Flat Spectral Energy Distribution Research Articles

Observational Appearance and Spectrum of Black-Hole Winds with Supercritical Accretion Disks

Abstract We examine the observational appearance and expected spectra of an optically thick, spherically symmetric, relativistic wind (a black-hole wind), blowing off from the central region of a geometrically thick supercritical accretion disk. Due to the relativistic limb-darkening (center-brightening) effect as well as the usual Doppler boost, the black-hole wind becomes luminous as the wind speed increases. We found that the spectra of the black-hole wind is somewhat similar to a blackbody one, while the spectra of a naked supercritical accretion disk is known to have a flat spectral energy distribution (SED) when viewing from the pole-on direction. In the case of a black-hole wind with a supercritical accretion disk, the spectra generally becomes a hybrid type, but we found that in almost all cases the characteristic flat SED of a supercritical disk disappears. That is, when the speed of the wind gas is low, the apparent photosphere of the black-hole wind becomes larger than the supercritical region. In this case the spectra of a black-hole wind dominates. As the wind speed increases, the apparent photosphere shrinks and the spectra of the disk emergent. However, when the wind speed becomes sufficiently high, the inner part of the naked supercritical disk is not seen, even if the inclination angle is small.

EARLY SPECTROSCOPIC IDENTIFICATION OF SN 2008D

SN 2008D was discovered while following up an unusually bright X-ray transient in the nearby spiral galaxy NGC 2770. We present early optical spectra (obtained 1.75 days after the X-ray transient) which allowed the first identification of the object as a supernova at redshift z = 0.007. These spectra were acquired during the initial declining phase of the light curve, likely produced in the stellar envelope cooling after shock breakout, and rarely observed. They exhibit a rather flat spectral energy distribution with broad undulations, and a strong, W-shaped feature with minima at 3980 and 4190 AA (rest frame). We also present extensive spectroscopy and photometry of the supernova during the subsequent photospheric phase. Unlike supernovae associated with gamma-ray bursts, SN 2008D displayed prominent He features and is therefore of Type Ib.

On the Nature of the Extended Radio Emission Surrounding T Tauri South

At centimeter wavelengths, the young stellar system T Tauri is known to be composed of two sources, the northern one associated with the optical star T Tau itself, and the southern one related to the infrared companion T Tau S. Here we reexamine the origin of the radio emission from these two components using archival 2 cm, 3.6 cm, and 6 cm VLA observations. The emission from the northern member is confirmed to be largely dominated by free-free radiation from an ionized wind, while the southern radio source is confirmed to consist of a compact component of magnetic origin, surrounded by an extended halo. Only moderately variable, the extended structure associated with the southern source is most likely the result of free-free radiation related to stellar winds. However, its flat spectral energy distribution, its extent, and the lack of variation of its size with the frequency of observation are incompatible with the classical picture of a fully ionized wind with constant velocity and mass-loss rate leading to an electron density distribution of ne(r) ∝ r-2. Instead, we propose a model in which the ionization results from the impact of a supersonic wind driven by T Tau Sb onto dense surrounding material, possibly associated with the circumbinary disk recently identified around the T Tau Sa/T Tau Sb pair. The timescales for cooling and recombination in such a situation are in good agreement with the observed morphological changes undergone by the extended structure as its driving source moves through the environment.

Mid‐InfraredSpitzerSpectra of X‐Ray–Selected Type 2 QSOs: QSO2s Are Not Ultraluminous Infrared Galaxies

We have performed a spectroscopic study of seven Type 2 QSOs using the mid-infrared spectrometer IRS on board the Spitzer Space Telescope. These are (to our knowledge) the first mid-IR spectra of X-ray-selected QSO2s taken. The objects have been selected according to their high intrinsic luminosities and column densities in X-rays. Their spectra strongly differ from template spectra of Type 2 active galactic nuclei (AGNs) at lower luminosities. They do not exhibit strong PAH dust emission features from circumnuclear star-forming regions, typical for lower luminosity Type 2 Seyfert galaxies or other previously used QSO2 templates, such as the (ultra)luminous infrared galaxy ([U]LIRG) NGC 6240. They also do not show the ice and silicate absorption features of highly luminous but deeply embedded compact nuclei seen in some ULIRGs. Instead, they reveal a relatively featureless, rising continuum similar to luminous Type 1 AGNs. We also find evidence for a 10 μm silicate feature in emission. Models of dusty tori in the AGN unification scenario predict this only for Type 1 AGNs. The ratio of the AGN continuum luminosity at 6 μm to the absorption-corrected 2-10 keV X-ray AGN luminosity is very similar to that found in Seyfert galaxies. X-ray-selected QSO2s are thus characterized by powerful AGNs in hosts with a luminosity due to star formation 1011 L☉. The dominance of the AGN light in the mid-IR spectra of QSO2s together with their flatter spectral energy distributions (SEDs) places important constraints on models of the cosmic infrared background and of the star formation history of the universe.

