arXiv subjects

David Hollenbach

Publications and source records attributed to David Hollenbach.

At least 19 recordsLinked to original sources

arXiv2025

GMCs and Star Formation in the Galaxy: I. Effects of an HII region on a GMC

The destruction of Giant Molecular Clouds is a key component in galaxy evolution. We theoretically model the destruction of GMCs by HII regions, which evaporate ionized gas and eject neutral gas during their expansion. HII regions follow one of three tracks, depending on the EUV luminosity, $S$, of the ionizing OB association: the expansion can stall inside the cloud; it can break out, forming a blister (champagne) flow; or, for $S>S_{\rm com}$, it can result in the formation of a cometary cloud. We present results for the accumulated mass loss, $M_{\rm loss}(t)$, and the final mass loss, $M_{{\rm loss},f}$, by evaporation and ejection for a range of cloud masses ($10^4 S_{\rm com}$ associations destroy at least 70% of the initial cloud. We find a critical cloud mass $M_{\rm survive}$ above which clouds never become cometary and lose $<$ 70% of their mass via a single association. Low mass clouds mostly lose mass via ejection of neutral gas.

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arXiv2021

C$^{18}$O emission as an effective measure of gas masses of protoplanetary disks

Many astrochemical models of observed CO isotopologue line emission, earlier considered a good proxy measure of H$_2$ and hence disk gas mass, favor large deviations in the carbon and oxygen gas phase abundances and argue that severe gas phase CO depletion makes it a poor mass tracer. Here, we show that C$^{18}$O line emission is an effective measure of the gas mass, and despite its complex chemistry, a possibly better tracer than HD. Our models are able to reproduce C$^{18}$O emission from recent ALMA surveys and the TW Hya disk to within a factor of $\sim 2-3$ using carbon and oxygen abundances characteristic of the interstellar medium (C/H$=1.4 \times 10^{-4}$; O/H$=3.2\times 10^{-4}$) without having to invoke unusual chemical processing. Our gas and dust disk structure calculations considering hydrostatic pressure equilibrium and our treatment of the CO conversion on grains are primarily responsible for the very different conclusions on disk masses and CO depletion. As did previous studies, we find that a gas phase C/O of $\sim 1-2$ can explain observed hydrocarbon emission from the TW Hya disk; but significantly, we find that CO isotopologue emission is only marginally affected by the C/O ratio. We therefore conclude that C$^{18}$O emission provides estimates of disk masses that are uncertain only to within a factor of a few, and describe a simplified modeling procedure to obtain gas disk masses from C$^{18}$O emission lines.

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arXiv2018

The High Mass Slope of the IMF

Recent papers have found that the inferred slope of the high-mass ($>1.5$ M$_\odot$) IMF for field stars in the solar vicinity has a larger value ($\sim 1.7-2.1$) than the slopes ($\sim 1.2-1.7$; Salpeter= 1.35) inferred from numerous studies of young clusters. We attempt to reconcile this apparent contradiction. Stars mostly form in Giant Molecular Clouds, and the more massive stars ($\gtrsim 3$ M$_\odot$) may have insufficient time before their deaths to uniformly populate the solar circle of the Galaxy. We examine the effect of small sample volumes on the {\it apparent} slope, $\Gamma_{\rm app}$, of the high-mass IMF by modeling the present day mass function (PDMF) over the mass range $1.5-6$ M$_\odot$. Depending on the location of the observer along the solar circle and the size of the sample volume, the apparent slope of the IMF can show a wide variance, with typical values steeper than the underlying universal value $\Gamma$. We show, for example, that the PDMFs observed in a small (radius $\sim 200$ pc) volume randomly placed at the solar circle have a $\sim 15-30$\% likelihood of resulting in $\Gamma_{\rm app} \gtrsim \Gamma+ 0.35$ because of inhomogeneities in the surface densities of more massive stars. If we add the a priori knowledge that the Sun currently lies in an interarm region, where the star formation rate is lower than the average at the solar circle, we find an even higher likelihood ($\sim 50-60\%$ ) of $\Gamma_{\rm app} \gtrsim \Gamma+0.35$, corresponding to $\Gamma_{\rm app} \gtrsim 1.7$ when the underlying $\Gamma= 1.35$.

