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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,279 records · Page 71
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Super massive black hole in galactic nuclei with tidal disruption of stars

Tidal disruption of stars by super massive central black holes from dense star clusters is modeled by high-accuracy direct N-body simulation. The time evolution of the stellar tidal disruption rate, the effect of tidal disruption on the stellar density profile, and, for the first time, the detailed origin of tidally disrupted stars are carefully examined and compared with classic papers in the field. Up to 128k particles are used in simulation to model the star cluster around a super massive black hole, and we use the particle number and the tidal radius of the black hole as free parameters for a scaling analysis. The transition from full to empty loss-cone is analyzed in our data, and the tidal disruption rate scales with the particle number, N, in the expected way for both cases. For the first time in numerical simulations (under certain conditions) we can support the concept of a critical radius of Frank and Rees, which claims that most stars are tidally accreted on highly eccentric orbits originating from regions far outside the tidal radius. Due to the consumption of stars moving on radial orbits, a velocity anisotropy is found inside the cluster. Finally we estimate the real galactic center based on our simulation results and the scaling analysis.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The progenitors of local ultra-massive galaxies across cosmic time: from dusty star-bursting to quiescent stellar populations

Using the UltraVISTA catalogs, we investigate the evolution in the 11.4 Gyr since z = 3 of the progenitors of local ultra-massive galaxies (log (M {sub star}/M {sub ☉}) ≈ 11.8; UMGs), providing a complete and consistent picture of how the most massive galaxies at z = 0 have assembled. By selecting the progenitors with a semi-empirical approach using abundance matching, we infer a growth in stellar mass of 0.56{sub −0.25}{sup +0.35} dex, 0.45{sub −0.20}{sup +0.16} dex, and 0.27{sub −0.12}{sup +0.08} dex from z = 3, z = 2, and z = 1, respectively, to z = 0. At z < 1, the progenitors of UMGs constitute a homogeneous population of only quiescent galaxies with old stellar populations. At z > 1, the contribution from star-forming galaxies progressively increases, with the progenitors at 2 < z < 3 being dominated by massive (M {sub star} ≈ 2 × 10{sup 11} M {sub ☉}), dusty (A {sub V} ∼ 1-2.2 mag), star-forming (SFR ∼ 100-400 M {sub ☉} yr{sup –1}) galaxies with a large range in stellar ages. At z = 2.75, ∼15% of the progenitors are quiescent, with properties typical of post-starburst galaxies with little dust extinction and strong Balmer break, and showing a large scatter in color. Our findings indicate that at least half of the stellar content of local UMGs was assembled at z > 1, whereas the remaining was assembled via merging from z ∼ 1 to the present. Most of the quenching of the star-forming progenitors happened between z = 2.75 and z = 1.25, in good agreement with the typical formation redshift and scatter in age of z = 0 UMGs as derived from their fossil records. The progenitors of local UMGs, including the star-forming ones, never lived on the blue cloud since z = 3. We propose an alternative path for the formation of local UMGs that refines previously proposed pictures and that is fully consistent with our findings.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

FROM BLUE STAR-FORMING TO RED PASSIVE: GALAXIES IN TRANSITION IN DIFFERENT ENVIRONMENTS

Exploiting a mass-complete (M {sub *} > 10{sup 10.25} M {sub ☉}) sample at 0.03 <z < 0.11 drawn from the Padova Millennium Galaxy Group Catalog, we use the (U – B) {sub rf} color and morphologies to characterize galaxies, in particular those that show signs of an ongoing or recent transformation of their star-formation activity and/or morphology: green galaxies, red passive late types, and blue star-forming early types. Color fractions depend on mass and only for M {sub *} < 10{sup 10.7} M {sub ☉} on environment. The incidence of red galaxies increases with increasing mass, and, for M {sub *} < 10{sup 10.7} M {sub ☉}, decreases toward the group outskirts and in binary and single galaxies. The relative abundance of green and blue galaxies is independent of environment and increases monotonically with galaxy mass. We also inspect galaxy structural parameters, star-formation properties, histories, and ages and propose an evolutionary scenario for the different subpopulations. Color transformations are due to a reduction and suppression of the star-formation rate in both bulges and disks that does not noticeably affect galaxy structure. Morphological transitions are linked to an enhanced bulge-to-disk ratio that is due to the removal of the disk, not to an increase of the bulge. Our modeling suggests that green colors might be due to star-formation histories declining with long timescales, as an alternative scenario to the classical ''quenching'' processes. Our results suggest that galaxy transformations in star-formation activity and morphology depend neither on the environment nor on being a satellite or the most massive galaxy of a halo. The only environmental dependence we find is the higher fast quenching efficiency in groups giving origin to poststarburst signatures.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

