168 research outputs found
Rusty old stars: a source of the missing interstellar iron?
Iron, the Universe's most abundant refractory element, is highly depleted in
both circumstellar and interstellar environments, meaning it exists in solid
form. The nature of this solid is unknown. In this Letter, we provide evidence
that metallic iron grains are present around oxygen-rich AGB stars, where it is
observationally manifest as a featureless mid-infrared excess. This
identification is made using Spitzer Space Telescope observations of evolved
globular cluster stars, where iron dust production appears ubiquitous and in
some cases can be modelled as the only observed dust product. In this context,
FeO is examined as the likely carrier for the 20-micron feature observed in
some of these stars. Metallic iron appears to be an important part of the dust
condensation sequence at low metallicity, and subsequently plays an influential
role in the interstellar medium. We explore the stellar metallicities and
luminosities at which iron formation is observed, and how the presence of iron
affects the outflow and its chemistry. The conditions under which iron can
provide sufficient opacity to drive a wind remain unclear.Comment: 7 pages. 4 figures, accepted ApJ
Spatially resolved mid-infrared observations of the triple system T Tauri
To enhance our knowledge of the characteristics and distribution of the
circumstellar dust associated with the individual components of the young
hierarchical triple system T Tau, observations in the N-band with MIDI at the
VLTI were performed. Our study is based on both the interferometric and the
spectrophotometric measurements and is supplemented by new visual and infrared
photometry. Also, the phases were investigated to determine the dominating
mid-infrared source in the close southern binary. The data were fit with the
help of a sophisticated physical disc model. This model utilises the radiative
transfer code MC3D that is based on the Monte-Carlo method. Extended
mid-infrared emission is found around all three components of the system.
Simultaneous fits to the photometric and interferometric data confirm the
picture of an almost face-on circumstellar disc around T Tau N. Towards this
star, the silicate band is seen in emission. This emission feature is used to
model the dust content of the circumstellar disc. Clear signs of dust
processing are found. Towards T Tau S, the silicate band is seen in absorption.
This absorption is strongly pronounced towards the infrared companion T Tau Sa
as can be seen from the first individual N-band spectra for the two southern
components. Our fits support the previous suggestion that an almost edge-on
disc is present around T Tau Sa. This disc is thus misaligned with respect to
the circumstellar disc around T Tau N. The interferometric data indicate that
the disc around T Tau Sa is oriented in the north-south direction, which
favours this source as launching site for the east-western jet. We further
determine from the interferometric data the relative positions of the
components of the southern binary.Comment: 24 pages, 19 figures, accepted for publication in A&
HD 144432: a young triple system
We present new imaging and spectroscopic data of the young Herbig star HD
144432 A, which was known to be a binary star with a separation of 1.47 arcsec.
High-resolution NIR imaging data obtained with NACO at the VLT reveal that HD
144432 B itself is a close binary pair with a separation of 0.1 arcsec.
High-resolution optical spectra, acquired with FEROS at the 2.2m MPG/ESO
telescope in La Silla, of the primary star and its co-moving companions were
used to determine their main stellar parameters such as effective temperature,
surface gravity, radial velocity, and projected rotational velocity by fitting
synthetic spectra to the observed stellar spectra. The two companions, HD
144432 B and HD 144432 C, are identified as low-mass T Tauri stars of spectral
type K7V and M1V, respectively. From the position in the HRD the triple system
appears to be co-eval with a system age of 6+/-3 Myr.Comment: Accepted for publication in Astronomy & Astrophysics, 4 pages, 4
figure
MBM 12: young protoplanetary discs at high galactic latitude
(abridged) We present Spitzer infrared observations to constrain disc and
dust evolution in young T Tauri stars in MBM 12, a star-forming cloud at high
latitude with an age of 2 Myr and a distance of 275 pc. The region contains 12
T Tauri systems, with primary spectral types between K3 and M6; 5 are weak-line
and the rest classical T Tauri stars. We first use MIPS and literature
photometry to compile spectral energy distributions for each of the 12 members
in MBM 12, and derive their IR excesses. The IRS spectra are analysed with the
newly developed two-layer temperature distribution (TLTD) spectral
decomposition method. For the 7 T Tauri stars with a detected IR excess, we
analyse their solid-state features to derive dust properties such as
mass-averaged grain size, composition and crystallinity. We find a spatial
gradient in the forsterite to enstatite range, with more enstatite present in
the warmer regions. The fact that we see a radial dependence of the dust
properties indicates that radial mixing is not very efficient in the discs of
these young T Tauri stars. The SED analysis shows that the discs in MBM 12, in
general, undergo rapid inner disc clearing, while the binary sources have
faster discevolution. The dust grains seem to evolve independently from the
stellar properties, but are mildly related to disc properties such as flaring
and accretion rates.Comment: 14 pages, accepted by Astronomy and Astrophysic
Variable accretion as a mechanism for brightness variations in T Tau S
(Note: this is a shortened version of the original A&A-style structured
abstract). The physical nature of the strong photometric variability of T Tau
Sa, the more massive member of the Southern "infrared companion" to T Tau, has
long been debated. Intrinsic luminosity variations due to variable accretion
were originally proposed but later challenged in favor of apparent fluctuations
due to time-variable foreground extinction. In this paper we use the timescale
of the variability as a diagnostic for the underlying physical mechanism.