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arXiv2017

A likely planet-induced gap in the disk around T Cha

We present high resolution ($0.11"\times0.06"$) 3mm ALMA observations of the highly inclined transition disk around the star T Cha. Our continuum image reveals multiple dust structures: an inner disk, a spatially resolved dust gap, and an outer ring. When fitting sky-brightness models to the real component of the 3mm visibilities, we infer that the inner emission is compact ($\le1$au in radius), the gap width is between 18-28 au, and the emission from the outer ring peaks at $\sim36$ au. We compare our ALMA image with previously published 1.6$\mu$m VLT/SPHERE imagery. This comparison reveals that the location of the outer ring is wavelength dependent. More specifically, the peak emission of the 3mm ring is at a larger radial distance than that of the 1.6$\mu$m ring, suggesting that millimeter-sized grains in the outer disk are located further away from the central star than micron-sized grains. We discuss different scenarios to explain our findings, including dead zones, star-driven photoevaporation, and planet-disk interactions. We find that the most likely origin of the dust gap is from an embedded planet, and estimate --- for a single planet scenario --- that T Cha's gap is carved by a $1.2M_{jup}$ planet.

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arXiv2016

Measuring Turbulence in TW Hya with ALMA: Methods and Limitations

We obtain high spatial and spectral resolution images of the CO J=2-1, CN N=2-1 and CS J=5-4 emission with ALMA in Cycle~2. The radial distribution of the turbulent broadening is derived with three approaches: two `direct' and one modelling. The first requires a single transition and derives \Tex{} directly from the line profile, yielding a \vturb{}. The second assumes two different molecules are co-spatial thus their relative linewidths allow for a calculation of \Tkin{} and \vturb{}. Finally we fit a parametric disk model where physical properties of the disk are described by power laws, to compare our `direct' methods with previous values. The two direct methods were limited to the outer $r > 40$~au disk due to beam smear. The direct method found \vturb{} ranging from $\approx$~\vel{130} at 40~au, dropping to $\approx$~\vel{50} in the outer disk, qualitatively recovered with the parametric model fitting. This corresponds to roughly $0.2 - 0.4~c_s$. CN was found to exhibit strong non-LTE effects outside $r \approx 140$~au, so \vturb{} was limited to within this radius. The assumption that CN and CS are co-spatial is consistent with observed linewidths only within $r \lesssim 100$~au, within which \vturb{} was found to drop from \vel{100} ($\approx~0.4~c_s$) to nothing at 100~au. The parametric model yielded a near constant \vel{50} for CS ($0.2 - 0.4~c_s$). We demonstrate that absolute flux calibration is and will be the limiting factor in all studies of turbulence using a single molecule. The magnitude of the dispersion is comparable with or below that predicted by the magneto-rotational instability theory. A more precise comparison would require to reach an absolute calibration precision of order 3\%, or to find a suitable combination of light and heavy molecules which are co-located in the disk.

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arXiv2015

The Impact of Dust Evolution and Photoevaporation on Disk Dispersal

Protoplanetary disks are dispersed by viscous evolution and photoevaporation in a few million years; in the interim small, sub-micron sized dust grains must grow and form planets. The time-varying abundance of small grains in an evolving disk directly affects gas heating by far-ultraviolet photons, while dust evolution affects photoevaporation by changing the disk opacity and resulting penetration of FUV photons in the disk. Photoevaporative flows, in turn, selectively carry small dust grains leaving the larger particles---which decouple from the gas---behind in the disk. We study these effects by investigating the evolution of a disk subject to viscosity, photoevaporation by EUV, FUV and X-rays, dust evolution, and radial drift using a 1-D multi-fluid approach (gas + different dust grain sizes) to solve for the evolving surface density distributions. The 1-D evolution is augmented by 1+1D models constructed at each epoch to obtain the instantaneous disk structure and determine photoevaporation rates. The implementation of a dust coagulation/fragmentation model results in a marginal decrease in disk lifetimes when compared to models with no dust evolution; the disk lifetime is thus found to be relatively insensitive to the evolving dust opacity. We find that photoevaporation can cause significant reductions in the gas/dust mass ratio in the planet-forming regions of the disk as it evolves, and may result in a corresponding increase in heavy element abundances relative to hydrogen. We discuss implications for theories of planetesimal formation and giant planet formation, including the formation of gas-poor giants. After gas disk dispersal, $\sim 3\times 10^{-4}$ \ms\ of mass in solids typically remain, comparable to the solids inventory of our solar system.

