16 research outputs found
Multiwavelength Variability of Sagittarius A* in 2019 July
We report timing analysis of near-infrared (NIR), X-ray, and sub-millimeter
(submm) data during a three-day coordinated campaign observing Sagittarius A*.
Data were collected at 4.5 micron with the Spitzer Space Telescope, 2-8 keV
with the Chandra X-ray Observatory, 3-70 keV with NuSTAR, 340 GHz with ALMA,
and at 2.2 micron with the GRAVITY instrument on the Very Large Telescope
Interferometer. Two dates show moderate variability with no significant lags
between the submm and the infrared at 99% confidence. July 18 captured a
moderately bright NIR flare (F_K ~ 15 mJy) simultaneous with an X-ray flare (F
~ 0.1 cts/s) that most likely preceded bright submm flux (F ~ 5.5 Jy) by about
+34 (+14 -33) minutes at 99% confidence. The uncertainty in this lag is
dominated by the fact that we did not observe the peak of the submm emission. A
synchrotron source cooled through adiabatic expansion can describe a rise in
the submm once the synchrotron-self-Compton NIR and X-ray peaks have faded.
This model predicts high GHz and THz fluxes at the time of the NIR/X-ray peak
and electron densities well above those implied from average accretion rates
for Sgr A*. However, the higher electron density postulated in this scenario
would be in agreement with the idea that 2019 was an extraordinary epoch with a
heightened accretion rate. Since the NIR and X-ray peaks can also be fit by a
non-thermal synchrotron source with lower electron densities, we cannot rule
out an unrelated chance coincidence of this bright submm flare with the
NIR/X-ray emission.Comment: Accepted for publication in The Astrophysical Journa
Companions of Stars: From Other Stars to Brown Dwarfs to Planets: The Discovery of the First Methane Brown Dwarf
The discovery of the first methane brown dwarf provides a framework for
describing the important advances in both fundamental physics and astrophysics
that are due to the study of companions of stars. I present a few highlights of
the history of this subject along with details of the discovery of the brown
dwarf Gliese 229B. The nature of companions of stars is discussed with an
attempt to avoid biases induced by anthropocentric nomenclature. With the newer
types of remote reconnaissance of nearby stars and their systems of companions,
an exciting and perhaps even more profound set of contributions to science is
within reach in the near future. This includes an exploration of the diversity
of planets in the universe and perhaps soon the first solid evidence for
biological activity outside our Solar System.Comment: 31 pages, 13 figure
Understanding the Chemical Complexity in Circumstellar Envelopes of C-rich AGB Stars: the Case of IRC +10216
The circumstellar envelopes of carbon-rich AGB stars show a chemical
complexity that is exemplified by the prototypical object IRC +10216, in which
about 60 different molecules have been detected to date. Most of these species
are carbon chains of the type CnH, CnH2, CnN, HCnN. We present the detection of
new species (CH2CHCN, CH2CN, H2CS, CH3CCH and C3O) achieved thanks to the
systematic observation of the full 3 mm window with the IRAM 30m telescope plus
some ARO 12m observations. All these species, known to exist in the
interstellar medium, are detected for the first time in a circumstellar
envelope around an AGB star. These five molecules are most likely formed in the
outer expanding envelope rather than in the stellar photosphere. A pure gas
phase chemical model of the circumstellar envelope is reasonably successful in
explaining the derived abundances, and additionally allows to elucidate the
chemical formation routes and to predict the spatial distribution of the
detected species.Comment: 4 pages, 4 figures; to appear in Astrophysics and Space Science,
special issue of "Science with ALMA: a new era for Astrophysics" conference,
November, 13-17 2006, ed. R. Bachille
Star Formation and Dynamics in the Galactic Centre
The centre of our Galaxy is one of the most studied and yet enigmatic places
in the Universe. At a distance of about 8 kpc from our Sun, the Galactic centre
(GC) is the ideal environment to study the extreme processes that take place in
the vicinity of a supermassive black hole (SMBH). Despite the hostile
environment, several tens of early-type stars populate the central parsec of
our Galaxy. A fraction of them lie in a thin ring with mild eccentricity and
inner radius ~0.04 pc, while the S-stars, i.e. the ~30 stars closest to the
