833 research outputs found
Identification of Emission Lines in the Low-Ionization Strontium Filament Near Eta Carinae
We have obtained deep spectra from 1640 to 10100A with the Space Telescope
Imaging Spectrograph (STIS) of the Strontium Filament, a largely neutral
emission nebulosity lying close to the very luminous star Eta Carinae and
showing an uncommon spectrum. Over 600 emission lines, both permitted and
forbidden, have been identified. The majority originates from neutral or
singly-ionized iron group elements (Sc, Ti, V, Cr, Mn, Fe, Co, Ni). Sr is the
only neutron capture element detected. The presence of Sr II, numerous strong
Ti II and V II lines and the dominance of Fe I over Fe II are notable
discoveries. While emission lines of hydrogen, helium, and nitrogen are
associable with other spatial structures at other velocities within the
Homunculus, no emission lines from these elements correspond to the spatial
structure or velocity of the Sr Filament. Moreover, no identified Sr Filament
emission line requires an ionization or excitation energy above approximately 8
eV. Ionized gas extends spatially along the aperture, oriented along the polar
axis of the Homunculus, and in velocity around the Strontium Filament. We
suggest that the Strontium Filament is shielded from ultraviolet radiation at
energies above 8 eV, but is intensely irradiated by the central star at
wavelengths longward of 1500A.Comment: 28 pages, 5 figures, 4 tables. Accepted by A&A. High resolution
pictures can be found at http://www.astro.lu.se/~henrikh/srpaper/srpaper.pd
A New Calculation of Ne IX Line Diagnostics
We describe the effect that new atomic calculations, including
fully-relativistic R-matrix calculations of collisional excitation rates and
level-specific dielectronic and radiative recombination rates, have on line
ratios from the astrophysically significant ion Ne IX. The new excitation rates
systematically change some predicted Ne IX line ratios by 25% at temperatures
at or below the temperature of maximum emissivity (4x10^6 K), while the new
recombination rates lead to systematic changes at higher temperatures. The new
line ratios are shown to agree with observations of Capella and sigma^2 CrB
significantly better than older line ratios, showing that 25-30% accuracy in
atomic rates is inadequate for high-resolution X-ray observations from existing
spectrometers.Comment: 5 pages, 6 figure
Time variations of the narrow FeII and HI spectral emission lines from the close vicinity of Eta Carinae during the spectral event of 2003
The spectrum of Eta Carinae and its ejecta shows slow variations over a
period of 5.5 years. However, the spectrum changes drastically on a time scale
of days once every period called the 'spectral event'. We report on variations
in the narrow emission line spectrum of gas condensations (the Weigelt blobs)
close to the central star during a spectral event. The rapid changes in the
stellar radiation field illuminating the blobs make the blobs a natural
astrophysical laboratory to study atomic photoprocesses. The different
responses of the HI Paschen lines, fluorescent lines and forbidden
[FeII] lines allow us to identify the processes and estimate physical
conditions in the blobs. This paper is based on observations from the Pico dos
Dias Observatory (LNA/Brazil) during the previous event in June 2003.Comment: Accepted for publication in A&
Estimating the furrow infiltration characteristic from a single advance point
Management and control of surface irrigation, in particular furrow irrigation, is limited by spatio-temporal soil infiltration variability as well as the high cost and time associated with collecting intensive field data for estimation of the infiltration characteristics. Recent work has proposed scaling the commonly used infiltration function by using a model infiltration curve and a single advance point for every other furrow in an irrigation event. Scaling factors were calculated for a series of furrows at two sites and at four points down the length of the field (0.25 L, 0.5 L, 0.75 L and L). Differences in the value of the scaling factor with distance were found to be a function of the shape of the advance curves. It is concluded that use of points early in the advance results in a substantial loss of accuracy and should be avoided. The scaling factor was also strongly correlated with the furrow-wetted perimeter suggesting that the scaling is an appropriate way of both predicting and accommodating the effect of the hydraulic variability
Eta Carinae -- Physics of the Inner Ejecta
Eta Carinae's inner ejecta are dominated observationally by the bright
Weigelt blobs and their famously rich spectra of nebular emission and
absorption lines. They are dense (n_e ~ 10^7 to 10^8 cm^-3), warm (T_e ~ 6000
to 7000 K) and slow moving (~40 km/s) condensations of mostly neutral (H^0)
gas. Located within 1000 AU of the central star, they contain heavily
CNO-processed material that was ejected from the star about a century ago.
