Irradiated Disk Model Research Articles

SPITZEROBSERVATIONS OF MF 16 NEBULA AND THE ASSOCIATED ULTRALUMINOUS X-RAY SOURCE

We present Spitzer Infrared Spectrograph (IRS) observations of the ultra-luminous X-ray source (ULX) NGC 6946 X-1 and its associated nebula MF 16. This ULX has very similar properties to the famous Holmberg II ULX, the first ULX to show a prominent infrared [O IV] emission line comparable to those found in AGN. This paper attempts to constrain the ULX Spectral Energy Distribution (SED) given the optical/UV photometric fluxes and high-resolution X-ray observations. Specifically, Chandra X-ray data and published Hubble optical/UV data are extrapolated to produce a model for the full optical to X-ray SED. The photoionization modeling of the IR lines and ratios is then used to test different accretion spectral models. While either an irradiated disk model or an O-supergiant plus accretion disk model fit the data very well, we prefer the latter because it fits the nebular parameters slightly better. In this second case the accretion disk alone dominates the extreme-UV and X-ray emission, while an O-supergiant is responsible for most of the far-UV emission.

THE NATURE OF THE UV/OPTICAL EMISSION OF THE ULTRALUMINOUS X-RAY SOURCE IN HOLMBERG II

We report on UV and X-ray spectroscopy and broad-band optical observations of the ultraluminous X-ray source in Holmberg II. Fitting various stellar spectral models to the combined, non-simultaneous data set, we find that normal metallicity stellar spectra are ruled out by the data, while low metallicity, Z = 0.1 Z_{\odot}, late O-star spectra provide marginally acceptable fits, if we allow for the fact that X-ray ionization from the compact object may reduce or eliminate UV absorption/emission lines from the stellar wind. By contrast, an irradiated disk model fits both UV and optical data with chi^2/dof=175.9/178, and matches the nebular extinction with a reddening of E(B-V)=0.05^{+0.05}_{-0.04}. These results suggest that the UV/optical flux of Holmberg II X-1 may be dominated by X-ray irradiated disk emission.

ON THE TRANSITIONAL DISK CLASS: LINKING OBSERVATIONS OF T TAURI STARS AND PHYSICAL DISK MODELS

Two decades ago "transitional disks" described spectral energy distributions (SEDs) of T Tauri stars with small near-IR excesses, but significant mid- and far-IR excesses. Many inferred this indicated dust-free holes in disks, possibly cleared by planets. Recently, this term has been applied disparately to objects whose Spitzer SEDs diverge from the expectations for a typical full disk. Here we use irradiated accretion disk models to fit the SEDs of 15 such disks in NGC 2068 and IC 348. One group has a "dip" in infrared emission while the others' continuum emission decreases steadily at all wavelengths. We find that the former have an inner disk hole or gap at intermediate radii in the disk and we call these objects "transitional" and pre-transitional" disks, respectively. For the latter group, we can fit these SEDs with full disk models and find that millimeter data are necessary to break the degeneracy between dust settling and disk mass. We suggest the term "transitional" only be applied to objects that display evidence for a radical change in the disk's radial structure. Using this definition, we find that transitional and pre-transitional disks tend to have lower mass accretion rates than full disks and that transitional disks have lower accretion rates than pre-transitional disks. These reduced accretion rates onto the star could be linked to forming planets. Future observations of transitional and pre-transitional disks will allow us to better quantify the signatures of planet formation in young disks.