Flaring vs. self-shadowed disks: The SEDs of Herbig Ae/Be stars

Isolated Herbig Ae stars can be divided into two groups (Meeus et al. 2001): those with an almost flat spectral energy distribution in the mid-infrared (`group I'), and those with a strong decline towards the far-infrared (`group II'). In this paper we show that the group I vs. II distinction can be understood as arising from flaring vs. self-shadowed disks. We show that these two types of disks are natural solutions of the 2-D radiation-hydrostatic structure equations. Disks with high optical depth turn out to be flaring and have a strong far-IR emission, while disks with an optical depth below a certain threshold drop into the shadow of their own puffed-up inner rim and are weak in the far-IR. In spite of not having a directly irradiated surface layer, self-shadowed disks still display dust features in emission, in agreement with observations of group II sources. We propose an evolutionary scenario in which a disk starts out with a flaring shape (group I source), and then goes through the process of grain growth, causing the optical depth of the disk to drop and the disk to become self-shadowed (group II source). We show that this scenario predicts that the (sub-)millimeter slope of the disk changes from steep (small grains) to Rayleigh-Jeans-like (large grains) in the early stages of evolution, so that all group II sources are expected to have Rayleigh-Jeans-like slopes, while some group I sources may still have steep (sub-)millimeter slopes.

Spectral Energy Distributions of Seyfert Nuclei

We present nuclear spectral energy distributions (SEDs) in the range 0.4–16 μm for an expanded CfA sample of Seyfert galaxies. The spectral indexes (fν ∝ ν) from 1 to 16 μm range from αIR ∼ 0.9 to 3.8. The shapes of the spectra are correlated with Seyfert type in the sense that steeper nuclear SEDs (νfν increasing with increasing wavelength) tend to be found in Seyfert 2's, and flatter SEDs (νfν is constant) in Seyfert 1–1.5's. The galaxies optically classified as Seyferts 1.8's and 1.9's display values of αIR as in type 1 objects, or values intermediate between those of Seyfert 1's and Seyfert 2's. The intermediate SEDs of many Seyfert 1.8–1.9's may be consistent with the presence of a pure Seyfert 1 viewed through a moderate amount (AV ≲ 5 mag) of foreground galaxy extinction. We find, however, that between 10% and 20% of galaxies with broad optical line components have steep infrared SEDs. Torus models usually adopt high equatorial opacities to reproduce the infrared properties of Seyfert 1's and 2's, resulting in a dichotomy of infrared SEDs (flat for type 1's, and steep for type 2's). Such a dichotomy, however, is not observed in our sample. The wide range of spectral indexes observed in the type 2 objects, the lack of extremely steep SEDs, and the large numbers of objects with intermediate spectral indexes cannot be reconciled with predictions from existing optically thick torus models. We discuss possible modifications to improve torus models, including low optical depth tori, clumpy dusty tori, and high optical depth tori with an extended optically thin component.

The Environment and Nature of the Class I Protostar Elias 29: Molecular Gas Observations and the Location of Ices

A (sub-)millimeter line and continuum study of the Class I protostar Elias 29 in the ρ Ophiuchi molecular cloud is presented whose goals are to understand the nature of this source and to locate the ices that are abundantly present along this line of sight. Within 15-60 beams, several different components contribute to the line emission. Two different foreground clouds are detected, an envelope/disk system and a dense ridge of HCO^+-rich material. The latter two components are spatially separated in millimeter interferometer maps. We analyze the envelope/disk system by using inside-out collapse and flared disk models. The disk is in a relatively face-on orientation (<60°), which explains many of the remarkable observational features of Elias 29, such as its flat spectral energy distribution, its brightness in the near-infrared, the extended components found in speckle interferometry observations, and its high-velocity molecular outflow. It cannot account for the ices seen along the line of sight, however. A small fraction of the ices is present in a (remnant) envelope of mass 0.12-0.33 M_☉, but most of the ices (~70%) are present in cool (T < 40 K) quiescent foreground clouds. This explains the observed absence of thermally processed ices (crystallized H_2O) toward Elias 29. Nevertheless, the temperatures could be sufficiently high to account for the low abundance of apolar (CO, N_2, O_2) ices. This work shows that it is crucial to obtain spectrally and spatially resolved information from single-dish and interferometric molecular gas observations in order to determine the nature of protostars and to interpret Infrared Space Observatory and future Space Infrared Telescope Facility observations of ices and silicates along a pencil beam.