Outside the blobs, the inner ejecta include absorption-line clouds with similar
conditions, plus emission-line gas that has generally lower densities and a
wider range of speeds (reaching a few hundred km/s) compared to the blobs. The
blobs appear to contain a negligible amount of dust and have a nearly dust-free
view of the central source, but our view across the inner ejecta is severely
affected by uncertain amounts of dust having a patchy distribution in the
foreground. Emission lines from the inner ejecta are powered by photoionization
and fluorescent processes. The variable nature of this emission, occurring in a
5.54 yr event cycle, requires specific changes to the incident flux that hold
important clues to the nature of the central object.Comment: This is Chapter 5 in a book entitled: Eta Carinae and the Supernova
Impostors, Kris Davidson and Roberta M. Humphreys, editors Springe
The FERRUM Project: experimental and theoretical transition rates of forbidden [Sc II] lines and radiative lifetimes of metastable Sc II levels
Context. In many plasmas, long-lived metastable atomic levels are depopulated
by collisions (quenched) before they decay radiatively. In low-density regions,
however, the low collision rate may allow depopulation by electric dipole (E1)
forbidden radiative transitions, so-called forbidden lines (mainly M1 and E2
transitions). If the atomic transition data are known, these lines are
indicators of physical plasma conditions and used for abundance determination.
Aims. Transition rates can be derived by combining relative intensities between
the decay channels, so-called branching fractions (BFs), and the radiative
lifetime of the common upper level. We use this approach for forbidden [Sc ii]
lines, along with new calculations. Methods. Neither BFs for forbidden lines,
nor lifetimes of metastable levels, are easily measured in a laboratory.
Therefore, astrophysical BFs measured in Space Telescope Imaging Spectrograph
(STIS) spectra of the strontium filament of Eta Carinae are combined with
lifetime measurements using a laser probing technique on a stored ion-beam
(CRYRING facility,MSL, Stockholm). These quantities are used to derive the
absolute transition rates (A-values). New theoretical transition rates and
lifetimes are calulated using the CIV3 code. Results. We report experimental
lifetimes of the Sc ii levels 3d2 a3P0,1,2 with lifetimes 1.28, 1.42, and 1.24
s, respectively, and transition rates for lines from these levels down to 3d4s
a3D in the region 8270-8390 A. These are the most important forbidden [Sc ii]
transitions. New calculations for lines and metastable lifetimes are also
presented, and are in good agreement with the experimental data.Comment: 5 pages. Accepted for A&
A Near-Infrared Spectroscopic Survey of Class I Protostars
We present the results of a near-IR spectroscopic survey of 110 Class I
protostars observed from 0.80 microns to 2.43 microns at a spectroscopic
resolution of R=1200. We find that Class I objects exhibit a wide range of
lines and the continuum spectroscopic features. 85% of Class I protostars
exhibit features indicative of mass accretion, and we found that the veiling
excess, CO emission, and Br Gamma emission are closely related. We modeled the
spectra to estimate the veiling excess (r_k) and extinction to each target. We
also used near-IR colors and emission line ratios, when available, to also
estimate extinction. In the course of this survey, we observed the spectra of
10 FU Orionis-like objects, including 2 new ones, as well as 3 Herbig Ae type
stars among our Class I YSOs. We used photospheric absorption lines, when
available, to estimate the spectral type of each target. Although most targets
are late type stars, there are several A and F-type stars in our sample.
Notably, we found no A or F class stars in the Taurus-Auriga or Perseus star
forming regions. There are several cases where the observed CO and/or water
absorption bands are deeper than expected from the photospheric spectral type.
We find a correlation between the appearance of the reflection nebula, which
traces the distribution of material on very large scales, and the near-IR
spectrum, which probes smaller scales. The spectra of the components of
spatially resolved protostellar binaries tend to be very similar. In particular
both components tend to have similar veiling and H_2 emission, inconsistent
with random selection from the sample as a whole. There is a strong correlation
between [Fe II] and H_2 emission, supporting previous results showing that H_2
emission in the spectra of young stars is usually shock excited by stellar
winds.Comment: 89 pages, 13 figures, 7 Table
Highly Variable Extinction and Accretion in the Jet-driving Class I Type Young Star PTF 10nvg (V2492 Cyg, IRAS 20496+4354)
We report extensive new photometry and spectroscopy of the highly variable
young stellar object PTF 10nvg including optical and near-infrared time series
data as well as mid-infrared and millimeter data. Following the previously
reported 2010 rise, during 2011 and 2012 the source underwent additional
episodes of brightening and dimming events including prolonged faint states.