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arXiv2014

Herschel HIFI observations of O$_2$ toward Orion: special conditions for shock enhanced emission

We report observations of molecular oxygen (O$_2$) rotational transitions at 487 GHz, 774 GHz, and 1121 GHz toward Orion Peak A. The O2 lines at 487 GHz and 774 GHz are detected at velocities of 10-12 km/s with line widths 3 km/s; however, the transition at 1121 GHz is not detected. The observed line characteristics, combined with the results of earlier observations, suggest that the region responsible for the O$_2$ emission is 9" (6e16 cm) in size, and is located close to the H2 Peak 1position (where vibrationally-excited H$_2$ emission peaks), and not at Peak A, 23" away. The peak O2 column density is 1.1e18/cm2. The line velocity is close to that of 621 GHz water maser emission found in this portion of the Orion Molecular Cloud, and having a shock with velocity vector lying nearly in the plane of the sky is consistent with producing maximum maser gain along the line-of-sight. The enhanced O$_2$ abundance compared to that generally found in dense interstellar clouds can be explained by passage of a low-velocity C-shock through a clump with preshock density 2e4/cm3, if a reasonable flux of UV radiation is present. The postshock O$_2$ can explain the emission from the source if its line of sight dimension is ~10 times larger than its size on the plane of the sky. The special geometry and conditions required may explain why O$_2$ emission has not been detected in the cores of other massive star-forming molecular clouds.

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arXiv2014

The Carbon Inventory in a Quiescent, Filamentary Molecular Cloud in G328

We present spectral line images of [CI] 809 GHz, CO J=1-0 115 GHz and HI 1.4 GHz line emission, and calculate the corresponding C, CO and H column densities, for a sinuous, quiescent Giant Molecular Cloud about 5 kpc distant along the l=328{\deg} sightline (hereafter G328) in our Galaxy. The [CI] data comes from the High Elevation Antarctic Terahertz (HEAT) telescope, a new facility on the summit of the Antarctic plateau where the precipitable water vapor falls to the lowest values found on the surface of the Earth. The CO and HI datasets come from the Mopra and Parkes/ATCA telescopes, respectively. We identify a filamentary molecular cloud, ~75 x 5 pc long with mass ~4 x 10E4 Msun and a narrow velocity emission range of just 4 km/s. The morphology and kinematics of this filament are similar in CO, [CI] and HI, though in the latter appears as self-absorption. We calculate line fluxes and column densities for the three emitting species, which are broadly consistent with a PDR model for a GMC exposed to the average interstellar radiation field. The [C/CO] abundance ratio averaged through the filament is found to be approximately unity. The G328 filament is constrained to be cold (Tdust < 20K) by the lack of far-IR emission, to show no clear signs of star formation, and to only be mildly turbulent from the narrow line width. We suggest that it may represent a GMC shortly after formation, or perhaps still be in the process of formation.

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arXiv2013

The Mopra Southern Galactic Plane CO Survey

We present the first results from a new carbon monoxide (CO) survey of the southern Galactic plane being conducted with the Mopra radio telescope in Australia. The 12CO, 13CO and C18O J=1-0 lines are being mapped over the l = 305-345 deg, b = +/- 0.5 deg portion of the 4th quadrant of the Galaxy, at 35" spatial and 0.1 km/s spectral resolution. The survey is being undertaken with two principal science objectives: (i) to determine where and how molecular clouds are forming in the Galaxy and (ii) to probe the connection between molecular clouds and the "missing" gas inferred from gamma-ray observations. We describe the motivation for the survey, the instrumentation and observing techniques being applied, and the data reduction and analysis methodology. In this paper we present the data from the first degree surveyed, l = 323-324 deg, b = +/- 0.5 deg. We compare the data to the previous CO survey of this region and present metrics quantifying the performance being achieved; the rms sensitivity per 0.1 km/s velocity channel is ~1.5K for 12CO and ~0.7K for the other lines. We also present some results from the region surveyed, including line fluxes, column densities, molecular masses, 12CO/13CO line ratios and 12CO optical depths. We also examine how these quantities vary as a function of distance from the Sun when averaged over the 1 square degree survey area. Approximately 2 x 10E6 MSun of molecular gas is found along the G323 sightline, with an average H2 number density of nH2 ~ 1 cm-3 within the Solar circle. The CO data cubes will be made publicly available as they are published.