WARM DUSTY DEBRIS DISKS AND DISTANT COMPANION STARS: V488 PER AND 2M1337

A possible connection between the presence of large quantities of warm (T ≥ 200 K) circumstellar dust at youthful stars and the existence of wide-separation companion stars has been noted in the literature. Here we point out the existence of a distant companion star to V488 Per, a K-type member of the α Persei cluster with the largest known fractional excess infrared luminosity (∼16%) of any main sequence star. We also report the presence of a distant companion to the previously recognized warm dust star 2M1337. With these discoveries the existence of a cause and effect relationship between a distant companion and large quantities of warm dust in orbit around youthful stars now seems compelling.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

STRONG C{sup +} EMISSION IN GALAXIES AT z ∼ 1-2: EVIDENCE FOR COLD FLOW ACCRETION POWERED STAR FORMATION IN THE EARLY UNIVERSE

We have recently detected the [C II] 157.7 μm line in eight star-forming galaxies at redshifts 1 to 2 using the redshift (z) Early Universe Spectrometer (ZEUS). Our sample targets star formation dominant sources detected in PAH emission. This represents a significant addition to [C II] observations during the epoch of peak star formation. We have augmented this survey with observations of the [O I] 63 μm line and far infrared photometry from the PACS and SPIRE Herschel instruments as well as Spitzer IRS spectra from the literature showing PAH features. Our sources exhibit above average gas heating efficiency, many with both [O I]/FIR and [C II]/FIR of ∼1% or more. The relatively strong [C II] emission is consistent with our sources being dominated by star formation powered photo-dissociation regions, extending to kiloparsec scales. We suggest that the star formation mode in these systems follows a Schmidt-Kennicutt law similar to local systems, but at a much higher rate due to molecular gas surface densities 10-100 times that of local star-forming systems. The source of the high molecular gas surface densities may be the infall of neutral gas from the cosmic web. In addition to the high [C II]/FIR values, we also find high [C II]/PAH ratios and, in at least one source, a cool dust temperature. This source, SWIRE 4-5, bears a resemblance in these diagnostics to shocked regions of Stephan's Quintet, suggesting that another mode of [C II] excitation in addition to normal photoelectric heating may be contributing to the observed [C II] line.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

SUB-STELLAR COMPANIONS AND STELLAR MULTIPLICITY IN THE TAURUS STAR-FORMING REGION

We present results from a large, high-spatial-resolution near-infrared imaging search for stellar and sub-stellar companions in the Taurus-Auriga star-forming region. The sample covers 64 stars with masses between those of the most massive Taurus members at ∼3 M {sub ☉} and low-mass stars at ∼0.2 M {sub ☉}. We detected 74 companion candidates, 34 of these reported for the first time. Twenty-five companions are likely physically bound, partly confirmed by follow-up observations. Four candidate companions are likely unrelated field stars. Assuming physical association with their host star, estimated companion masses are as low as ∼2 M {sub Jup}. The inferred multiplicity frequency within our sensitivity limits between ∼10-1500 AU is 26.3{sub −4.9}{sup +6.6}%. Applying a completeness correction, 62% ± 14% of all Taurus stars between 0.7 and 1.4 M {sub ☉} appear to be multiple. Higher order multiples were found in 1.8{sub −1.5}{sup +4.2}% of the cases, in agreement with previous observations of the field. We estimate a sub-stellar companion frequency of ∼3.5%-8.8% within our sensitivity limits from the discovery of two likely bound and three other tentative very low-mass companions. This frequency appears to be in agreement with what is expected from the tail of the stellar companion mass ratio distribution, suggesting that stellar and brown dwarf companions share the same dominant formation mechanism. Further, we find evidence for possible evolution of binary parameters between two identified sub-populations in Taurus with ages of ∼2 Myr and ∼20 Myr, respectively.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ON THE NATURE OF RAPIDLY ROTATING SINGLE EVOLVED STARS

