230 research outputs found
Coordinated NIR/mm observations of flare emission from Sagittarius A*
Context. We report on a successful, simultaneous observation and modelling of the millimeter (mm) to near-infrared (NIR) flare emission of the Sgr A* counterpart associated with the supermassive (4 × 10^6 M_☉) black hole at the Galactic centre (GC). We present a mm/sub-mm light curve of Sgr A* with one of the highest quality continuous time coverages.
Aims. We study and model the physical processes giving rise to the variable emission of Sgr A*.
Methods. Our non-relativistic modelling is based on simultaneous observations carried out in May 2007 and 2008, using the NACO adaptive optics (AO) instrument at the ESO's VLT and the mm telescope arrays CARMA in California, ATCA in Australia, and the 30 m IRAM telescope in Spain. We emphasize the importance of multi-wavelength simultaneous fitting as a tool for imposing adequate constraints on the flare modelling. We present a new method for obtaining concatenated light curves of the compact mm-source Sgr A* from single dish telescopes and interferometers in the presence of significant flux density contributions from an extended and only partially resolved source.
Results. The observations detect flaring activity in both the mm domain and the NIR. Inspection and modelling of the light curves show that in the case of the flare event on 17 May 2007, the mm emission follows the NIR flare emission with a delay of 1.5±0.5 h. On 15 May 2007, the NIR flare emission is also followed by elevated mm-emission. We explain the flare emission delay by an adiabatic expansion of source components. For two other NIR flares, we can only provide an upper limit to any accompanying mm-emission of about 0.2 Jy. The derived physical quantities that describe the flare emission give a source component expansion speed of ν_(exp) ~ 0.005c–0.017c, source sizes of about one Schwarzschild radius, flux densities of a few Janskys, and spectral indices of α = 0.6 to 1.3. These source components peak in the THz regime.
Conclusions. These parameters suggest that either the adiabatically expanding source components have a bulk motion greater than ν_(exp) or the expanding material contributes to a corona or disk, confined to the immediate surroundings of Sgr A*. Applying the flux density values or limits in the mm- and X-ray domain to the observed flare events constrains the turnover frequency of the synchrotron components that are on average not lower than about 1 THz, such that the optically thick peak flux densities at or below these turnover frequencies do not exceed, on average, about ~1 Jy
A Near-Infrared Survey of the Inner Galactic Plane for Wolf-Rayet Stars II. Going Fainter: 71 More New WR Stars
We are continuing a J, K and narrow-band imaging survey of 300 square degrees
of the plane of the Galaxy, searching for new Wolf-Rayet stars. Our survey
spans 150 degrees in Galactic longitude and reaches 1 degree above and below
the Galactic plane. The survey has a useful limiting magnitude of K = 15 over
most of the observed Galactic plane, and K = 14 (due to severe crowding) within
a few degrees of the Galactic center. Thousands of emission line candidates
have been detected. In spectrographic follow-ups of 146 relatively bright WR
star candidates we have re-examined 11 previously known WC and WN stars and
discovered 71 new WR stars, 17 of type WN and 54 of type WC. Our latest image
analysis pipeline now picks out WR stars with a 57% success rate. Star subtype
assignments have been confirmed with K band spectra, and distances approximated
using the method of spectroscopic parallax. Some of the new WR stars are
amongst the most distant known in our Galaxy. The distribution of these new WR
stars is beginning to trace the locations of massive stars along the distant
spiral arms of the Milky Way.Comment: 48 pages, 25 figures, 8 Tables, Accepted by AJ 4-18-201
Intra-day Variability of Sagittarius A* at 3 Millimeters
We report observations and analysis of flux monitoring of Sagittarius A* at
3-mm wavelength using the OVRO millimeter interferometer over a period of eight
days (2002 May 23-30). Frequent phase and flux referencing (every 5 minutes)
with the nearby calibrator source J1744-312 was employed to control for
instrumental and atmospheric effects. Time variations are sought by computing
and subtracting, from each visibility in the database, an average visibility
obtained from all the data acquired in our monitoring program having similar uv
spacings. This removes the confusing effects of baseline-dependent, correlated
flux interference caused by the static, thermal emission from the extended
source Sgr A West. Few-day variations up to ~20% and intra-day variability of
\~20% and in some cases up to ~40% on few-hour time scales emerge from the
differenced data on SgrA*. Power spectra of the residuals indicate the presence
of hourly variations on all but two of the eight days. Monte Carlo simulation
of red-noise light curves indicates that the hourly variations are well
described by a red-noise power spectrum with P(f) ~ f^(-1). Of particular
interest is a ~2.5 hour variation seen prominently on two consecutive days. An
average power spectrum from all eight days of data reveals noteworthy power on
this time scale. There is some indication that few-hour variations are more
pronounced on days when the average daily flux is highest. We briefly discuss
the possibility that these few-hour variations are due to the dynamical
modulation of accreting gas around the central supermassive black hole, as well
as the implications for the structure of the SgrA* photosphere at 3 mm.