Subrelativistic Jets from Black Hole Accretion Vortices. I. The Extreme‐Ultraviolet and X‐Ray Emission from Radio‐quiet Quasars

This article and its companion describe a theory of quasars that differentiates radio-loud quasars from radio-quiet quasars by the physical structures extant in the accretion vortices above the poles of a supermassive, rapidly rotating black hole. Quasars are manifestly radio loud as a consequence of large-scale magnetic flux threading these funnels and relativistic magnetic winds of plasma that propagate outward (magnetic wind models of radio-loud quasars exist in the literature). Radio-quiet quasars are devoid of this large-scale magnetic flux, and radiation pressure in the funnel drives hypersonic, underexpanded, subrelativistic jets. These jets are relatively feeble compared to the radiation-driven relativistic jets from the funnels of superluminous accretion disks that were popular in the theoretical literature of the 1980s. The primary emphases of this article are the near-field dynamics and the observational implications of the radiation-driven jets from these vortices conjectured to exist in radio-quiet quasars. The physical state of the ejecta is governed by the force of gravity, the initiating continuum radiation pressure, and the line-driving forces from ultraviolet accretion disk radiation. These hypersonic jets form a bipolar broad absorption line region (BALR) with a small covering factor dictated by the collimation of the jets. Consequently, radio-quiet quasars have broad absorption lines in the UV region of their spectra if the line of sight passes through a jet. Radio-loud quasars do not have BALRs in this model. It is demonstrated in the companion article that absorbing columns with a range of outflow velocities from ≈ 0 km s-1 to more than 35,000 km s-1 result in these putative jets. In this article it is shown that the frictional and compressional heating in the turbulent boundary layer between the base of the jet and the funnel walls creates a hot corona above the innermost regions of the disk. The corona cools by Compton-scattering disk UV radiation, creating the far-UV and soft X-ray excess that radio-quiet quasars have relative to radio-loud quasars as found in composite spectra of the Hubble Space Telescope (HST) and ROSAT archives, respectively. Furthermore, Compton cooling of the hot outer regions of the corona creates the flat spectral energy distribution of hard X-rays observed in radio-quiet quasars. By the geometry of an X-ray source collaring the base of the jet, if the line of sight passes through the jet (a BAL quasar), X-ray emission is obscured. Thus, soft X-ray emission is suppressed in BAL quasars relative to the rest of the radio-quiet population in the model as observed. Seyfert galaxies do not have disks that are luminous to drive winds from the vortex and hence do not have BALRs in the model consistent with IUE searches.

Aperture Synthesis 13CO (J = 1-0) Observations of the Molecular Gas around DG Tauri: Evidence for a Dispersing Gas Disk

view Abstract Citations (37) References (36) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Aperture Synthesis 13CO (J = 1-0) Observations of the Molecular Gas around DG Tauri: Evidence for a Dispersing Gas Disk Kitamura, Yoshimi ; Kawabe, Ryohei ; Saito, Masao Abstract We made aperture synthesis 13CO (J = 1-0) observations of the molecular gas around a young star DG Tau, using the Nobeyama Millimeter Array. We have detected the gas in the velocity range υLSR = 4.3-7.3 km s-1, and have revealed the velocity structure of the gas with velocity resolution 0.43 km s - 1 The molecular gas around DG Tau is found to have a disklike structure, with the major axis perpendicular to the blueshifted optical jet ejected from the star. The radius, mass, and inclination angle of the disk are estimated to be ∼2800 AU, 0.03 Msun, and 40°, respectively. It is noted that an expanding motion is distinct in the disk. The expanding velocity is about 1.5 km s-1, which is larger than both the Kepler and free-fall velocities around DG Tau. Since the mechanical luminosity of the expanding motion is estimated to be 6 × 10-4 Lsun, much smaller than the stellar luminosity of ∼1 Lsun, the expansion is possibly driven by the stellar wind from the central star. The disk model for the expanding gas around DG Tau is quite consistent with the structures of the reflection nebula and the blueshifted optical jet that were observed around the star. It would be reasonable to make the interpretation that the outer part of the large gas disk around the star, or the disk-shaped remnant of the envelope surrounding the star, -is now being dispersed owing to the stellar wind during the evolution of a large protostellar disk into a compact protoplanetary one, because DG Tau has a flat spectral energy distribution and is considered to be evolving from a protostar to a T Tauri star. Publication: The Astrophysical Journal Pub Date: January 1996 DOI: 10.1086/176728 Bibcode: 1996ApJ...457..277K Keywords: ISM: JETS AND OUTFLOWS; STARS: MASS LOSS; STARS: PRE-MAIN-SEQUENCE; STARS: CIRCUMSTELLAR MATTER; ISM: MOLECULES; STARS: INDIVIDUAL CONSTELLATION NAME: DG TAURI full text sources ADS | data products SIMBAD (5)