Because the IR emission emerging from Sa is dominantly thermal emission from
circumstellar dust at <=1500K, we can derive a minimum size of the region
responsible for the time-variable emission. In the context of the variable
foreground extinction scenario, this region must be (un-) covered within the
variability timescale, which implies a minimum velocity for the obscuring
foreground material. If this velocity supercedes the local Kepler velocity we
can reject foreground extinction as a valid variability mechanism. The variable
accretion scenario allows for shorter variability timescales since the
variations in luminosity occur on much smaller scales, essentially at the
surface of the star, and the disk surface can react almost instantly on the
changing irradiation with a higher or lower dust temperature and according
brightness. We have detected substantial variations at long wavelengths in T
Tau S: +26% within four days at 12.8 micron. We show that this short-term
variability cannot be due to variable extinction and instead must be due to
variable accretion. Using a radiative transfer model of the Sa disk we show
that variable accretion can in principle also account for the much larger
(several magnitude) variations observed on timescales of several years. For the
long-term variability, however, also variable foreground extinction is a viable
mechanism.Comment: 15 pages, 8 figures, Accepted for publication in Astronomy and
Astrophysic
Two-photon exchange effect on deuteron electromagnetic form factors
Corrections of two-photon exchange to proton and neutron electromagnetic form
factors are employed to study the effect of two-photon exchange on the deuteron
electromagnetic form factors. Numerical results of the effect are given. It is
suggested to test the effect in the measurement of in a small angle
limit.Comment: 12 pages 9 figure
Star and protoplanetary disk properties in Orion's suburbs
(Note: this is a shortened version of the original "structured" A&A format
abstract.) We performed a large optical spectroscopic and photometric survey of
the Lynds~1630N and 1641 clouds. We provide a catalog of 132 confirmed young
stars in L1630N and 267 such objects in L1641. We identify 28 transition disk
systems, 20 of which were previously unknown, as well as 42 new transition disk
candidates for which we have broad-band photometry but no optical spectroscopy.
We estimate mass accretion rates M_acc from the equivalent widths of the
H_alpha, H_beta, and HeI 5876\AA emission lines, and find a dependence on
stellar mass of M_acc propto Mstar^alpha, with alpha~3.1 in the subsolar mass
range that we probe. An investigation of a large literature sample of mass
accretion rate estimates yields a similar slope of alpha~2.8 in the subsolar
regime, but a shallower slope of alpha~2.0 if the whole mass range of 0.04
M_sun-5 Msun is included. Among the transition disk objects, the fraction of
stars that show significant accretion activity is relatively low compared to
stars with still optically thick disks (26\pm11% vs. 57\pm6%, respectively).
However, those transition disks that do show significant accretion have the
same median accretion rate as normal optically thick disks of 3-4*10^{-9}
M_sun/yr. We find that the ages of the transition disks and the WTTSs without
disks are statistically indistinguishable, and both groups are significantly
older than the CTTSs. These results argue against disk-binary interaction or
gravitational instability as mechanisms causing a transition disk appearance.
Our observations indicate that disk lifetimes in the clustered population are
shorter than in the distributed population. We propose refined Halpha
equivalent width criteria to distinguish WTTSs from CTTSs.Comment: 52 pages, 16 tables, 29 figures. Accepted by A&A. Table numbering
error correcte
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