We present an analysis of the nature of the rapidly rotating, apparently single giant based on rotational and radial velocity measurements carried out by the CORAVEL spectrometers. From the analyzed sample, composed of 2010 spectroscopic, apparently single, evolved stars of luminosity classes IV, III, II, and Ib with spectral types G and K, we classified 30 stars that presented unusual, moderate to rapid rotation. This work reports, for the first time, the presence of these abnormal rotators among subgiant, bright giant, and Ib supergiant stars. To date, this class of stars was reported only among giant stars of luminosity class III. Most of these abnormal rotators present an IRAS infrared excess, which, in principle, can be related to dust around these stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

PLANETS AROUND LOW-MASS STARS (PALMS). V. AGE-DATING LOW-MASS COMPANIONS TO MEMBERS AND INTERLOPERS OF YOUNG MOVING GROUPS

We present optical and near-infrared adaptive optics (AO) imaging and spectroscopy of 13 ultracool (>M6) companions to late-type stars (K7–M4.5), most of which have recently been identified as candidate members of nearby young moving groups (YMGs; 8–120 Myr) in the literature. Three of these are new companions identified in our AO imaging survey, and two others are confirmed to be comoving with their host stars for the first time. The inferred masses of the companions (∼10–100 M{sub Jup}) are highly sensitive to the ages of the primary stars; therefore we critically examine the kinematic and spectroscopic properties of each system to distinguish bona fide YMG members from old field interlopers. The new M7 substellar companion 2MASS J02155892–0929121 C (40–60 M{sub Jup}) shows clear spectroscopic signs of low gravity and, hence, youth. The primary, possibly a member of the ∼40 Myr Tuc-Hor moving group, is visually resolved into three components, making it a young low-mass quadruple system in a compact (≲100 AU) configuration. In addition, Li i λ6708 absorption in the intermediate-gravity M7.5 companion 2MASS J15594729+4403595 B provides unambiguous evidence that it is young (≲200 Myr) and resides below the hydrogen-burning limit. Three new close-separation (<1″) companions (2MASS J06475229–2523304 B, PYC J11519+0731 B, and GJ 4378 Ab) orbit stars previously reported as candidate YMG members, but instead are likely old (≳1 Gyr) tidally locked spectroscopic binaries without convincing kinematic associations with any known moving group. The high rate of false positives in the form of old active stars with YMG-like kinematics underscores the importance of radial velocity and parallax measurements to validate candidate young stars identified via proper motion and activity selection alone. Finally, we spectroscopically confirm the cool temperature and substellar nature of HD 23514 B, a recently discovered M8 benchmark brown dwarf orbiting the dustiest-known member of the Pleiades.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

The relation between luminous AGNs and star formation in their host galaxies

We study the relation of active galactic nuclei (AGNs) to star formation in their host galaxies. Our sample includes 205 Type-1 and 85 Type-2 AGNs, 162 detected with Herschel, from fields surrounding 30 galaxy clusters in the Local Cluster Substructure Survey. The sample is identified by optical line widths and ratios after selection to be brighter than 1 mJy at 24 μm. We show that Type-2 AGN [O iii]λ5007 line fluxes at high z can be contaminated by their host galaxies with typical spectrograph entrance apertures (but our sample is not compromised in this way). We use spectral energy distribution (SED) templates to decompose the galaxy SEDs and estimate star formation rates (SFRs), AGN luminosities, and host galaxy stellar masses (described in an accompanying paper). The AGNs arise from massive black holes (∼3×10{sup 8}M{sub ⊙}) accreting at ∼10% of the Eddington rate and residing in galaxies with stellar mass >3×10{sup 10}M{sub ⊙}; those detected with Herschel have IR luminosity from star formation in the range of L{sub SF,IR}∼10{sup 10}−10{sup 12}L{sub ⊙}. We find that (1) the specific SFRs in the host galaxies are generally consistent with those of normal star-forming (main sequence) galaxies; (2) there is a strong correlation between the luminosities from star formation and the AGN; and (3) the correlation may not result from a causal connection, but could arise because the black hole mass (and hence AGN Eddington luminosity) and star formation are both correlated with the galaxy mass.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

