigate key propertiesof stars and brown dwarfs. From photometric monitoring we are able toobtain information about rotation and magnetic activity, which areexpected to change in the mass range below 0.3 solar masses, since thesefully convective objects cannot host a solar-type dynamo. On the otherhand, spectroscopic variability information can be used to obtain adetailed view on the accretion process in very young objects. In thispaper, we report about our observational efforts to analyse thevariability and rotational evolution of young brown dwarfs and verylow-mass stars.

Photometric Accretion Signatures Near the Substellar Boundary
Multiepoch imaging of the Orion equatorial region by the Sloan DigitalSky Survey has revealed that significant variability in the bluecontinuum persists into the late-M spectral types, indicating thatmagnetospheric accretion processes occur below the substellar boundaryin the Orion OB1 association. We investigate the strength of theaccretion-related continuum veiling by comparing the reddening-invariantcolors of the most highly variable stars against those of main-sequenceM dwarfs and evolutionary models. A gradual decrease in the g-bandveiling is seen for the cooler and less massive members, as expected fora declining accretion rate with decreasing mass. We also see evidencethat the temperature of the accretion shock decreases in the very lowmass regime, reflecting a reduction in the energy flux carried by theaccretion columns. We find that the near-IR excess attributed tocircumstellar disk thermal emission drops rapidly for spectral typeslater than M4. This is likely due to the decrease in color contrastbetween the disk and the cooler stellar photosphere. Since accretion,which requires a substantial stellar magnetic field and the presence ofa circumstellar disk, is inferred for masses down to 0.05Msolar, we surmise that brown dwarfs and low-mass stars sharea common mode of formation.

Velocity field and star formation in the Horsehead nebula
Using large scale maps in C18O(2-1) and in the continuum at1.2 mm obtained at the IRAM-30 m antenna with the Heterodyne ReceiverArray (HERA) and MAMBO2, we investigated the morphology and the velocityfield probed in the inner layers of the Horsehead nebula. The datareveal a non-self-gravitating (m/m_vir≈ 0.3) filament of dust and gas(the “neck”, \varnothing = 0.15-0.30 pc) connecting theHorsehead western ridge, a Photon-Dominated Region illuminated byσOri, to its parental cloud L1630. Several dense cores areembedded in the ridge and the neck. One of these cores appearsparticularly peaked in the 1.2 mm continuum map and corresponds to afeature seen in absorption on ISO maps around 7 μm. ItsC18O emission drops at the continuum peak, suggestive ofmolecular depletion onto cold grains. The channel maps of the Horseheadexhibit an overall north-east velocity gradient whose orientationswivels east-west, showing a somewhat more complex structure than wasrecently reported by Pound et al. (2003) using BIMA CO(1-0) mapping. Inboth the neck and the western ridge, the material is rotating around anaxis extending from the PDR to L1630 (angular velocity =1.5-4.0 kms-1). Moreover, velocity gradients along the filament appearto change sign regularly (3 km s-1 pc-1, period =0.30 pc) at the locations of embedded integrated intensity peaks. Thenodes of this oscillation are at the same velocity. Similar transversecuts across the filament show a sharp variation of the angular velocityin the area of the main dense core. The data also suggest thatdifferential rotation is occurring in parts of the filament. We presenta new scenario for the formation and evolution of the nebula and discussdense core formation inside the filament.

Quantitative H and K band spectroscopy of Galactic OB-stars at medium resolution
In this paper we have analyzed 25 Galactic O and early B-stars by meansof H and K band spectroscopy, with the primary goal to investigate towhat extent a lone near-IR spectroscopy is able to recover stellar andwind parameters derived in the optical. Most of the spectra have beentaken with subaru-ircs, at an intermediate resolution of 12 000, andwith a very high S/N, mostly on the order of 200 or better. In order tosynthesize the strategic H/He lines, we have used our recent,line-blanketed version of fastwind (Puls et al. 2005, A&A, 435,669). In total, seven lines have been investigated, where for two starswe could make additional use of the Hei2.05 singlet which has beenobserved with irtf-cshell. Apart from Brγ and Heii2.18, the otherlines are predominately formed in the stellar photosphere, and thusremain fairly uncontaminated from more complex physical processes,particularly clumping. First we investigated the predicted behaviour ofthe strategic lines. In contradiction to what one expects from theoptical in the O-star regime, almost all photospheric H/Hei/Heii H/Kband lines become stronger if the gravity decreases. Concerning H andHeii, this finding is related to the behaviour of Stark broadening as afunction of electron density, which in the line cores is different formembers of lower (optical) and higher (IR) series. Regarding Hei, thepredicted behaviour is due to some subtle NLTE effects resulting in astronger overpopulation of the lower level when the gravity decreases.We have compared our calculations with results from the alternative NLTEmodel atmosphere code cmfgen (Hillier & Miller 1998, ApJ, 496, 407).In most cases, we found reasonable or nearly perfect agreement. Only theHei2.05 singlet for mid O-types suffers from some discrepancy, analogouswith findings for the optical Hei singlets. For most of our objects, weobtained good fits, except for the line cores of Brγ in earlyO-stars with significant mass-loss. Whereas the observations showBrγ mostly as rather symmetric emission lines, the models predicta P Cygni type profile with strong absorption. This discrepancy (whichalso appears in lines synthesized by cmfgen) might be an indirect effectof clumping. After having derived the stellar and wind parameters fromthe IR, we have compared them to results from previous optical analyses.Overall, the IR results coincide in most cases with the optical oneswithin the typical errors usually quoted for the correspondingparameters, i.e., an uncertainty in T_eff of 5%, in log g of 0.1 dex andin {dot M} of 0.2 dex, with lower errors at higher wind densities.Outliers above the 1-σ level where found in four cases withrespect to log g and in two cases for {dot M}.