SMBH (<0.04 pc), have randomly oriented and highly eccentric orbits. The
formation of such early-type stars has been a puzzle for a long time: molecular
clouds should be tidally disrupted by the SMBH before they can fragment into
stars. We review the main scenarios proposed to explain the formation and the
dynamical evolution of the early-type stars in the GC. In particular, we
discuss the most popular in situ scenarios (accretion disc fragmentation and
molecular cloud disruption) and migration scenarios (star cluster inspiral and
Hills mechanism). We focus on the most pressing challenges that must be faced
to shed light on the process of star formation in the vicinity of a SMBH.Comment: 68 pages, 35 figures; invited review chapter, to be published in
expanded form in Haardt, F., Gorini, V., Moschella, U. and Treves, A.,
'Astrophysical Black Holes'. Lecture Notes in Physics. Springer 201
The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets
This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics
The Theory of Brown Dwarfs and Extrasolar Giant Planets
Straddling the traditional realms of the planets and the stars, objects below
the edge of the main sequence have such unique properties, and are being
discovered in such quantities, that one can rightly claim that a new field at
the interface of planetary science and and astronomy is being born. In this
review, we explore the essential elements of the theory of brown dwarfs and
giant planets, as well as of the new spectroscopic classes L and T. To this
end, we describe their evolution, spectra, atmospheric compositions, chemistry,
physics, and nuclear phases and explain the basic systematics of
substellar-mass objects across three orders of magnitude in both mass and age
and a factor of 30 in effective temperature. Moreover, we discuss the
distinctive features of those extrasolar giant planets that are irradiated by a
central primary, in particular their reflection spectra, albedos, and transits.
Aspects of the latest theory of Jupiter and Saturn are also presented.
Throughout, we highlight the effects of condensates, clouds, molecular
abundances, and molecular/atomic opacities in brown dwarf and giant planet
atmospheres and summarize the resulting spectral diagnostics. Where possible,
the theory is put in its current observational context.Comment: 67 pages (including 36 figures), RMP RevTeX LaTeX, accepted for
publication in the Reviews of Modern Physics. 30 figures are color. Most of
the figures are in GIF format to reduce the overall size. The full version
with figures can also be found at:
http://jupiter.as.arizona.edu/~burrows/papers/rm
The properties and environment of the giant, infrared-luminous galaxy IRAS 09104 + 4109
IRAS 09104 + 4109 is the most luminous object yet discovered by means of the IRAS survey. It is identified with a cD galaxy having a strong emission-line spectrum at a redshift of 0.442 and emits 6 x 10 to the 12th solar luminosities/sq h, 99 percent of which emerges at infrared wavelengths. One component of a double radio source is coincident with the center of the galaxy. The high luminosity of this source may be related to an interaction with one or more members of the rich cluster in which it lies. There is a secondary peak in the emission-line image of the galaxy. Emission lines from both regions are broad but narrower than those characteristic of the only other objects known to have such high luminosities, such as Seyfert 1's and QSOs. It is suggested that the strong infrared excess of IRAS 09104 + 4109 is produced by dust obscuring a broad line region
IR observations of the double quasar 0957 + 561 A, B and the intervening galaxy
The properties of the remarkable double quasar 0957 + 561 were first described by Walsh et al. Recently Young et al. have described CCD observations of a distant galaxy associated with the quasar pair, and have identified this galaxy as a gravitational lens forming a double image of a single quasar. We report here 1.2–2.2-µm observations of the system that support the conclusion that the twin quasars are a pair of images of a single object; the quasar has an energy distribution that is unusual. The intervening galaxy is shown to be highly luminous with a bolometric luminosity of about 2 × 10^(11) L
High spatial resolution imaging of NGC 1068 in the mid infrared
Accepted for publication in AJConsiglio Nazionale delle Ricerche - Biblioteca Centrale - P.le Aldo Moro, 7 Rome; International School for Advanced Studies, Via Beirut, 7 Trieste / CNR - Consiglio Nazionale delle RichercheSIGLEITItal