THE INTERNAL ROTATION PROFILE OF THE B-TYPE STAR KIC 10526294 FROM FREQUENCY INVERSION OF ITS DIPOLE GRAVITY MODES

The internal angular momentum distribution of a star is the key to determining its evolution. Fortunately, stellar internal rotation can be probed through studies of rotationally split nonradial oscillation modes. In particular, the detection of nonradial gravity modes (g modes) in massive young stars has recently become feasible thanks to the Kepler space mission. Our goal is to derive the internal rotation profile of the Kepler B8V star KIC 10526294 through asteroseismology. We interpret the observed rotational splittings of its dipole g modes using four different approaches based on the best seismic models of the star and their rotational kernels. We show that these kernels can resolve differential rotation within the radiative envelope if a smooth rotational profile is assumed and if the observational errors are small. Based on Kepler data, we find that the rotation rate near the core-envelope boundary is well constrained to 163 ± 89 nHz. The seismic data are consistent with rigid rotation but a profile with counter-rotation within the envelope has a statistical advantage over constant rotation. Our study should be repeated for other massive stars with a variety of stellar parameters in order to determine the physical conditions that control the internal rotation profile of young massive stars, with the aim of improving the input physics of their models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

OPTICAL MASS FLOW DIAGNOSTICS IN HERBIG AE/BE STARS

We examine a broad range of mass flow diagnostics in a large sample of Herbig Ae/Be stars (HAEBES) using high resolution optical spectra. The Hβ and He i 5876 Å lines show the highest incidence of P Cygni (30%) and inverse P Cygni (14%) morphologies, respectively. The Fe ii 4924 Å line also shows a large incidence of P Cygni profiles (11%). We find support for many of the conclusions reached in a study based on the analysis of the He i λ10830 line in a large sample of HAEBES. Namely, HAEBES exhibit smaller fractions of both blueshifted absorption (i.e., mass outflow) and redshifted absorption (i.e., mass infall or accretion) than their lower mass cousins, the classical T Tauri stars (CTTSs). In particular, the optical data supports the conclusion that HAEBES displaying redshifted absorption, in general, show maximum redshifted absorption velocities that are smaller fractions of their stellar escape velocities than is found for CTTSs. This suggests that HAEBE accretion flows are originating deeper in the gravitational potentials of their stars than in CTTS systems. In addition, we find a lack of inner disk wind signatures in the blueshifted absorption objects; only stellar wind signatures are clearly observed. These findings, along with the lack of detected magnetic fields around HAEBES, support the idea that large magnetospheres are not prevalent around HAEBES and that accretion flows are instead mediated by significantly smaller magnetospheres with relatively smaller truncation radii (e.g., 1–2 R{sub *}). Redshifted absorption is much more common around Herbig Ae stars than Be stars, suggesting that Herbig Be stars may accrete via a boundary layer rather than along magnetic field lines.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

X-ray properties of low-mass pre-main sequence stars in the Orion trapezium cluster

