Gas Accretion Process Research Articles

The Westerbork HALOGAS Survey: Status and Early Results

AbstractWe present early results from the ongoing Hydrogen Accretion in LOcal GAlaxieS (HALOGAS) Survey, which is being performed with the Westerbork Synthesis Radio Telescope (WSRT). The HALOGAS Survey aims to detect and characterize the cold gas accretion process in nearby spirals, through sensitive observations of neutral hydrogen (Hi) 21-cm line emission. In this contribution, we present an overview of ongoing analyses of several HALOGAS targets.

The chemical evolution of IC 10

Dwarf irregular galaxies are relatively simple unevolved objects where it is easy to test models of galactic chemical evolution. We attempt to determine the star formation and gas accretion history of IC10, a local dwarf irregular for which abundance, gas, and mass determinations are available. We apply detailed chemical evolution models to predict the evolution of several chemical elements (He, O, N, S) and compared our predictions with the observational data. We consider additional constraints such as the present-time gas fraction, the star formation rate (SFR), and the total estimated mass of IC10. We assume a dark matter halo for this galaxy and study the development of a galactic wind. We consider different star formation regimes: bursting and continuous. We explore different wind situations: i) normal wind, where all the gas is lost at the same rate and ii) metal-enhanced wind, where metals produced by supernovae are preferentially lost. We study a case without wind. We vary the star formation efficiency (SFE), the wind efficiency, and the time scale of the gas infall, which are the most important parameters in our models. We find that only models with metal-enhanced galactic winds can reproduce the properties of IC10. The star formation must have proceeded in bursts rather than continuously and the bursts must have been less numerous than ~10 over the whole galactic lifetime. Finally, IC10 must have formed by a slow process of gas accretion with a timescale of the order of 8 Gyr.

UGC8802: A MASSIVE DISK GALAXY IN FORMATION

We report new observations of the galaxy UGC8802 obtained through GASS, the GALEX Arecibo SDSS Survey, which show this galaxy to be in a remarkable evolutionary state. UGC8802 (GASS35981) is a disk galaxy with stellar mass M*=2x10^10 Msolar which appears to contain an additional 2.1x10^10 Msolar of HI gas. New millimeter observations with the IRAM 30m telescope indicate a molecular gas mass only a tenth this large. Using deep long-slit spectroscopy, we examine the spatially resolved star formation rate and metallicity profiles of GASS35981 for clues to its history. We find that the star formation surface density in this galaxy is low (Sigma_SFR=0.003 Msolar/yr/kpc^2) and that the star formation is spread remarkably evenly across the galaxy. The low molecular gas masses measured in our three IRAM pointings are largely consistent with the total star formation measured within the same apertures. Our MMT long-slit spectrum reveals a sharp drop in metallicity in the outer disk of GASS35981. The ratio of current star formation rate to existing stellar mass surface density in the outer disk is extremely high, implying that all the stars must have formed within the past ~1Gyr. At current star formation rates, however, GASS35981 will not consume its HI reservoir for another 5-7 Gyr. Despite its exceptionally large gas fraction for a galaxy this massive, GASS35981 has a regular rotation curve and exhibits no sign of a recent interaction or merger. We speculate that GASS35981 may have acquired its gas directly from the inter-galactic medium, and that it and other similar galaxies identified in the GASS survey may provide rare local glimpses of gas accretion processes that were more common during the prime epoch of disk galaxy formation at z~1.

Clumpy galaxies at z∼0.6: kinematics, stability and comparison with analogues at other redshifts

Distant clumpy galaxies are thought to be Jeans-unstable disks, and an important channel for the formation of local galaxies, as suggested by recent spatially-resolved kinematic observations of z~2 galaxies. I study the kinematics of clumpy galaxies at z~0.6, and compare their properties with those of counterparts at higher and lower redshifts. I selected a sample of 11 clumpy galaxies at z~0.6 from the representative sample of emission line, intermediate-mass galaxies IMAGES. Selection was based on rest-frame UV morphology from HST/ACS images, mimicking the selection criteria commonly used at higher redshifts. Their spatially-resolved kinematics were derived in the frame of the IMAGES survey, using the VLT/FLAMES-GIRAFFE multi-integral field spectrograph. For those showing large-scale rotation, I derived the Toomre Q parameter, which characterizes the stability of their gaseous and stellar phases. I find that the fraction of UV-selected clumpy galaxies at z~0.6 is 20+/-12%. Roughly half of them (45+/-30%) have complex kinematics inconsistent with Jeans-unstable disks, while those in the remaining half (55+/-30%) show large-scale rotations. The latter reveal a stable gaseous phase, but the contribution of their stellar phase makes them globally unstable to clump formation. Clumpy galaxies appear to be less unstable at z~0.6 than at z~2, which could explain why the UV clumps tend to vanish in rest-frame optical images of z~0.6 clumpy galaxies, conversely to z~2 clumpy galaxies, in which the stellar phase can substantially fragment. This suggests that the former correspond to patchy star-formation regions superimposed on a smoother mass distribution. A possible and widespread scenario for driving clump formation relies on instabilities by cold streams penetrating the dark matter halos where clumpy galaxies inhabit. While such a gas accretion process is predicted to be significant in massive, z~2 haloes, it is also predicted to be strongly suppressed in similar, z~0.6 haloes, which could explain why lowest-z clumpy galaxies appear to be driven by a different mechanism. Instead, I found that interactions are probably the dominant driver leading to the formation of clumpy galaxies at z<1. I argue that the nature of z>1 clumpy galaxies remains more uncertain. While cold flows could be an important driver at z~2, I also argue that the observed and cumulative merger fraction between z=2 and z=3 is large enough so that every z~2 galaxy might be the result of a merger that occurred within their past 1 Gyr. I conclude that it is premature to rule out mergers as a universal driver for galaxy evolution from z~2 down to z=0.