Circularly polarized radio emission from the T Tauri star Hubble 4

view Abstract Citations (30) References (33) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Circularly Polarized Radio Emission from the T Tauri Star Hubble 4 Skinner, Stephen L. Abstract Multiwavelength VLA observations of the weak-lined T Tauri star (WTTS) Hubble 4, which is located in the nearby (about 140 pc) Taurus-Auriga star-forming region, are reported. The emission is characterized by a nearly flat spectral energy distribution with a spectral index of 0 (Snu varies as nu exp alpha). Right circular polarization was detected at both 3.6 and 6 cm at levels of 19 +/- 5 percent and 18 +/- 3 percent, respectively, with an upper limit of not more than 15 percent (3 sigma) at 20 cm. The star was in a state of low radio activity, with flux densities of 1.3-1.7 mJy and a radio luminosity of 10 exp 16.5 ergs/Hz s. The unambiguous detection of circularly polarized radio emission from Hubble 4 provides direct evidence for the existence of ordered magnetic fields in this WTTS. Publication: The Astrophysical Journal Pub Date: May 1993 DOI: 10.1086/172626 Bibcode: 1993ApJ...408..660S Keywords: Circular Polarization; Radio Emission; Radio Sources (Astronomy); Stellar Radiation; T Tauri Stars; Cyclotron Resonance; Spectral Energy Distribution; Stellar Magnetic Fields; Very Large Array (Vla); Astrophysics; POLARIZATION; RADIO CONTINUUM: STARS; STARS: INDIVIDUAL NAME: HUBBLE 4; STARS: PRE--MAIN-SEQUENCE full text sources ADS | data products SIMBAD (5)

Far-ultraviolet radiation from disk globular clusters

IUE spectra obtained in a survey of the metal-rich disk system of globular clusters are presented. Significant FUV fluxes were detected in the 1200-2000-A short-wavelength (SWP) range of the IUE Observatory in several disk globular clusters. These clusters are the most metal-rich known to have an FUV flux. Three clusters show spectral energy distrbutions (SEDs) clearly rising at shorter wavelengths, not unlike the upturns observed in the bulges of metal-rich elliptical galaxies. Several others with weak SWP detections appear to have flat or uncertain spectral energy distributions. Blue stragglers provide a possible explanation for flux redder than 2000 A in clusters showing weaker flux in the SWP region, and with flat or declining SEDs.

Nonthermal radio emission from the X-ray-bright K5 IV star 1E 1751 + 7046

VLA observations are analyzed of the K5 IV star 1E 1751 + 7046, which is known to be an X-ray source on the basis of Einstein observations and which shows some similarity to stars of the FK Comae type. Radio continuum emission has been detected at 3.6, 6, and 20 cm, and the spectral energy distribution is nearly flat. There is no evidence for circular polarization, and upper limits are given. The 6-cm flux is roughly one order of magnitude less than reported in a previous VLA observation, indicating that this star may flare at radio wavelengths. The inferred radio variability and the nearly flat spectral energy distribution suggest that the emission is of nonthermal origin. A variety of possible emission mechanisms is examined, and it is concluded that the emission is best explained in terms of a gyrosynchrotron/synchrotron model with the electrons following a power-law energy distribution. A brief comparison of the radio properties of 1E 1751 + 7046 with FK Comae-like stars and RS CVn systems is given.

Far-infrared properties of optically selected quasars

The far-infrared properties of 10, optically selected quasars were studied on the basis of pointed IRAS observations and ground-based near-infrared and radio measurements. Nine of these quasars were detected in at least three IRAS bands. The flat spectral energy distributions characterizing these optically selected quasars together with large 60-100-micron luminosities suggest that the infrared emission is dominated by nonthermal radiation. Seven of the nine quasars with far-infrared detections were found to have low-frequency turnovers.