Constraining the properties of spots on Pleiades very low mass stars
We present results of a multi-filter monitoring campaign for very lowmass (VLM) stars in the Pleiades. Simultaneous to our I-band time series(Scholz & Eislöffel 2004b), which delivered photometric periodsfor nine VLM stars, we obtained light curves in the J- and H-band. OneVLM star with M≈0.15 Mȯ (BPL129) shows a period inall three wavelength bands. The amplitudes in I, J, and H are 0.035,0.035, and 0.032 mag. These values are compared to simulations, in whichwe compute the photometric amplitude as a function of spot temperatureand filling factor. The best agreement between observations and modelsis found for cool spots with a temperature contrast of 18-31% and a verylow surface filling factor of 4-5%. We suggest that compared to moremassive stars VLM objects may have either very few spots or a rathersymmetric spot distribution. This difference might be explained with achange from a shell to a distributed dynamo in the VLM regime.

Density structure of the Horsehead nebula photo-dissociation region
We present high angular resolution images of the H2 1-0 S(1) lineemission obtained with the Son of ISAAC (SOFI) at the New TechnologyTelescope (NTT) of the Horsehead nebula. These observations are analysedin combination with Hα line emission, aromatic dust, CO and dustcontinuum emissions. The Horsehead nebula illuminated by the O9.5V starσ Ori (χ ˜ 60) presents a typical photodissociation region(PDR) viewed nearly edge-on and offers an ideal opportunity to study thegas density structure of a PDR. The H2 fluorescent emission observationsreveal extremely sharp and bright filaments associated with theilluminated edge of the nebula which spatially coincides with thearomatic dust emission. Analysis of the H2 fluorescent emission,sensitive to both the far-UV radiation field and the gas density, inconjunction with the aromatic dust and Hα line emission, bringsnew constraints on the illumination conditions and the gas density inthe outer PDR region. Furthermore, combination of this data withmillimeter observations of CO and dust continuum emission allows us totrace the penetration of the far-UV radiation field into the cloud andprobe the gas density structure throughout the PDR. From comparison withPDR model calculations, we find that i) the gas density follows a steepgradient at the cloud edge, with a scale length of 0.02 pc (or 10'') andnH˜ 104 and 105 cm-3 inthe H2 emitting and inner cold molecular layers respectively; and ii)this density gradient model is essentially a constant pressure model,with P˜ 4 × 106 K cm-3. The constraintsderived here on the gas density profile are important for the study ofphysical and chemical processes in PDRs and provide new insight into theevolution of interstellar clouds. Also, this work shows the stronginfluence of the density structure on the PDR spatial stratification andillustrates the use of different tracers to determine this densitystructure.

A near-infrared survey for new low-mass members in α Per
We present a near-infrared (K'-band) survey of 0.7 square degree area inthe α Persei open cluster (age = 90 Myr, distance = 182 pc)carried out with the Omega-Prime camera on the Calar Alto 3.5-mtelescope. Combining optical data (Rc and I_c) obtained withthe KPNO/MOSA detector and presented in Stauffer et al. (1999) with theK' observations, a sample of new candidate members has been extractedfrom the optical-infrared colour-magnitude diagram. The location ofthese candidates in the colour-colour diagram suggests that two-thirdsof them are actually reddened background giants. About 20 new candidatemembers with masses between 0.3 and 0.04 Mȯ are added tothe ~400 known α Per cluster members. If they are indeed αPer members, four of the new candidates would be brown dwarfs. Wediscuss the advantages and drawbacks of the near-infrared survey ascompared to the optical selection method. We also describe the outcomeof optical spectroscopy obtained with the Twin spectrograph on the CalarAlto 3.5-m telescope for about 30 candidates, including selected membersfrom the optical sample presented in Barrado y Navascués et al.(2002) and from our joint optical/infrared catalogue. These resultsargue in favour of the optical selection method for this particularcluster.