The Chandra HETG Orion Legacy Project (HOLP) is the first comprehensive set of observations of a very young massive stellar cluster that provides high-resolution X-ray spectra of very young stars over a wide mass range (0.7–2.3 M{sub ⊙}). In this paper, we focus on the six brightest X-ray sources with T Tauri stellar counterparts that are well-characterized at optical and infrared wavelengths. All stars show column densities which are substantially smaller than expected from optical extinction, indicating that the sources are located on the near side of the cluster with respect to the observer as well as that these stars are embedded in more dusty environments. Stellar X-ray luminosities are well above 10{sup 31} erg s{sup −1}, in some cases exceeding 10{sup 32} erg s{sup −1} for a substantial amount of time. The stars during these observations show no flares but are persistently bright. The spectra can be well fit with two temperature plasma components of 10 MK and 40 MK, of which the latter dominates the flux by a ratio 6:1 on average. The total emission measures range between 3–8 × 10{sup 54} cm{sup −3} and are comparable to active coronal sources. The fits to the Ne ix He-Like K-shell lines indicate forbidden to inter-combination line ratios consistent with the low-density limit. Observed abundances compare well with active coronal sources underlying the coronal nature of these sources. The surface flux in this sample of 0.6–2.3 M{sub ⊙} classical T Tauri stars shows that coronal activity increases significantly between ages 0.1 and 10 Myr. The results demonstrate the power of X-ray line diagnostics to study coronal properties of T Tauri stars in young stellar clusters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

A PILOT DEEP SURVEY FOR X-RAY EMISSION FROM fuvAGB STARS

We report the results of a pilot survey for X-ray emission from a newly discovered class of AGB stars with far-ultraviolet excesses (fuvAGB stars) using XMM-Newton and Chandra. We detected X-ray emission in three of six fuvAGB stars observed—the X-ray fluxes are found to vary in a stochastic or quasi-periodic manner on roughly hour-long timescales, and simultaneous UV observations using the Optical Monitor on XMM for these sources show similar variations in the UV flux. These data, together with previous studies, show that X-ray emission is found only in fuvAGB stars. From modeling the spectra, we find that the observed X-ray luminosities are ∼(0.002–0.2) L{sub ⊙} and the X-ray-emitting plasma temperatures are ∼(35–160) × 10{sup 6} K. The high X-ray temperatures argue against the emission arising in stellar coronae, or directly in an accretion shock, unless it occurs on a WD companion. However, none of the detected objects is a known WD-symbiotic star, suggesting that if WD companions are present, they are relatively cool (<20,000 K). In addition, the high X-ray luminosities specifically argue against emission originating in the coronae of main-sequence companions. We discuss several models for the X-ray emission and its variability and find that the most likely scenario for the origin of the X-ray (and FUV) emission involves accretion activity around a companion star, with confinement by strong magnetic fields associated with the companion and/or an accretion disk around it.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

ASTEROSEISMOLOGY OF EVOLVED STARS WITH KEPLER: A NEW WAY TO CONSTRAIN STELLAR INTERIORS USING MODE INERTIAS

The asteroseismology of evolved solar-like stars is experiencing growing interest due to the wealth of observational data from space-borne instruments such as the CoRoT and Kepler spacecraft. In particular, the recent detection of mixed modes, which probe both the innermost and uppermost layers of stars, paves the way for inferring the internal structure of stars along their evolution through the subgiant and red giant phases. Mixed modes can also place stringent constraints on the physics of such stars and on their global properties (mass, age, etc.). Here, using two Kepler stars (KIC 4351319 and KIC 6442183), we demonstrate that measurements of mixed mode characteristics allow us to estimate the mode inertias, providing a new and additional diagnostics on the mode trapping and subsequently on the internal structure of evolved stars. We however stress that the accuracy may be sensitive to non-adiabatic effects.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