MULTIPLICITY, DISKS, AND JETS IN THE NGC 2071 STAR-FORMING REGION

We present centimeter and millimeter observations of the NGC 2071 star-forming region performed with the VLA and CARMA. We detected counterparts at 3.6 cm and 3 mm for the previously known sources IRS 1, IRS 2, IRS 3, and VLA 1. All these sources show SEDs dominated by free-free thermal emission at cm wavelengths, and thermal dust emission at mm wavelengths, suggesting that all of them are associated with YSOs. IRS 1 shows a complex morphology at 3.6 cm, with changes in the direction of its elongation. We discuss two possible explanations to this morphology: the result of changes in the direction of a jet due to interactions with a dense ambient medium, or that we are actually observing the superposition of two jets arising from two components of a binary system. Higher angular resolution observations at 1.3 cm support the second possibility, since a double source is inferred at this wavelength. IRS 3 shows a clear jet-like morphology at 3.6 cm. Over a time-span of four years, we observed changes in the morphology of this source that we interpret as due to ejection of ionized material in a jet. The emission at 3 mm of IRS 3 is angularly resolved, with a deconvolved size (FWHM) of ~120 AU, and seems to be tracing a dusty circumstellar disk perpendicular to the radio jet. An irradiated accretion disk model around an intermediate-mass YSO can account for the observed SED and spatial intensity profile at 3 mm, supporting this interpretation.

OPTICAL EMISSION OF THE ULTRALUMINOUS X-RAY SOURCE NGC 5408 X-1: DONOR STAR OR IRRADIATED ACCRETION DISK?

We obtained 3 epochs of simultaneous Hubble Space Telescope (HST)/Wide Field Camera 3 and Chandra observations of the ultraluminous X-ray source NGC 5408 X-1. The counterpart of the X-ray source is seen in all HST filters, from the UV through the NIR, and for the first time, we resolve the optical nebula around the ULX. We identified a small OB association near the ULX that may be the birthplace of the system. The stellar association is young, ~ 5 Myr, contains massive stars up to 40 solar masses, and is thus similar to associations seen near other ULXs, albeit younger. The UV/Optical/NIR spectral energy distribution (SED) of the ULX counterpart is consistent with that of a B0I supergiant star. We are also able to fit the whole SED from the X-rays to the NIR with an irradiated disk model. The three epochs of data show only marginal variability and thus, we cannot firmly conclude on the nature of the optical emission.

ASPITZERIRS STUDY OF INFRARED VARIABILITY IN TRANSITIONAL AND PRE-TRANSITIONAL DISKS AROUND T TAURI STARS

We present a Spitzer IRS study of variability in 14 T Tauri stars in the Taurus and Chamaeleon star-forming regions. The sample is composed of transitional and pre-transitional objects which contain holes and gaps in their disks. We detect variability between 5 and 38 μm in all but two of our objects on timescales of 2–3 years. Most of the variability observed can be classified as seesaw behavior, whereby the emission at shorter wavelengths varies inversely with the emission at longer wavelengths. For many of the objects we can reasonably reproduce the observed variability using irradiated disk models, particularly by changing the height of the inner disk wall by ∼20%. When the inner wall is taller, the emission at the shorter wavelengths is higher since the inner wall dominates the emission at 2–8 μm. The taller inner wall casts a larger shadow on the outer disk wall, leading to less emission at wavelengths beyond 20 μm where the outer wall dominates. We discuss how the possible presence of planets in these disks could lead to warps that cause changes in the height of the inner wall. We also find that crystalline silicates are common in the outer disks of our objects and that in the four disks in the sample with the most crystalline silicates, variability on timescales of 1 week is present. In addition to explaining the infrared variability described above, planets can create shocks and collisions which can crystallize the dust and lead to short timescale variability.

Ne II FINE-STRUCTURE LINE EMISSION FROM THE OUTFLOWS OF YOUNG STELLAR OBJECTS

The flux and line shape of the fine-structure transitions of \NeII\ and \NeIII\ at 12.8 and 15.55\,$\mu$m and of the forbidden transitions of \OI\ $\lambda6300$ are calculated for young stellar objects with a range of mass-loss rates and X-ray luminosities using the X-wind model of jets and the associated wide-angle winds. For moderate and high accretion rates, the calculated \NeII\ line luminosity is comparable to or much larger than produced in X-ray irradiated disk models. All of the line luminosities correlate well with the main parameter in the X-wind model, the mass-loss rate, and also with the assumed X-ray luminosity --- and with one another. The line shapes of an approaching jet are broad and have strong blue-shifted peaks near the effective terminal velocity of the jet. They serve as a characteristic and testable aspect of jet production of the neon fine-structure lines and the \OI\ forbidden transitions.