The observed high-amplitude variations are largely consistent with extinction
changes having a 220 day quasi-periodic signal. Spectral evolution includes not
only changes in the spectral slope but correlated variation in the prominence
of TiO/VO/CO bands and atomic line emission, as well as anticorrelated
variation in forbidden line emission which, along with H_2, dominates optical
and infrared spectra at faint epochs. Neutral and singly-ionized atomic species
are likely formed in an accretion flow and/or impact while the origin of
zero-velocity atomic LiI 6707 in emission is unknown. Forbidden lines,
including several rare species, exhibit blueshifted emission profiles and
likely arise from an outflow/jet. Several of these lines are also seen
spatially offset from the continuum source position, presumably in a shocked
region of an extended jet. CARMA maps resolve on larger scales a spatially
extended outflow in mm-wavelength CO. We attribute the observed photometric and
spectroscopic behavior in terms of occultation of the central star as well as
the bright inner disk and the accretion/outflow zones that renders shocked gas
in the inner part of the jet amenable to observation at the faint epochs. We
discuss PTF 10nvg as a source exhibiting both accretion-driven (perhaps
analogous to V1647 Ori) and extinction-driven (perhaps analogous to UX Ori or
GM Cep) high-amplitude variability phenomena.Comment: accepted to AJ - in press (74 pages
A connection between star formation activity and cosmic rays in the starburst galaxy M 82
Although Galactic cosmic rays (protons and nuclei) are widely believed to be
dominantly accelerated by the winds and supernovae of massive stars, definitive
evidence of this origin remains elusive nearly a century after their discovery
[1]. The active regions of starburst galaxies have exceptionally high rates of
star formation, and their large size, more than 50 times the diameter of
similar Galactic regions, uniquely enables reliable calorimetric measurements
of their potentially high cosmic-ray density [2]. The cosmic rays produced in
the formation, life, and death of their massive stars are expected to
eventually produce diffuse gamma-ray emission via their interactions with
interstellar gas and radiation. M 82, the prototype small starburst galaxy, is
predicted to be the brightest starburst galaxy in gamma rays [3, 4]. Here we
report the detection of >700 GeV gamma rays from M 82. From these data we
determine a cosmic-ray density of 250 eV cm-3 in the starburst core of M 82, or
about 500 times the average Galactic density. This result strongly supports
that cosmic-ray acceleration is tied to star formation activity, and that
supernovae and massive-star winds are the dominant accelerators.Comment: 18 pages, 4 figures; published in Nature; Version is prior to
Nature's in-house style editing (differences are minimal
Multiwavelength Observations of LS I +61 303 with VERITAS, Swift and RXTE
We present results from a long-term monitoring campaign on the TeV binary LSI
+61 303 with VERITAS at energies above 500 GeV, and in the 2-10 keV hard X-ray
bands with RXTE and Swift, sampling nine 26.5 day orbital cycles between
September 2006 and February 2008. The binary was observed by VERITAS to be
variable, with all integrated observations resulting in a detection at the 8.8
sigma (2006/2007) and 7.3 sigma (2007/2008) significance level for emission
above 500 GeV. The source was detected during active periods with flux values
ranging from 5 to 20% of the Crab Nebula, varying over the course of a single
orbital cycle. Additionally, the observations conducted in the 2007-2008
observing season show marginal evidence (at the 3.6 sigma significance level)
for TeV emission outside of the apastron passage of the compact object around
the Be star. Contemporaneous hard X-ray observations with RXTE and Swift show
large variability with flux values typically varying between 0.5 and 3.0*10^-11
ergs cm^-2 s^-1 over a single orbital cycle. The contemporaneous X-ray and TeV
data are examined and it is shown that the TeV sampling is not dense enough to
detect a correlation between the two bands.Comment: 30 pages, 5 figures, 2 table, Accepted for publication in The
Astrophysical Journa
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