Absorption properties and evolution of active galactic nuclei

Aims. Intrinsic absorption is a fundamental physical property for understanding the evolution of active galactic nuclei (AGN). Here we study a sample of 1290 AGN, selected in the 2–10 keV band from different flux-limited surveys with very high optical identification completeness.

Abundances of lithium, sodium and potassium in Vega★

Vega's photospheric abundances of Li, Na and K were determined by using considerably weak lines measured on the very high signal-to-noise ratio spectrum, while the non-local thermodynamic equilibrium (non-LTE) correction and the gravity-darkening correction were adequately taken into account. It was confirmed that these alkali elements are mildly underabundant ([Li/H]≃−0.6, [Na/H]≃−0.3 and [K/H]≃−0.2) compared to the Solar system values, as generally seen also in other metals. Since the tendency of Li being more deficient than Na and K is qualitatively similar to what is seen in typical interstellar cloud, the process of interstellar gas accretion may be related with the abundance anomaly of Vega, as suspected in the case of λ Boo stars.

Toward a Deterministic Model of Planetary Formation. I. A Desert in the Mass and Semimajor Axis Distributions of Extrasolar Planets

In an attempt to develop a deterministic theory for planet formation, we examine the accretion of cores of giant planets from planetesimals, gas accretion onto the cores, and their orbital migration. We adopt a working model for nascent protostellar disks with a wide variety of surface density distributions in order to explore the range of diversity among extrasolar planetary systems. We evaluate the cores' mass growth rate c through runaway planetesimal accretion and oligarchic growth. The accretion rate of cores is estimated with a two-body approximation. In the inner regions of disks, the cores' eccentricity is effectively damped by their tidal interaction with the ambient disk gas and their early growth is stalled by isolation. In the outer regions, the cores' growth rate is much smaller. If some cores can acquire more mass than a critical value of several Earth masses during the persistence of the disk gas, they would be able to rapidly accrete gas and evolve into gas giant planets. The gas accretion process is initially regulated by the Kelvin-Helmholtz contraction of the planets' gas envelope. Based on the assumption that the exponential decay of the disk gas mass occurs on the timescales ~106-107 yr and that the disk mass distribution is comparable to those inferred from the observations of circumstellar disks of T Tauri stars, we carry out simulations to predict the distributions of masses and semimajor axes of extrasolar planets. In disks as massive as the minimum-mass disk for the solar system, gas giants can form only slightly outside the ice boundary at a few AU. However, cores can rapidly grow above the critical mass inside the ice boundary in protostellar disks with 5 times more heavy elements than those of the minimum-mass disk. Thereafter, these massive cores accrete gas prior to its depletion and evolve into gas giants. The limited persistence of the disk gas and the decline in the stellar gravity prevent the formation of cores capable of efficient gas accretion outside 20-30 AU. Unimpeded dynamical accretion of gas is a runaway process that is terminated when the residual gas is depleted either globally or locally in the form of a gap in the vicinity of their orbits. Since planets' masses grow rapidly from 10 to 100 M?, the gas giant planets rarely form with asymptotic masses in this intermediate range. Our model predicts a paucity of extrasolar planets with mass in the range 10-100 M? and semimajor axis less than 3 AU. We refer to this deficit as a planet desert. We also examine the dynamical evolution of protoplanets by considering the effect of orbital migration of giant planets due to their tidal interactions with the gas disks, after they have opened up gaps in the disks. The effect of migration is to sharpen the boundaries and to enhance the contrast of the planet desert. It also clarifies the separation between the three populations of rocky, gas giant, and ice giant planets. Based on our results, we suggest that the planets' mass versus semimajor axes diagram can provide strong constraints on the dominant formation processes of planets analogous to the implications of the color-magnitude diagram on the paths of stellar evolution. We show that the mass and semimajor axis distributions generated in our simulations for the gas giants are consistent with those of the known extrasolar planets. Our results also indicate that a large fraction (90%-95%) of the planets that have migrated to within 0.05 AU must have perished. Future observations can determine the existence and the boundaries of the planet desert in this diagram, which can be used to extrapolate the ubiquity of rocky planets around nearby stars. Finally, the long-term dynamical interaction between planets of various masses can lead to both eccentricity excitation and scattering of planets to large semimajor axes. These effects are to be included in future models.