Molecular hydrogen emission from protoplanetary disks: UV and X-ray irradiated disk model with dust evolution

We have modelled a detailed physical structure of protoplanetary disks, taking into account X-ray and UV irradiation from a central star, as well as dust size growth and settling towards the disk midplane. In addition, we have calculated the level populations and line emission of molecular hydrogen in the disks. As a result, we reproduce the observed strong H 2 line flux if the disks are influenced by strong UV and X-ray irradiation. Also, the dust evolution changes the physical properties of the disk, and thus the H 2 line ratios.

Results from DROXO

The infrared [Ne II] and [Ne III] fine structure lines at 12.81um and 15.55um are predicted to trace the circumstellar disk gas subject to X-ray heating and ionization. We investigate the origin of these lines by comparing observations with models of X-ray irradiated disks and by searching for empirical correlations between the line luminosities and stellar and circumstellar parameters. We measure neon line fluxes and X-ray luminosities for 28 young stellar objects in the Rho Ophiuchi star formation region for which good quality infrared spectra and X-ray data have been obtained, the former with the Spitzer IRS and the latter with the Deep Rho Ophiuchi XMM-Newton Observation. We detect the [Ne II] and the [Ne III] lines in 10 and 1 cases, respectively. Line luminosities show no correlation with X-ray emission. The luminosity of the [Ne II] line for one star, and that of both the [Ne II] and [Ne III] lines for a second star, match the predictions of published models of X-ray irradiated disks; for the remaining 8 objects the [Ne II] emission is 1-3 dex higher than predicted on the basis of their L_X. Class I objects show significantly stronger [Ne II] lines than Class II and Class III ones. A correlation is moreover found between the [Ne II] line emission and the disk mass accretion rates. This might point toward a role of accretion-generated UV emission in the generation of the line or to other mechanisms related to mass inflows from circumstellar disks and envelopes and/or to the associated mass outflows (winds and jets). We conclude that the X-ray luminosity is not the only parameter that determines the [Ne II] emission. Explaining the strong [Ne II] emission of Class I objects likely requires the inclusion in the models of additional physical components such as the envelope, inflows, and outflows.

A PHOTOIONIZED NEBULA SURROUNDING AND VARIABLE OPTICAL CONTINUUM EMISSION FROM THE ULTRALUMINOUS X-RAY SOURCE IN NGC 5408

We obtained optical spectra of the counterpart of the ultraluminous X-ray source NGC 5408 X-1 using the FORS spectrograph on the VLT. The spectra show strong high excitation emission lines, He{\sc ii} $\lambda$4686 and [Ne{\sc V}] $\lambda$3426, indicative of X-ray photoionization. Using the measured X-ray spectrum as input to a photoionization model, we calculated the relation between the He{\sc ii} and X-ray luminosities and found that the He{\sc ii} flux implies a lower bound on the X-ray luminosity of $3 \times 10^{39} \rm erg s^{-1}$. The [Ne{\sc v}] flux requires a similar X-ray luminosity. After subtraction of the nebular emission, the continuum appears to have a power-law form with a spectral slope of $-2.0^{+0.1}_{-0.2}$. This is similar to low-mass X-ray binaries where the optical spectra are dominated by reprocessing of X-rays in the outer accretion disk. In one observation, the continuum, He{\sc ii} $\lambda$4686, and [Ne{\sc V}] $\lambda$3426 fluxes are about 30% lower than in the other five observations. This implies that part of the line emission originates within one light-day of the compact object. Fitting the optical continuum emission and archival X-ray data to an irradiated disk model, we find that $(6.5 \pm 0.7) \times 10^{-3}$ of the total bolometric luminosity is thermalized in the outer accretion disk. This is consistent with values found for stellar-mass X-ray binaries and larger than expected in models of super-Eddington accretion flows. We find no evidence for absorption lines that would permit measurement of the radial velocity of the companion star.