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arXiv2013

Interstellar H$_2$O Masers from J Shocks

We present a model in which the 22 GHz H$_2$O masers observed in star-forming regions occur behind shocks propagating in dense regions (preshock density $n_0 \sim 10^6 - 10^8$ cm$^{-3}$). We focus on high-velocity ($v_s > 30$ km s$^{-1}$) dissociative J shocks in which the heat of H$_2$ re-formation maintains a large column of $\sim 300-400$ K gas; at these temperatures the chemistry drives a considerable fraction of the oxygen not in CO to form H$_2$O. The H$_2$O column densities, the hydrogen densities, and the warm temperatures produced by these shocks are sufficiently high to enable powerful maser action. The observed brightness temperatures (generally $\sim 10^{11} - 10^{14}$ K) are the result of coherent velocity regions that have dimensions in the shock plane that are 10 to 100 times the shock thickness of $\sim 10^{13}$ cm. The masers are therefore beamed towards the observer, who typically views the shock "edge-on", or perpendicular to the shock velocity; the brightest masers are then observed with the lowest line of sight velocities with respect to the ambient gas. We present numerical and analytic studies of the dependence of the maser inversion, the resultant brightness temperature, the maser spot size and shape, the isotropic luminosity, and the maser region magnetic field on the shock parameters and the coherence path length; the overall result is that in galactic H$_2$O 22 GHz masers these observed parameters can be produced in J shocks with $n_0\sim 10^6 - 10^8$ cm$^{-3}$ and $v_s \sim 30 -200$ km s$^{-1}$. A number of key observables such as maser shape, brightness temperature, and global isotropic luminosity depend only on the particle flux into the shock, $j=n_0v_s$, rather than on $n_0$ and $v_s$ separately.

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arXiv2013

Understanding the origin of the [OI] low-velocity component from T Tauri stars

The formation time, masses, and location of planets are strongly impacted by the physical mechanisms that disperse protoplanetary disks and the timescale over which protoplanetary material is cleared out. Accretion of matter onto the central star, protostellar winds/jets, magnetic disk winds, and photoevaporative winds operate concurrently. Hence, disentangling their relative contribution to disk dispersal requires identifying diagnostics that trace different star-disk environments. Here, we analyze the low velocity component (LVC) of the Oxygen optical forbidden lines, which is found to be blueshifted by a few km/s with respect to the stellar velocity. We find that the [OI] LVC profiles are different from those of [NeII] at 12.81\mu m and CO at 4.7\mu m lines pointing to different origins for these gas lines. We report a correlation between the luminosity of the [OI] LVC and the accretion luminosity Lacc. We do not find any correlation with the X-ray luminosity, while we find that the higher is the stellar FUV luminosity, the higher is the luminosity of the [OI] LVC. In addition, we show that the [OI]6300\AA/5577\AA\ ratio is low (ranging between 1 and 8). These findings favor an origin of the [OI] LVC in a region where OH is photodissociated by stellar FUV photons and argue against thermal emission from an X-ray-heated layer. Detailed modeling of two spectra with the highest S/N and resolution shows that there are two components within the LVC: a broad, centrally peaked component that can be attributed to gas arising in a warm disk surface in Keplerian rotation (with FWHM between ~40 and ~60 km/s), and a narrow component (with FWHM ~10 km/s and small blueshifts of ~2 km/s) that may arise in a cool (<1,000 K) molecular wind.

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arXiv2012

Remnant gas in evolved circumstellar disks: Herschel PACS observations of 10-100 Myr old disk systems

We present Herschel PACS spectroscopy of the [OI] 63 micron gas-line for three circumstellar disk systems showing signs of significant disk evolution and/or planet formation: HR 8799, HD 377 and RX J1852.3-3700. [OI] is undetected toward HR 8799 and HD 377 with 3 sigma upper limits of 6.8 x 10^-18 W m^-2 and 9.9 x 10^-18 W m^-2 respectively. We find an [OI] detection for RX J1852.3-3700 at 12.3 +- 1.8 x 10^-18 W m^-2. We use thermo-chemical disk models to model the gas emission, using constraints on the [OI] 63 micron, and ancillary data to derive gas mass upper limits and constrain gas-to-dust ratios. For HD 377 and HR 8799, we find 3 sigma upper limits on the gas mass of 0.1-20 Mearth. For RX J1852.3-3700, we find two distinct disk scenarios that could explain the detection of [OI] 63 micron and CO(2-1) upper limits reported from the literature: (i) a large disk with gas co-located with the dust (16-500 AU), resulting in a large tenuous disk with ~16 Mearth of gas, or (ii) an optically thick gas disk, truncated at ~70 AU, with a gas mass of 150 Mearth. We discuss the implications of these results for the formation and evolution of planets in these three systems.

