35 research outputs found
On the origin of [Ne II] emission in young stars: mid-infrared and optical observations with the Very Large Telescope
{Abridged version for ArXiv}. We provide direct constraints on the origin of
the [Ne II] emission in 15 young stars using high-spatial and spectral
resolution observations with VISIR at the VLT that allow us to study the
kinematics of the emitting gas. In addition we compare the [Ne II] line with
optical forbidden lines observed for three stars with UVES. The [Ne II] line
was detected in 7 stars, among them the first confirmed detection of [Ne II] in
a Herbig Be star, V892 Tau. In four cases, the large blueshifted lines indicate
an origin in a jet. In two stars, the small shifts and asymmetric profiles
indicate an origin in a photo-evaporative wind. CoKu Tau 1, seen close to
edge-on, shows a spatially unresolved line centered at the stellar rest
velocity, although cross-dispersion centroids move within 10 AU from one side
of the star to the other as a function of wavelength. The line profile is
symmetric with wings extending up to about +-80 km/s. The origin of the [Ne II]
line could either be due to the bipolar jet or to the disk. For the stars with
VLT-UVES observations, in several cases, the optical forbidden line profiles
and shifts are very similar to the profile of the [Ne II] line, suggesting that
the lines are emitted in the same region. A general trend observed with VISIR
is a lower line flux when compared with the fluxes obtained with Spitzer. We
found no correlation between the line full-width at half maximum and the line
peak velocity. The [Ne II] line remains undetected in a large part of the
sample, an indication that the emission detected with Spitzer in those stars is
likely extended.Comment: Accepted for publication in Astronomy & Astrophysics; revised
version: corrected minor typos, corrected center values (col 3) for CoKuTau1
in Table
HDE 245059: A Weak-Lined T Tauri Binary Revealed by Chandra and Keck
We present the Chandra High Energy Transmission Grating Spectrometer (HETGS)
and Keck observations of HDE 245059, a young weak-lined T Tauri star (WTTS),
member of the pre-main sequence group in the Lambda Orionis Cluster. Our high
spatial resolution, near-infrared observations with Keck reveal that HDE 245059
a binary separated by 0.87". Based on this new information we have obtained an
estimate of the masses of the binary components; 3M_{sun} and 2.5M_{sun} for
the north and south components, respectively. We have estimated the age of the
system to be ~2-3 Myr. We detect both components of the binary in the zeroth
order Chandra image and in the grating spectra. Our fits to the spectrum of the
binary have shown that the emission is dominated by a plasma between 8 and 15
MK, a soft component at 4 MK and a hard component at 50 MK are also detected.
The value of the hydrogen column density was low, 8 x 10^{19} cm^{-2}, likely
due to the clearing of the inner region of the Lambda Orionis cloud. The
abundance pattern shows an inverse First Ionization Potential (FIP) effect for
all elements from O to Fe, the only exception being Ca. A 3-T model was fitted
to the individual zeroth order spectra using the abundances derived for the
binary. We have also obtained several lines fluxes from the grating spectra.
The fits to the triplets show no evidence of high densities. We conclude that
the X-ray properties of the weak-lined T Tau binary HDE 245059 are similar to
those generally observed in other weak-lined T Tau stars. Although its
accretion history may have been affected by the clearing of the interstellar
material around Lambda Ori, its coronal properties appears not to have been
strongly modified.Comment: 12 pages, 10 figures Accepted for publication in Ap
High-Resolution Chandra X-Ray Imaging And Spectroscopy Of The Sigma Orionis Cluster
We present results of a 90 ks Chandra X-ray observation of the young sigma Orionis cluster ( age similar to 3 Myr) obtained with the HETGS. We use the high-resolution grating spectrum and moderate-resolution CCD spectrum of the massive central star sigma Ori AB (O9.5 V + B0.5 V) to test wind shock theories of X-ray emission and also analyze the high spatial resolution zero-order ACIS-S image of the central cluster region. Chandra detected 42 X-ray sources on the primary CCD (ACIS-S3). All but five have near-IR or optical counterparts and about one-fourth are variable. Notable high-mass stellar detections are sigma Ori AB, the magnetic B star sigma Ori E, and the B5 V binary HD 37525. Most of the other detections have properties consistent with lower mass K- or M-type stars. We present the first X-ray spectrum of the unusual infrared source IRS 1, located approximate to 3 \u27\u27 north of sigma Ori AB. Its X-ray properties and elongated mid-IR morphology suggest that it is an embedded low-mass T Tauri star whose disk/envelope is being photoevaporated by sigma Ori AB. We focus on the radiative wind shock interpretation of the soft luminous X-ray emission from sigma Ori AB, but also consider possible alternatives including magnetically confined wind shocks and colliding wind shocks. Its emission lines show no significant asymmetries or centroid shifts and are moderately broadened to HWHM approximate to 264 km s(-1), or one-fourth the terminal wind speed. Forbidden lines in He-like ions are formally undetected, implying strong UV suppression. The Mg XI triplet forms in the wind acceleration zone within one stellar radius above the surface. These X-ray properties are consistent in several respects with the predictions of radiative wind shock theory for an optically thin wind, but explaining the narrow line widths presents a challenge to the theory
Dust amorphization in protoplanetary disks
High-energy irradiation of the circumstellar material might impact the
structure and the composition of a protoplanetary disk and hence the process of
planet formation. In this paper, we present a study on the possible influence
of the stellar irradiation, indicated by X-ray emission, on the crystalline
structure of the circumstellar dust. The dust crystallinity is measured for 42
class II T Tauri stars in the Taurus star-forming region using a decomposition
fit of the 10 micron silicate feature, measured with the Spitzer IRS
instrument. Since the sample includes objects with disks of various
evolutionary stages, we further confine the target selection, using the age of
the objects as a selection parameter. We correlate the X-ray luminosity and the
X-ray hardness of the central object with the crystalline mass fraction of the
circumstellar dust and find a significant anti-correlation for 20 objects
within an age range of approx. 1 to 4.5 Myr. We postulate that X-rays represent
the stellar activity and consequently the energetic ions of the stellar winds
which interact with the circumstellar disk. We show that the fluxes around 1 AU
and ion energies of the present solar wind are sufficient to amorphize the
upper layer of dust grains very efficiently, leading to an observable reduction
of the crystalline mass fraction of the circumstellar, sub-micron sized dust.