Constraining jet/disc geometry and radiative processes in stellar black holes XTE J1118+480 and GX 339−4

We present results from modeling of quasi-simultaneous broad band (radio through X-ray) observations of the galactic stellar black hole (BH) transient X-ray binary (XRB) systems XTE J1118+480 and GX 339-4 using an irradiated disc + compact jet model. In addition to quantifying the physical properties of the jet, we have developed a new irradiated disc model which also constrains the geometry and temperature of the outer accretion disc by assuming a disc heated by viscous energy release and X-ray irradiation from the inner regions. For the source XTE J1118+480, which has better spectral coverage of the two in optical and near-IR (OIR) wavelengths, we show that the entire broad band continuum can be well described by an outflow-dominated model + an irradiated disc. The best-fit radius of the outer edge of the disc is consistent with the Roche lobe geometry of the system, and the temperature of the outer edge of the accretion disc is similar to those found for other XRBs. Irradiation of the disc by the jet is found to be negligible for this source. For GX 339-4, the entire continuum is well described by the jet-dominated model only, with no disc component required. For the two XRBs, which have very different physical and orbital parameters and were in different accretion states during the observations, the sizes of the jet base are similar and both seem to prefer a high fraction of non-thermal electrons in the acceleration/shock region and a magnetically dominated plasma in the jet. These results, along with recent similar results from modeling other galactic XRBs and AGNs, may suggest an inherent unity in diversity in the geometric and radiative properties of compact jets from accreting black holes.

Detection of an Inner Gaseous Component in a Herbig Be Star Accretion Disk: Near‐ and Mid‐Infrared Spectrointerferometry and Radiative Transfer modeling of MWC 147

Westudythegeometryandthephysicalconditionsintheinner(AU-scale)circumstellarregionaroundtheyoung HerbigBestarMWC147usinglong-baselinespectrointerferometryinthenear-infrared(NIR)K-band,VLTI/AMBER observations, and PTI archive data, as well as the mid-infrared (MIR) N-band, VLTI/MIDI observations. The emission from MWC 147 is clearly resolved and has a characteristic physical size of � 1.3 and � 9 AU at 2.2 and 11 � m, respectively (Gaussian diameter). The MIR emission reveals asymmetry consistent with a disk structure seen under intermediate inclination. The spectrally dispersed AMBER and MIDI interferograms both show a strong increase in the characteristic size toward longer wavelengths, much steeper than predicted by analytic disk models assuming power-law radial temperature distributions. We model the interferometric data and the spectral energy distribution of MWC 147 withtwo-dimensional, frequency-dependent radiationtransfer simulations. Thisanalysis showsthat models of spherical envelopes or passive irradiated Keplerian disks (with vertical or curved puffed-up inner rim) can easily fit the SED, but predict much lower visibilities than observed; the angular size predicted by such models is 2Y4 times larger than the size derived from the interferometric data, so these models can clearly be ruled out. Models of a Keplerian disk with optically thick gas emission from an active gaseous disk (inside the dust sublimation zone), however, yield a good fit of the SED and simultaneously reproduce the absolute level and the spectral dependence of the NIR and MIR visibilities. We conclude that the NIR continuum emission from MWC 147 is dominated by accretion luminosity emerging from an optically thick inner gaseous disk, while the MIR emission also contains contributions from the outer, irradiated dust disk. Subject headingg accretion, accretion disks — stars: formation — stars: individual (MWC 147) — stars: preYmain-sequence — techniques: interferometric

Observations of Disks around Brown Dwarfs in the TW Hydra Association with the Spitzer Infrared Spectrograph