Finally, these data have enabled us to produce a high sensitivity 3-mm map of
the extended thermal emission surrounding SgrA*.Comment: Accepted for publication in The Astrophysical Journal Letters, 8
pages, 4 figure
Isolated Wolf-Rayet Stars and O Supergiants in the Galactic Center Region Identified via Paschen-alpha Excess
We report the discovery of 19 hot, evolved, massive stars near the Galactic
center region (GCR). These objects were selected for spectroscopy owing to
their detection as strong sources of Paschen-alpha emission-line excess,
following a narrowband imaging survey of the central 0.65 x 0.25 degress (l, b)
around Sgr A* with the Hubble Space Telescope. Discoveries include 5
carbon-type (WC) and 6 nitrogen-type (WN) Wolf-Rayet stars, 6 O supergiants,
and 2 B supergiants. Two of the O supergiants have X-ray counterparts, the
properties of which are broadly consistent with colliding-wind binaries and
solitary O stars. The infrared photometry of 17 stars is consistent with the
Galactic center distance, but 2 of them are located in the foreground. Several
WC stars exhibit a relatively large infrared excesses, which is possibly the
signature of thermal emission from hot dust. Most of the stars appear scattered
throughout the GCR, with no relation to the three known massive young clusters;
several others lie near the Arches and Quintuplet clusters and may have
originated within one of these systems. The results of this work bring the
total sample of Wolf-Rayet stars in the GCR to 92. All sources of strong
P-alpha excess have been identified in the area surveyed with HST, which
implies that the sample of WN stars in this region is near completion, and is
dominated by late (WNL) types. The current WC sample, although probably not
complete, is almost exclusively dominated by late (WCL) types. The observed
Wolf-Rayet subtype distribution in the GCR is a reflection of the intrinsic
rarity of early subtypes (WNE and WCE) in the inner Galaxy, an effect that is
driven by metallicity.Comment: Correction to Fig 4,5,6,10, and Tables 3, 5. The spectrum of the
previously known WC9 star WR101q had been mistakenly labeled as Star #19 in
Fig. 5 of the original manuscript. Star #19 is actually a newly discovered
WN5b star, whose spectrum was accidentally not included in the original
paper. The correct WN5b spectrum is now included in Fig. 4. An erratum was
submitted to ApJ on Aug 15, 201
Near-infrared spectroscopy of SN 2009ip's 2012 brightening reveals a dusty pre-supernova environment
We present low-resolution near-infrared (IR) 0.8–2.5 μm spectra of Supernova (SN) 2009ip, taken immediately before, during and just after its rapid brightening in late September/October 2012. The first epoch shows the same general spectral characteristics as the later epochs (smooth continuum, narrow H and He I emission lines), but the IR continuum shape is substantially redder than the later epochs. The epoch 1 continuum can be approximated by reddening the peak-luminosity (epoch 3) spectrum by E(B − V) = 1.0 mag, but the blue colour seen in visual-wavelength spectra at the same time indicates that strong wavelength-dependent extinction by circumstellar dust is not the correct explanation. Instead, we favour the hypothesis that the redder colour before the brightening arises from excess emission from hot ∼2000 K circumstellar dust. The minimum radius ( ≳ 120 au) deduced from the dust temperature and observed luminosity of the transient, combined with the observed expansion speed in the precursor outbursts of SN 2009ip, is consistent with an ejection at least 1.1 yr earlier. The mass of hot dust indicated by the IR excess is ∼4 × 10^(−7) M⊙, although this is only a lower limit since the near-IR data do not constrain the mass of cooler dust. Thus, the observed pre-SN outbursts of this object were able to efficiently form dust into which the SN ejecta and radiation now propagate. This is consistent with the notion that the same pre-SN eruptions that generally give rise to SNe IIn also give rise to the dust needed for their commonly observed IR echoes. We also discuss some aspects of the IR line profiles, including He I λ10 830
HST Paschen-alpha Survey of the Galactic Center: Data Reduction and Products
Our HST/NICMOS Pa survey of the Galactic center (GC) provides a uniform,
panoramic, high-resolution map of stars and ionized diffuse gas in the central
416 arcmin^2 of the Galaxy. This survey was carried out with 144 HST orbits
using two narrow-band filters at 1.87 and 1.90 micron in NICMOS Camera 3. In
this paper, we describe in detail the data reduction and mosaicking procedures
followed, including background level matching and astrometric corrections. We