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arXiv2012

The Chemistry of Interstellar OH+, H2O+, and H3O+: Inferring the Cosmic Ray Ionization Rates from Observations of Molecular Ions

We model the production of OH+, H2O+, and H3O+ in interstellar clouds, using a steady state photodissociation region code that treats the freeze-out of gas species, grain surface chemistry, and desorption of ices from grains. The code includes PAHs, which have important effects on the chemistry. All three ions generally have two peaks in abundance as a function of depth into the cloud, one at A_V<~1 and one at A_V~3-8, the exact values depending on the ratio of incident ultraviolet flux to gas density. For relatively low values of the incident far ultraviolet flux on the cloud ({\chi}<~ 1000; {\chi}= 1= local interstellar value), the columns of OH+ and H2O+ scale roughly as the cosmic ray primary ionization rate {\zeta}(crp) divided by the hydrogen nucleus density n. The H3O+ column is dominated by the second peak, and we show that if PAHs are present, N(H3O+) ~ 4x10^{13} cm^{-2} independent of {\zeta}(crp) or n. If there are no PAHs or very small grains at the second peak, N(H3O+) can attain such columns only if low ionization potential metals are heavily depleted. We also model diffuse and translucent clouds in the interstellar medium, and show how observations of N(OH+)/N(H) and N(OH+)/N(H2O+) can be used to estimate {\zeta}(crp)/n, {\chi}/n and A_V in them. We compare our models to Herschel observations of these two ions, and estimate {\zeta}(crp) ~ 4-6 x 10^-16 (n/100 cm^-3) s^-1 and \chi/n = 0.03 cm^3 for diffuse foreground clouds towards W49N.

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arXiv2011

Herschel Measurements of Molecular Oxygen in Orion

We report observations of three rotational transitions of molecular oxygen (O2) in emission from the H2 Peak 1 position of vibrationally excited molecular hydrogen in Orion. We observed the 487 GHz, 774 GHz, and 1121 GHz lines using HIFI on the Herschel Space Observatory, having velocities of 11 km s-1 to 12 km s-1 and widths of 3 km s-1. The beam-averaged column density is N(O2) = 6.5\times1016 cm-2, and assuming that the source has an equal beam filling factor for all transitions (beam widths 44, 28, and 19"), the relative line intensities imply a kinetic temperature between 65 K and 120 K. The fractional abundance of O2 relative to H2 is 0.3 - 7.3\times10-6. The unusual velocity suggests an association with a ~ 5" diameter source, denoted Peak A, the Western Clump, or MF4. The mass of this source is ~ 10 M\odot and the dust temperature is \geq 150 K. Our preferred explanation of the enhanced O2 abundance is that dust grains in this region are sufficiently warm (T \geq 100 K) to desorb water ice and thus keep a significant fraction of elemental oxygen in the gas phase, with a significant fraction as O2. For this small source, the line ratios require a temperature \geq 180 K. The inferred O2 column density \simeq 5\times1018 cm-2 can be produced in Peak A, having N(H2) \simeq 4\times1024 cm-2. An alternative mechanism is a low-velocity (10 to 15 km s-1) C-shock, which can produce N(O2) up to 1017 cm-2.

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arXiv2011

Emission Lines from the Gas Disk around TW Hydra and the Origin of the Inner Hole

We compare line emission calculated from theoretical disk models with optical to sub-millimeter wavelength observational data of the gas disk surrounding TW Hya and infer the spatial distribution of mass in the gas disk. The model disk that best matches observations has a gas mass ranging from $\sim10^{-4}-10^{-5}$\ms\ for $0.06{\rm AU} 13.6$eV) flux from TW Hya. H$_2$ pure rotational line emission comes primarily from $r\sim 1-30$AU. [OI]63$\mu$m, HCO$^+$ and CO pure rotational lines all arise from the outer disk at $r\sim30-120$AU. We discuss planet formation and photoevaporation as causes for the decrease in surface density of gas and dust inside 4 AU. If a planet is present, our results suggest a planet mass $\sim 4-7$M$_J$ situated at $\sim 3$AU. Using our photoevaporation models and the best surface density profile match to observations, we estimate a current photoevaporative mass loss rate of $4\times10^{-9}$\ms\ yr$^{-1}$ and a remaining disk lifetime of $\sim 5$ million years.