This effect could also erase other relations between crystallinity and
disk/star parameters such as age or spectral type.Comment: accepted for publication by A&
The disk-bearing young star IM Lup
Classical T Tauri stars (CTTS) differ in their X-ray signatures from older
pre-main sequence stars, e.g. weak-lined TTS (WTTS). CTTS show a soft excess
and deviations from the low-density coronal limit in the He-like triplets. We
test whether these features correlate with accretion or the presence of a disk
by observing IM Lup, a disk-bearing object apparently in transition between
CTTS and WTTS without obvious accretion. We analyse a Chandra grating spectrum
and additional XMM-Newton data of IM Lup and accompanying optical spectra, some
of them taken simultaneously to the X-ray observations. We fit the X-ray
emission lines and decompose the Ha emission line in different components. In
X-rays IM Lup has a bright and hot active corona, where elements of low
first-ionisation potential are depleted. The He-like Ne IX triplet is in the
low-density state, but due to the small number of counts a high-density
scenario cannot be excluded on the 90% confidence level. In all X-ray
properties IM Lup resembles a main-sequence star, but it is also compatible
with CTTS signatures on the 90% confidence level, thus we cannot decide if the
soft excess and deviations from the low-density coronal limit in the He-like
triplets in CTTS require accretion or only the presence of a disk. IM Lup is
chromospherically active, which explains most of the emission in Ha. Despite
its low equivalent width, the complexity of the Ha line profile is reminiscent
of CTTS. We present an estimate for the mass accretion rate of 10e-11 Msun/yr.Comment: 10 pages, 6 figures, accepted by A&
A gas density drop in the inner 6 AU of the transition disk around the Herbig Ae star HD 139614
A. Carmona was partly supported by the Spanish Grant AYA 2011-26202. A. Carmona, A. Kospal and Zs. Regaly were partly supported by the Momentum grant of the MTA CSFK Lendulet Disk Research Group of the Hungarian Academy of Sciences. A. Carmona and C. Pinte acknowledge funding from the European Commissionâs 7th Framework Program (ECFP7) (contract PERG06-GA-2009-256513) and from Agence Nationale pour la Recherche (ANR) of France under contract ANR-2010-JCJC-0504-01. A. Carmona acknowledges financial support by the European Southern Observatory visitors program. The research leading to these results has received funding from the EC-FP7 under grant agreement no 284405. C. Eiroa is partly supported by the Spanish Grant AYA 2014-55840-P L. A. Cieza was supported by CONICYT-FONDECYT grant number 1140109 and the Millennium Science Initiative (Chilean Ministry of Economy), through grant Nucleus RC130007.Context. Quantifying the gas surface density inside the dust cavities and gaps of transition disks is important to establish their origin. Aims. We seek to constrain the surface density of warm gas in the inner disk of HD 139614, an accreting 9 Myr Herbig Ae star with a (pre-)transition disk exhibiting a dust gap from 2.30.1 to 5.60.3 AU. Methods. We observed HD 139614 with ESO/VLT CRIRES and obtained high-resolution (R~90 000) spectra of CO isotopologues ro-vibrational emission at 4.7 ÎŒm. We derive constraints on the diskâs structure by modeling the CO isotopologue line profiles, the spectroastrometric signal, and the rotational diagrams using grids of flat Keplerian disk models. Results. We detected v = 1 â 0 12CO, 2â1 12CO, 1â0 13CO, 1â0 C18O, and 1â0 C17O ro-vibrational lines. Lines are consistent with disk emission and thermal excitation. 12CO v = 1â 0 lines have an average width of 14 km s-1, Tgas of 450 K and an emitting region from 1 to 15 AU. 13CO and C18O lines are on average 70 and 100 K colder, 1 and 4 km s-1 narrower than 12CO v = 1â 0, and are dominated by emission at R â„ 6 AU. The 12CO = 1 â 0 composite line profile indicates that if there is a gap devoid of gas it must have a width narrower than 2 AU. We find that a drop in the gas surface density (ÎŽgas) at R = 5 â 6 AU is required to be able to reproduce the line profiles and rotational diagrams of the three CO isotopologues simultaneously. Models without a gas density drop generate 13CO and C18O emission lines that are too broad and warm. The value of gas can range from 10-2 to 10-4 depending on the gas-to-dust ratio of the outer disk. We find that the gas surface density profile at 1 2 AU) gas gap suggest the presence of an embedded < 2 MJ planet at around 4 AU.PostprintPeer reviewe
Puzzling thermonuclear burst behaviour from the transient low-mass X-ray binary IGR J17473-2721
We investigate the thermonuclear bursting behaviour of IGR J17473-2721, an
X-ray transient that in 2008 underwent a six month long outburst, starting