HST-COS SPECTROSCOPY OF THE COOLING FLOW IN A1795—EVIDENCE FOR INEFFICIENT STAR FORMATION IN CONDENSING INTRACLUSTER GAS

We present far-UV spectroscopy from the Cosmic Origins Spectrograph on the Hubble Space Telescope of a cool, star-forming filament in the core of A1795. These data, which span 1025 Å < λ{sub rest} < 1700 Å, allow for the simultaneous modeling of the young stellar populations and the intermediate-temperature (10{sup 5.5} K) gas in this filament, which is far removed (∼30 kpc) from the direct influence of the central active galactic nucleus. Using a combination of UV absorption line indices and stellar population synthesis modeling, we find evidence for ongoing star formation, with the youngest stars having ages of 7.5{sub −2.0}{sup +2.5} Myr and metallicities of 0.4{sub −0.1}{sup +0.2} Z {sub ☉}. The latter is consistent with the local metallicity of the intracluster medium. We detect the O VI λ1038 line, measuring a flux of f {sub O} {sub VI,} {sub 1038} = 4.0 ± 0.9 × 10{sup –17} erg s{sup –1} cm{sup –2}. The O VI λ1032 line is redshifted such that it is coincident with a strong Galactic H{sub 2} absorption feature, and is not detected. The measured O VI λ1038 flux corresponds to a cooling rate of 0.85 ± 0.2 (stat) ± 0.15 (sys) M {sub ☉} yr{sup –1} at ∼10{sup 5.5} K, assuming that the cooling proceeds isochorically, which is consistent with the classical X-ray luminosity-derived cooling rate in the same region. We measure a star formation rate of 0.11 ± 0.02 M {sub ☉} yr{sup –1} from the UV continuum, suggesting that star formation is proceeding at 13{sub −2}{sup +3}% efficiency in this filament. We propose that this inefficient star formation represents a significant contribution to the larger-scale cooling flow problem.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

NEW CANDIDATE ERUPTIVE YOUNG STARS IN LYNDS 1340

We report on the discovery of three candidate eruptive young stars, found during our comprehensive multi-wavelength study of the young stellar population of the dark cloud L1340. These stars are as follows. (1) IRAS 02224+7227 (2MASS 02270555+7241167, HH 487S) exhibited FUor-like spectrum in our low-resolution optical spectra. The available photometric data restrict its luminosity to 23 L {sub ☉} < L {sub bol} < 59 L {sub ☉}. (2) 2MASS 02263797+7304575, identified as a classical T Tauri star during our Hα survey, exhibited an EXor-type brightening in 2005 November at the time of the Sloan Digital Sky Survey observations of the region. (3) 2MASS 02325605+7246055, a low-mass embedded young star, associated with a fan-shaped infrared nebula, underwent an outburst between the DSS 1 and DSS 2 surveys, leading to the appearance of a faint optical nebula. Our [S II] and Hα images, as well as the Spitzer Infrared Array Camera 4.5 μm images, revealed Herbig-Haro objects associated with this star. Our results suggest that amplitudes and timescales of outbursts do not necessarily correlate with the evolutionary stage of the stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

KEPLER RAPIDLY ROTATING GIANT STARS

Rapidly rotating giant stars are relatively rare and may represent important stages of stellar evolution, resulting from stellar coalescence of close binary systems or accretion of substellar companions by their hosting stars. In the present Letter, we report 17 giant stars observed in the scope of the Kepler space mission exhibiting rapid rotation behavior. For the first time, the abnormal rotational behavior for this puzzling family of stars is revealed by direct measurements of rotation, namely from photometric rotation period, exhibiting a very short rotation period with values ranging from 13 to 55 days. This finding points to remarkable surface rotation rates, up to 18 times the rotation of the Sun. These giants are combined with six others recently listed in the literature for mid-infrared (IR) diagnostics based on Wide-field Infrared Survey Explorer information, from which a trend for an IR excess is revealed for at least one-half of the stars, but at a level far lower than the dust excess emission shown by planet-bearing main-sequence stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ON THE STAR FORMATION LAW FOR SPIRAL AND IRREGULAR GALAXIES

A dynamical model for star formation on a galactic scale is proposed in which the interstellar medium is constantly condensing to star-forming clouds on the dynamical time of the average midplane density, and the clouds are constantly being disrupted on the dynamical timescale appropriate for their higher density. In this model, the areal star formation rate scales with the 1.5 power of the total gas column density throughout the main regions of spiral galaxies, and with a steeper power, 2, in the far outer regions and in dwarf irregular galaxies because of the flaring disks. At the same time, there is a molecular star formation law that is linear in the main and outer parts of disks and in dIrrs because the duration of individual structures in the molecular phase is also the dynamical timescale, canceling the additional 0.5 power of surface density. The total gas consumption time scales directly with the midplane dynamical time, quenching star formation in the inner regions if there is no accretion, and sustaining star formation for ∼100 Gyr or more in the outer regions with no qualitative change in gas stability or molecular cloud properties. The ULIRG track follows from high densities in galaxy collisions.

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