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arXiv2010

The Dark Molecular Gas

The mass of molecular gas in an interstellar cloud is often measured using line emission from low rotational levels of CO, which are sensitive to the CO mass, and then scaling to the assumed molecular hydrogen H_2 mass. However, a significant H_2 mass may lie outside the CO region, in the outer regions of the molecular cloud where the gas phase carbon resides in C or C+. Here, H_2 self-shields or is shielded by dust from UV photodissociation, where as CO is photodissociated. This H_2 gas is "dark" in molecular transitions because of the absence of CO and other trace molecules, and because H_2 emits so weakly at temperatures 10 K < T < 100 K typical of this molecular component. This component has been indirectly observed through other tracers of mass such as gamma rays produced in cosmic ray collisions with the gas and far-infrared/submillimeter wavelength dust continuum radiation. In this paper we theoretically model this dark mass and find that the fraction of the molecular mass in this dark component is remarkably constant (~ 0.3 for average visual extinction through the cloud with mean A_V ~ 8) and insensitive to the incident ultraviolet radiation field strength, the internal density distribution, and the mass of the molecular cloud as long as mean A_V, or equivalently, the product of the average hydrogen nucleus column and the metallicity through the cloud, is constant. We also find that the dark mass fraction increases with decreasing mean A_V, since relatively more molecular H_2 material lies outside the CO region in this case.

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arXiv2010

Spitzer Spectroscopy of the Transition Object TW Hya

We report sensitive Spitzer IRS spectroscopy in the 10-20 micron region of TW Hya, a nearby T Tauri star. The unusual spectral energy distribution of the source, that of a transition object, indicates that the circumstellar disk in the system has experienced significant evolution, possibly as a result of planet formation. The spectrum we measure is strikingly different from that of other classical T Tauri stars reported in the literature, displaying no strong emission features of water, acetylene, or HCN. The difference indicates that the inner planet formation region (within 5 AU) of the gaseous disk has evolved physically and/or chemically away from the classical T Tauri norm. Nevertheless, TW Hya does show a rich spectrum of emission features of atoms (HI, [NeII], and [NeIII]) and molecules (H2, OH, CO2, HCO+, and possibly CH3), some of which are also detected in classical T Tauri spectra. The properties of the neon emission are consistent with an origin for the emission in a disk irradiated by X-rays (with a possible role for additional irradiation by stellar EUV). The OH emission we detect, which also likely originates in the disk, is hot, arising from energy levels up to 23,000 K above ground, and may be produced by the UV photodissociation of water. The HI emission is surprisingly strong, with relative strengths that are consistent with case B recombination. While the absence of strong molecular emission in the 10-20 micron region may indicate that the inner region of the gaseous disk has been partly cleared by an orbiting giant planet, chemical and/or excitation effects may be responsible instead. We discuss these issues and how our results bear on our understanding of the evolutionary state of the TW Hya disk.

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arXiv2009

Time-Evolution of Viscous Circumstellar Disks due to Photoevaporation by FUV, EUV and X-ray Radiation from the Central Star

We present the time evolution of viscously accreting circumstellar disks as they are irradiated by ultraviolet and X-ray photons from a low-mass central star. Our model is a hybrid of a 1D time-dependent viscous disk model coupled to a 1+1D disk vertical structure model used for calculating the disk structure and photoevaporation rates. We find that disks of initial mass 0.1M_o around 1M_o stars survive for 4x10^6 years, assuming a viscosity parameter $\alpha=0.01$, a time-dependent FUV luminosity $L_{FUV}~10^{-2}-10^{-3}$ L_o and with X-ray and EUV luminosities $L_X \sim L_{EUV} ~ 10^{-3}$L_o. We find that FUV/X-ray-induced photoevaporation and viscous accretion are both important in depleting disk mass. Photoevaporation rates are most significant at ~ 1-10 AU and at >~ 30 AU. Viscosity spreads the disk which causes mass loss by accretion onto the central star and feeds mass loss by photoevaporation in the outer disk. We find that FUV photons can create gaps in the inner, planet-forming regions of the disk (~ 1-10 AU) at relatively early epochs in disk evolution while disk masses are still substantial. EUV and X-ray photons are also capable of driving gaps, but EUV can only do so at late, low accretion-rate epochs after the disk mass has already declined substantially. Disks around stars with predominantly soft X-ray fields experience enhanced photoevaporative mass loss. We follow disk evolution around stars of different masses, and find that disk survival time is relatively independent of mass for stars with M <~ 3M_o; for M >~ 3M_o the disks are short-lived(~10^5 years).

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