Are PAHs precursors of small hydrocarbons in photo-dissociation regions? The Horsehead case
We present maps at high spatial and spectral resolution in emissionlines of CCH, c-C3H2, C4H, 12CO and C18O of theedge of the Horsehead nebula obtained with the IRAM Plateau de BureInterferometer (PdBI). The edge of the Horsehead nebula is aone-dimensional Photo-Dissociation Region (PDR) viewed almost edge-on.All hydrocarbons are detected at high signal-to-noise ratio in the PDRwhere intense emission is seen both in the H2 ro-vibrational lines andin the PAH mid-infrared bands. C18O peaks farther away fromthe cloud edge. Our observations demonstrate that CCH, c-C3H2 and C4Hare present in UV-irradiated molecular gas, with abundances nearly ashigh as in dense, well-shielded molecular cores. PDR models i) need alarge density gradient at the PDR edge to correctly reproduce the offsetbetween the hydrocarbons and H2 peaks; and ii) fail to reproduce thehydrocarbon abundances. We propose that a new formation path of carbonchains, in addition to gas phase chemistry, should be considered inPDRs: because of intense UV-irradiation, large aromatic molecules andsmall carbon grains may fragment and feed the interstellar medium withsmall carbon clusters and molecules in significant amounts.

Spectroscopic analysis of tremendous-outburst-nova candidates
In the course of a long-term project investigating classical novae withlarge outburst amplitudes, we have performed optical spectroscopy ofseveral old-nova candidates. We here present the spectra of thecandidates V630 Sgr, XX Tau, CQ Vel,V842 Cen, and V529 Ori, that hitherto lacked suchclassification.While the first four show spectra typical of cataclysmic variables andcan thus be identified as such, V529 Ori is probablymisclassified. Of special interest are the two systems XX Tau andV842 Cen, which show signs of being low mass transfer systems. Assuch they can be used to judge the evolution scenarios for novae. Inparticular, given the rather young age of their outbursts, it appearsmore likely that these systems are not on their way into hibernation(i.e., cutting off mass transfer for a longer period of time), but aresimply settling down towards their original configuration ofcomparatively low, but steady, mass transfer, such as for dwarf novae.Based on observations collected at the European Southern Observatory, LaSilla, Chile.

Rotation and variability of very low mass stars and brown dwarfs near ɛ Ori
We explore the rotation and activity of very low mass (VLM) objects bymeans of a photometric variability study. Our targets in the vicinity ofɛ Ori belong to the Ori OB1b population in the Orionstar-forming complex. In this region we selected 143 VLM stars and browndwarfs (BDs), whose photometry in RIJHK is consistent with membership ofthe young population. The variability of these objects was investigatedusing a densely sampled I-band time series covering four consecutivenights with altogether 129 data points per object. Our targets showthree types of variability: Thirty objects, including nine BDs, showsignificant photometric periods, ranging from 4 h up to 100 h, which weinterpret as the rotation periods. Five objects, including two BDs,exhibit variability with high amplitudes up to 1 mag which is at leastpartly irregular. This behaviour is most likely caused by ongoingaccretion and confirms that VLM objects undergo a T Tauri phase similarto solar-mass stars. Finally, one VLM star shows a strong flare event of0.3 mag amplitude. The rotation periods show dependence on mass, i.e.the average period decreases with decreasing object mass, consistentwith previously found mass-period relationships in younger and olderclusters. The period distribution of BDs extends down to the breakupperiod, where centrifugal and gravitational forces are balanced.Combining our BD periods with literature data, we found that the lowerperiod limit for substellar objects lies between 2 h and 4 h, more orless independent of age. Contrary to stars, these fast rotating BDs seemto evolve at constant rotation period from ages of 3 Myr to 1 Gyr, inspite of the contraction process. Thus, they should experience strongrotational braking.Based on observations collected at the European Southern Observatory,Chile, observing run 68.C-0213(A)Table 2 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr ( or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1007.

A Deep Objective Prism Survey for Classical T Tauri Stars in the σ Orionis Region
A very deep 1976 CTIO Curtis-Schmidt objective prism plate of theσ Orionis association taken under excellent conditions haspermitted us to search for fainter classical T Tauri stars associatedwith this cluster. We have discovered 63 Hα emission stars in thearea to a limiting magnitude of about 18. Most of these stars are newadditions to the growing list of pre-main-sequence objects identified inthis very dense, very young cluster. Since T Tauri stars vary in lightand emission-line strength on all timescales, these data also provide anhistorical record of the association. By comparing these with laterobservations, we are able to estimate the average space density of theHα emission-line stars in the surveyed area as between 0.52 and0.86 pc-3 down to a spectral class of dM0. This is about athird of the number of dwarf or subgiant stars in the region that couldhave been detected as emission-line stars in these surveys.