(unusually) with an X-ray burst. We detected a total of 57 thermonuclear bursts
throughout the outburst with AGILE, Swift, RXTE, and INTEGRAL. The wide range
of inferred accretion rates (between <1% and about 20% of the Eddington
accretion rate m-dot_Edd) spanned during the outburst allows us to study
changes in the nuclear burning processes and to identify up to seven different
phases. The burst rate increased gradually with the accretion rate until it
dropped (at a persistent flux corresponding to about 15% of m-dot_Edd) a few
days before the outburst peak, after which bursts were not detected for a
month. As the persistent emission subsequently decreased, the bursting activity
resumed with a much lower rate than during the outburst rise. This hysteresis
may arise from the thermal effect of the accretion on the surface nuclear
burning processes, and the timescale is roughly consistent with that expected
for the neutron star crust thermal response. On the other hand, an undetected
superburst, occurring within a data gap near the outburst peak, could have
produced a similar quenching of burst activity.Comment: 18 pages, 12 figures, 1 table, accepted for publication in MNRA
A multi-wavelength study of the young star V1118 Orionis in outburst
Abriged version for astroph: The young late-type star V1118 Orionis was in
outburst from 2005 to 2006. We followed the outburst with optical and
near-infrared photometry; the X-ray emission was further probed with
observations taken with XMM-Newton and Chandra during and after the outburst.
In addition, we obtained mid-infrared photometry and spectroscopy with Spitzer
at the peak of the outburst and in the post-outburst phase. The spectral energy
distribution of V1118 Ori varied significantly over the course of the outburst.
The optical flux showed the largest variations, most likely due to enhanced
emission by a hot spot. The latter dominated the optical and near-infrared
emission at the peak of the outburst, while the disk emission dominated in the
mid-infrared. The X-ray flux correlated with the optical and infrared fluxes,
indicating that accretion affected the magnetically active corona and the
stellar magnetosphere. The thermal structure of the corona was variable with
some indication of a cooling of the coronal temperature in the early phase of
the outburst with a gradual return to normal values. Color-color diagrams in
the optical and infrared showed variations during the outburst, with no obvious
signature of reddening due to circumstellar matter. Using MC realizations of
star+disk+hotspot models to fit the SED in ``quiescence'' and at the peak of
the outburst, we determined that the mass accretion rate varied from about
2.5E-7 Msun/yr to 1E-6 Msun/yr; in addition the fractional area of the hotspot
increased significantly as well. The multi-wavelength study of the V1118 Ori
outburst helped us to understand the variations in spectral energy
distributions and demonstrated the interplay between the disk and the stellar
magnetosphere in a young, strongly accreting star.Comment: Accepted in A&A, Tables will be published onlin
The Taurus Spitzer Survey: New Candidate Taurus Members Selected Using Sensitive Mid-Infrared Photometry
We report on the properties of pre-main-sequence objects in the Taurus
molecular clouds as observed in 7 mid- and far-infrared bands with the Spitzer
Space Telescope. There are 215 previously-identified members of the Taurus
star-forming region in our ~44 square degree map; these members exhibit a range
of Spitzer colors that we take to define young stars still surrounded by
circumstellar dust (noting that ~20% of the bonafide Taurus members exhibit no
detectable dust excesses). We looked for new objects in the survey field with
similar Spitzer properties, aided by extensive optical, X-ray, and ultraviolet
imaging, and found 148 candidate new members of Taurus. We have obtained
follow-up spectroscopy for about half the candidate sample, thus far confirming
34 new members, 3 probable new members, and 10 possible new members, an
increase of 15-20% in Taurus members. Of the objects for which we have
spectroscopy, 7 are now confirmed extragalactic objects, and one is a
background Be star. The remaining 93 candidate objects await additional
analysis and/or data to be confirmed or rejected as Taurus members. Most of the
new members are Class II M stars and are located along the same cloud filaments
as the previously-identified Taurus members. Among non-members with Spitzer
colors similar to young, dusty stars are evolved Be stars, planetary nebulae,
carbon stars, galaxies, and AGN.Comment: Accepted to ApJS. Two large online-only figures available with the
preprint here: http://web.ipac.caltech.edu/staff/rebull/research.htm