Using SpeX at the NASA Infrared Telescope Facility and the Spitzer Infrared Spectrograph, we have obtained infrared spectra from 0.7 to 40 μm for three young brown dwarfs in the TW Hydra association (τ ~ 10 Myr), 2MASSW J1207334–393254, 2MASSW J1139511–315921, and SSSPM J1102–3431. The spectral energy distribution for 2MASSW J1139511–315921 is consistent with a stellar photosphere for the entire wavelength range of our data, whereas the other two objects exhibit significant excess emission at λ > 5μm. We are able to reproduce the excess emission from each brown dwarf using our models of irradiated accretion disks. According to our model fits, both disks have experienced a high degree of dust settling. We also find that silicate emission at 10 and 20 μm is absent from the spectra of these disks, indicating that grains in the upper disk layers have grown to sizes larger than ~5 μm. Both of these characteristics are consistent with previous observations of decreasing silicate emission with lower stellar masses and older ages. These trends suggest that either (1) the growth of dust grains, and perhaps planetesimal formation, occurs faster in disks around brown dwarfs than in disks around stars or (2) the radii of the mid-IR-emitting regions of disks are smaller for brown dwarfs than for stars, and grains grow faster at smaller disk radii. Finally, we note the possible detection of an unexplained emission feature near 14 μm in the spectra of both of the disk-bearing brown dwarfs.

SpitzerObservations of the Orion OB1 Association: Disk Census in the Low‐Mass Stars

We present new Spitzer observations of two fields in the Orion OB1 association. We report IRAC/MIPS observations for 115 confirmed members and 41 photometric candidates of the ~10 Myr 25 Orionis aggregate in the OB1a subassociation, and 106 confirmed members and 65 photometric candidates of the 5 Myr region located in the OB1b subassociation. The 25 Orionis aggregate shows a disk frequency of 6%, while the field in the OB1b subassociation shows a disk frequency of 13%. Combining IRAC, MIPS, and 2MASS photometry, we place stars bearing disks in several classes: those with optically thick disks (class II systems), with an inner transitional disks (transitional disk candidates), and with evolved disks; the last exhibit smaller IRAC/MIPS excesses than class II systems. In all, we identify one transitional disk candidate in the 25 Orionis aggregate and three in the OB1b field; this represents ~10% of the disk-bearing stars, indicating that the transitional disk phase can be relatively fast. We find that the frequency of disks is a function of the stellar mass, suggesting a maximum around stars with spectral type M0. Comparing the infrared excess in the IRAC bands among several stellar groups, we find that inner disk emission decays with stellar age, showing a correlation with the respective disk frequencies. The disk emission at the IRAC and MIPS bands in several stellar groups indicates that disk dissipation takes place faster in the inner region of the disks. Comparison with models of irradiated accretion disks, computed with several degrees of settling, suggests that the decrease in the overall accretion rate observed in young stellar groups is not sufficient to explain the weak disk emission observed in the IRAC bands for disk-bearing stars with ages 5 Myr or older; larger degrees of dust settling are necessary to explain these objects.

AMBER and MIDI interferometric observations of the post-AGB binary IRAS 08544-4431: the circumbinary disc resolved

Context: For a large group of post-AGB binaries, the presence of a stable reservoir of dust is postulated. Although this reservoir will influence the final evolution stages of these objects significantly, its actual geometry and structure remains largely unknown. Aims: We aim at determining the dust morphology of a member of this group, IRAS 08544-4431. Methods: We use the interferometric capabilities of the AMBER and MIDI instruments, operating in the K and N-band respectively. The high spatial resolution measurements are used in conjunction with the broad band spectral characteristics to determine the dust geometry, based on self consistent 2D radiative transfer models. Results: We resolve the object in both K and N. Moreover, using the closure phase capabilities of AMBER, we measure in the K-band a large asymmetry of the dusty environment. The interferometric data are clearly incompatible with a spherical outflow. We model the dusty environment with a passive irradiated dusty disc model. Although this model is constrained mainly on the basis of the spectral energy distribution, it reproduces simultaneously the amplitude and closure phase of the visibilities, in both wavelength bands. Conclusions: Our model of a passive, irradiated disc in equilibrium gives an excellent fit to both the K and N-band visibilities and closure phase. The dust around this evolved binary star is indeed locked in a circumbinary disc with a significant scale height. Grain growth, settling, radial mixing and crystallization are efficient in such an environment. We conclude that the circumbinary disc of this evolved object, is governed by the same physical processes that govern the proto-planetary discs around young stellar objects. Based on observations collected at the European Southern Observatory, Chile: ESO.076.D-0274, ESO.078.D-0113.