XMM-Newton EPIC observations of stellar clusters and star forming regions
We report on observations of open clusters (OCs) and star formingregions (SFRs) obtained with the EPIC camera as part of the MissionScientist Guaranteed Time on XMM-Newton. These observations provide apowerful tool to investigate the evolution of coronal activity inlate-type convective stars and its dependence on magnetic fieldgeneration by dynamo processes. We discuss the motivations for thisprogram and present some results for the SFRs sigma Orionis (˜2-5Myr) and Taurus-Auriga (˜1-10 Myr) as well as for the OCs IC 2602(˜30 Myr), alpha Persei (˜50 Myr), Praesepe (˜600 Myr) andthe Hyades (˜600 Myr). We discuss imaging and spectral data providedby the EPIC MOS and PN detectors focussing on the determination of thecluster X-ray luminosity function and of the temperature structure,chemical abundances and time variability of cluster stars.Based on observations collected with the ESA mission XMM-Newton as partof the Mission Scientist (R. Pallavicini) Guaranteed Time

Extra-solar system planets: searches, discoveries and characteristics
The study of planets outside our solar system constitutes a new branchof astronomy that literally did not exist a decade ago. This discussionbegins with how people have thought about other worlds in the past andsome of the reported detections that turned out not to be true. Itcontinues with a brief description of several successful ways of findingexoplanets and the properties of the planets found and their host stars,and concludes with an attempt to look ahead. Most of the planets nowknown revealed themselves because their mutual orbits with their parentstars impose small, periodic radial velocity shifts in the stellarspectra, and most of the host stars are rich in heavy elements by thestandards of the solar neighbourhood. The inventory of actual andpotential detection methods has reached about two dozen.

A Galactic O Star Catalog
We have produced a catalog of 378 Galactic O stars with accuratespectral classifications that is complete for V<8 but includes manyfainter stars. The catalog provides cross-identifications with othersources; coordinates (obtained in most cases from Tycho-2 data);astrometric distances for 24 of the nearest stars; optical (Tycho-2,Johnson, and Strömgren) and NIR photometry; group membership,runaway character, and multiplicity information; and a Web-based versionwith links to on-line services.

The Substellar Population of the Young Cluster λ Orionis
By collecting optical and infrared photometry and low-resolutionspectroscopy, we have identified a large number of low-mass stars andbrown dwarf candidates belonging to the young cluster (~5 Myr)associated with the binary star λ Orionis. The lowest mass objectfound is an M8.5 with an estimated mass of 0.02 Msolar (~0.01Msolar for objects without spectroscopic confirmation). Forthose objects with spectroscopy, the measured strength of the Hαemission line follows a distribution similar to other clusters with thesame age range, with larger equivalent widths for cooler spectral types.Three of the brown dwarfs have Hα emission equivalent widths oforder 100 Å, suggesting that they may have accretion disks andthus are the substellar equivalent of classical T Tauri stars. We havederived the initial mass function for the cluster. For the substellarregime, the index of the mass spectrum is α=0.60+/-0.06, verysimilar to other young associations.

The Irradiated Herbig-Haro Jets Near σ Orionis
We present high-resolution echelle spectra of the irradiated Herbig-Harojets HH 444, 445, 446, and 447 from the Keck I telescope. Thestructural, kinematic, and physical properties of the jets near theirsources are analyzed in detail. In particular, spatio-kinematic electrondensity variations and evidence for excited, low-velocity entrainedmaterial are found. Despite their proximity to the massive stars in theσ Orionis system, the inner portions of these jets generallyexhibit low-excitation conditions, consistent with shadowing fromcircumstellar material. The outflow sources show Hα and forbiddenline emission on top of late-type photospheric spectra and stronglithium absorption, indicative of typical classical T Tauri stars. Thesespectroscopic data are discussed in the context of theoretical modelsthat attempt to account for the generation of outflows. In addition,narrowband Hubble Space Telescope (HST) WFPC2 images of HH 444 and 445are presented and compared with the spectroscopic data. In addition tothe jets, the images of both sources reveal extended emission structuresthat point directly away from σ Ori and are therefore interpretedas photoevaporating proplyds.

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Observation and Astrometry data

Right ascension:05h38m44.80s
Apparent magnitude:3.81
Distance:352.113 parsecs
Proper motion RA:0
Proper motion Dec:0
B-T magnitude:3.521
V-T magnitude:3.763

Catalogs and designations:
Proper NamesDaria Naumenko
Bayerσ Ori
Flamsteed48 Ori
HD 1989HD 37468
TYCHO-2 2000TYC 4771-1196-1
USNO-A2.0USNO-A2 0825-01619481
BSC 1991HR 1931
HIPHIP 26549

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