have detected ~570,000 near-IR sources and are able to quantify photometric
uncertainties of the detections. The source detection limit varies across the
survey field but the typical 50% completion limit is ~17th mag (Vega System) in
the 1.90 micron band. A comparison with the expected stellar magnitude
distribution shows that these sources are primarily Main-Sequence massive stars
(>7M) and evolved lower mass stars at the distance of the GC. In particular,
the observed source magnitude distribution exhibits a prominent peak, which
could represent the Red Clump stars within the GC. The observed magnitude and
color of these RC stars support a steep extinction curve in the near-IR toward
the GC. The flux ratios of our detected sources in the two bands also allow for
an adaptive and statistical estimate of extinction across the field. With the
subtraction of the extinction-corrected continuum, we construct a net Pa
emission map and identify a set of Pa-emitting sources, which should mostly be
evolved massive stars with strong stellar winds. The majority of the identified
Pa point sources are located within the three known massive GC stellar
clusters. However, a significant fraction of our Pa-emitting sources are
located outside the clusters and may represent a new class of `field' massive
stars, many of which may have formed in isolation and/or in small groups. The
maps and source catalogues presented here are available electronically.Comment: Accepted for publication in MNRA
HST Palpha Survey of the Galactic Center -- Searching the missing young stellar populations within the Galactic Center
We present preliminary results of our \hst Pa survey of the Galactic
Center (\gc), which maps the central 0.650.25 degrees around Sgr A*.
This survey provides us with a more complete inventory of massive stars within
the \gc, compared to previous observations. We find 157 Pa emitting
sources, which are evolved massive stars. Half of them are located outside of
three young massive star clusters near Sgr A*. The loosely spatial distribution
of these field sources suggests that they are within less massive star
clusters/groups, compared to the three massive ones. Our Pa mosaic not
only resolves previously well-known large-scale filaments into fine structures,
but also reveals many new extended objects, such as bow shocks and H II
regions. In particular, we find two regions with large-scale Pa diffuse
emission and tens of Pa emitting sources in the negative Galactic
longitude suggesting recent star formation activities, which were not known
previously. Furthermore, in our survey, we detect 0.6 million stars, most
of which are red giants or AGB stars. Comparisons of the magnitude distribution
in 1.90 m and those from the stellar evolutionary tracks with different
star formation histories suggest an episode of star formation process about 350
Myr ago in the \gc .Comment: 10 pages, 6 figures, Proceedings of the Galactic Center Workshop
2009, Shangha
Magnetism, X-rays, and Accretion Rates in WD 1145+017 and other Polluted White Dwarf Systems
This paper reports circular spectropolarimetry and X-ray observations of
several polluted white dwarfs including WD 1145+017, with the aim to constrain
the behavior of disk material and instantaneous accretion rates in these
evolved planetary systems. Two stars with previously observed Zeeman splitting,
WD 0322-019 and WD 2105-820, are detected above 5 sigma and > 1 kG, while
WD 1145+017, WD 1929+011, and WD 2326+049 yield (null) detections below this
minimum level of confidence. For these latter three stars, high-resolution
spectra and atmospheric modeling are used to obtain limits on magnetic field
strengths via the absence of Zeeman splitting, finding B* < 20 kG based on data
with resolving power R near 40 000. An analytical framework is presented for
bulk Earth composition material falling onto the magnetic polar regions of
white dwarfs, where X-rays and cyclotron radiation may contribute to accretion
luminosity. This analysis is applied to X-ray data for WD 1145+017, WD
1729+371, and WD 2326+049, and the upper bound count rates are modeled with
spectra for a range of plasma kT = 1 - 10 keV in both the magnetic and
non-magnetic accretion regimes. The results for all three stars are consistent
with a typical dusty white dwarf in a steady-state at 1e8 - 1e9 g/s. In
particular, the non-magnetic limits for WD 1145+017 are found to be well below
previous estimates of up to 1e12 g/s, and likely below 1e10 g/s, thus
suggesting the star-disk system may be average in its evolutionary state, and
only special in viewing geometry.Comment: 14 pages, 7 figures, 3 tables; accepted to MNRA
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