Anomalous X-ray pulsars: persistent states with fallback disks

The anomalous X-ray pulsar 4U 0142+61 was recently detected in the mid infrared bands with the SPITZER Observatory (Wang, Chakrabarty and Kaplan: Nature 440, 772 (2006)). This observation is the first instance for a disk around an AXP. From a reanalysis of optical and infrared data, we show that the observations indicate that the disk is likely to be an active disk rather than a passive dust disk beyond the light cylinder, as proposed in the discovering paper. Furthermore, we show that the irradiated accretion disk model can also account for all the optical and infrared observations of the anomalous X-ray pulsars in the persistent state.

Multiple Regression Analysis of the Variable Component in the Near-Infrared Region for Type 1 AGN MCG +08-11-011

We propose a new method of analyzing a variable component for type 1 active galactic nuclei (AGNs) in the near-infrared wavelength region. This analysis uses a multiple regression technique and divides the variable component into two components originating in the accretion disk at the center of an AGN and from the dust torus that far surrounds the disk. Applying this analysis to the long-term VHK monitoring data of MCG +08-11-011 that were obtained by the MAGNUM project, we found that the (H - K) color temperature of the dust component is T = 1635 ± 20 K, which agrees with the sublimation temperature of dust grains, and that the time delay of K to H variations is Δt ≈ 6 days, which indicates the existence of a radial temperature gradient in the dust torus. As for the disk component, we found that the power-law spectrum of fν ∝ να in the V to near-infrared HK bands varies with a fixed index of α ≈ -0.1 to +0.4, which is broadly consistent with the irradiated standard disk model. The outer part of the disk therefore extends out to a radial distance where the temperature decreases to radiate the light in the near-infrared.

Optical and Infrared Emission from the Anomalous X-Ray Pulsars and Soft Gamma-Ray Repeaters

We show that the irradiated accretion disk model can account for all the optical and infrared observations of the anomalous X-ray pulsars in the persistent state. Model fits do not constrain the outer disk radii, while placing an upper limit to the inner disk radii, and thus to the strength of the dipole component of the stellar magnetic field. While magnetar fields (B_* > 10^{14} G) in higher multipoles are compatible with the irradiated disk model, magnetic dipole components of magnetar strength are not consistent with optical data.

24 μm Detections of Circum(sub)stellar Disks in IC 348: Grain Growth and Inner Holes?

We present observations of six late-type members of the young cluster IC 348 detected at 24 μm with the Multiband Imaging Photometer for Spitzer (MIPS). At least four of the objects are probably substellar. Combining these data with ground-based optical and near-infrared photometry and complementary observations with the Infrared Array Camera (IRAC), we have modeled the spectral energy distributions using detailed models of irradiated accretion disks. We are able to fit the observations with models using a range of maximum grain sizes from ISM-type dust to grains as large as 1 mm. Two objects show a lack of excess emission at wavelengths shortward of 5.8-8 μm but significant excess at longer wavelengths, indicative of large optically thin or evacuated inner holes. Our models indicate an inner hole of radius ~0.5-0.9 AU for the brown dwarf L316; this is the first brown dwarf with evidence for an AU-scale inner disk hole. We examine several possible mechanisms for the inner disk clearing in this case, including photoevaporation and planet formation.

X‐Ray and Infrared Enhancement of Anomalous X‐Ray Pulsar 1E 2259+586

The long-term (~1.5 yr) X-ray enhancement and the accompanying infrared enhancement light curves of the anomalous X-ray pulsar 1E 2259+586 following the major bursting epoch can be accounted for by the relaxation of a fallback disk that has been pushed back by a gamma-ray flare. The required burst energy estimated from the results of our model fits is low enough for such a burst to have remained below the detection limits. We find that an irradiated disk model with a low irradiation efficiency is in good agreement with both X-ray and infrared data. Nonirradiated disk models also give a good fit to the X-ray light curve, but are not consistent with the infrared data for the first week of the enhancement.