Protoplanetary Formation. I. Neptune

Neptune has a gaseous envelope with mass larger than that of the Earth. We examine the possibility that proto-Neptune formed through an initial buildup of a core prior to the accretion of a gaseous envelope in the solar nebula environment. In this conventional scenario, the inferred formation timescale of proto-Neptune would be ~106-7 yr since the signature of protoplanetary disks are observed to vanish on such a timescale. We show that after their emergence through an initial epoch of runaway buildup, proto-Neptune's core would exert gravitational perturbations on and excite eccentricity among the planetesimals near its orbit. In the absence of dissipation due to inelastic collisions or gas drag, the planetesimals' velocity dispersion and the core's growth timescale would increase indefinitely. But close encounters between planetesimals and the core also lead to orbital drift and migration, which would replenish the planetesimals in the core's feeding zone and thereby sustain the core's rate of growth. In a subsequent paper, we show that when the core has acquired an adequate (comparable to that of Neptune's present) mass, it may capture those planetesimals along its migration path onto its strongest mean-motion resonances. Such a process would account for the observed distribution of trans-Neptunian objects. Eventually, this resonant barrier would prevent residual planetesimals from reaching the core's feeding zone and terminate its mass growth as well as its orbital migration. We also investigate the process of gas accretion onto the core. We show that the depletion of nebula gas may be required to limit the mass of proto-Neptune's gaseous envelope. Such a depletion may either be due to the global viscous evolution of the nebula or to tidally induced gap formation near proto-Neptune's orbit. The former process would require a 3 order of magnitude reduction of the surface density from that of the minimum-mass solar nebula model. The latter process would provide not only a natural mass limit for the gaseous envelope in both Neptune and Uranus but also a mechanism for Neptune's outward orbital migration on its formation timescale. The necessary conditions for gap formation are that the outer parts of the disk have a very low viscosity and a small thickness.

Formation of Giant Planets: Dependences on Core Accretion Rate and Grain Opacity

We have investigated the formation of gaseous envelopes of giant planets with wide ranges of parameters through quasi-static evolutionary simulations. In the nucleated instability model, rapid gas accretion is triggered when the solid core mass exceeds a critical mass. The gas accretion should be regulated essentially by core accretion rate and grain opacity in the outermost envelope. The conventional critical core mass ~5-20 M⊕ (M⊕: Earth's mass), however, is based on some nominal values of these quantities. The discovery of extrasolar giant planets requires investigation of the gas accretion processes under various circumstances. Furthermore, the current planetary accretion theory points out that the cores of Jupiter and Saturn would have been isolated from planetesimals and the core accretion would have almost stopped in their later stage of formation before their masses reached the conventional critical core mass. Through numerical simulations of quasi-static evolution of the gaseous envelope, we have investigated the characteristic growth times of the envelope mass for wide ranges of core accretion rate and grain opacity. We also studied the case where core accretion stops before onset of rapid gas accretion. Our main results are (1) the growth time of the envelope mass τg depends strongly on the core mass, moderately on the grain opacity, and weakly on the past core accretion process, and (2) τg is expressed approximately as τg ~ 108(Mcore /M⊕)-2.5(κ gr/1 cm 2 g-1) yr, where Mcore is the core mass and κgr is the grain opacity. Our results combined with the recent planetary accretion theory suggest surface density of solid materials twice as massive as that of the minimum-mass solar nebula model and the longer lifetime of the nebula than the 108 yr needed to form Jupiter and Saturn; otherwise migration of protoplanets may have to be considered. Our extensive parametric study not only confirms the difficulty in the formation of the giant planets quantitatively and rigorously, it also gives essential information in considering the problem of the formation, which is quite useful in applications.

Observations of Mass Flow, Disks, and Accretion Processes in Algol Binary Systems

As part of a multi-faceted study of gas streams, disks, and accretion processes in Algol type binaries, R.S. Polidan and I have obtained high resolution IUE observations of seven systems. Perhaps one of the most interesting results from these observations was the discovery of strong, variable absorption lines of N V, C IV, and Si IV in the spectra of stars whose photospheres are much too cool to form these ions. N V (1239,1243) and C IV (1548,1551) have been observed in AU Mon (B5V, 11ḍ1), CX Dra (B3V, 6ḍ7), and U CrB (B4V, 3ḍ5) while Si IV (1394, 1403) was found in TX UMa (B8V, 3ḍ1). The systems in which these ions have been observed generally contain primaries whose dimensions are greater than the Lubow and Shu (1975) parameter ϖmin.Radial velocity measurements of the N V, C IV, and Si IV features suggest that the line formation region is associated with the primary star and small in size. The strengths of the N V, C IV features appear to be variable on a time scale of several orbital periods and may be determined by the instantaneous rate of mass transfer. However, when the high ionization features are observed, their presence appears to be global and not restricted to the domain of gas stream impact.