Global modeling of radiatively driven accretion of metals from compact debris disks onto the white dwarfs

Recent infrared observations have revealed presence of compact (radii < R_Sun) debris disks around more than a dozen of metal-rich white dwarfs (WD), likely produced by tidal disruption of asteroids. Accretion of high-Z material from these disks may account for the metal contamination of these WDs. It was previously shown using local calculations that the Poynting-Robertson (PR) drag acting on the dense, optically thick disk naturally drives metal accretion onto the WD at the typical rate \dot M_{PR} \approx 10^8 g/s. Here we extend this local analysis by exploring global evolution of the debris disk under the action of the PR drag for a variety of assumptions about the disk properties. We find that massive disks (mass > 10^{20} g), which are optically thick to incident stellar radiation inevitably give rise to metal accretion at rates \dot M > 0.2\dot M_{PR}. The magnitude of \dot M and its time evolution are determined predominantly by the initial pattern of the radial distribution of the debris (i.e. ring-like vs. disk-like) but not by the total mass of the disk. The latter determines only the disk lifetime, which can be several Myr or longer. Evolution of an optically thick disk generically results in the development of a sharp outer edge of the disk. We also find that the low mass (< 10^{20} g), optically thin disks exhibit \dot M << \dot M_{PR} and evolve on characteristic timescale \sim 10^5-10^6 yr, independent of their total mass.Comment: 9 pages, 8 figures, submitted to Ap

A Tidally-Disrupted Asteroid Around the White Dwarf G29-38

The infrared excess around the white dwarf G29-38 can be explained by emission from an opaque flat ring of dust with an inner radius 0.14 of the radius of the Sun and an outer radius approximately equal to the Sun's. This ring lies within the Roche region of the white dwarf where an asteroid could have been tidally destroyed, producing a system reminiscent of Saturn's rings. Accretion onto the white dwarf from this circumstellar dust can explain the observed calcium abundance in the atmosphere of G29-38. Either as a bombardment by a series of asteroids or because of one large disruption, the total amount of matter accreted onto the white dwarf may have been comparable to the total mass of asteroids in the Solar System, or, equivalently, about 1% of the mass in the asteroid belt around the main sequence star zeta Lep.Comment: ApJ Letters, in pres

Ideals and finiteness conditions for subsemigroups

In this paper we consider a number of finiteness conditions for semigroups related to their ideal structure, and ask whether such conditions are preserved by sub- or supersemigroups with finite Rees or Green index. Specific properties under consideration include stability, D=J and minimal conditions on ideals.Comment: 25 pages, revised according to referee's comments, to appear in Glasgow Mathematical Journa

Global Models of Runaway Accretion in White Dwarf Debris Disks

A growing sample of white dwarfs (WDs) with metal-enriched atmospheres are accompanied by excess infrared emission, indicating that they are encircled by a compact dusty disk of solid debris. Such `WD debris disks' are thought to originate from the tidal disruption of asteroids or other minor bodies, but the precise mechanism(s) responsible for transporting matter to the WD surface remains unclear, especially in those systems with the highest inferred metal accretion rates dM_Z/dt ~ 1e8-1e10 g/s. Here we present global time-dependent calculations of the coupled evolution of the gaseous and solid components of WD debris disks. Solids transported inwards (initially due to PR drag) sublimate at tens of WD radii, producing a source of gas that accretes onto the WD surface and viscously spreads outwards in radius, where it overlaps with the solid disk. If the aerodynamic coupling between the solids and gaseous disks is sufficiently strong (and/or the gas viscosity sufficiently weak), then gas builds up near the sublimation radius faster than it can viscously spread away. Since the rate of drag-induced solid accretion increases with gas density, this results in a runaway accretion process, during which the WD accretion rate reaches values orders of magnitude higher than can be achieved by PR drag alone. We explore the evolution of WD debris disks across a wide range of physical conditions and calculate the predicted distribution of observed accretion rates dM_Z/dt, finding reasonable agreement with the current sample. Although the conditions necessary for runaway accretion are at best marginally satisfied given the minimal level of aerodynamic drag between circular gaseous and solid disks, the presence of other stronger forms of solid-gas coupling---such as would result if the gaseous disk is only mildly eccentric---substantially increase the likelihood of runaway accretion.Comment: 23 pages, 20 figures, submitted to MNRA

A Dusty Disk Around GD 362, a White Dwarf With a Uniquely High Photospheric Metal Abundance

Eighteen years after an infrared excess was discovered associated with the white dwarf G29-38, we report ground-based measurements (JHKL'N') with mJy-level sensitivity of GD 362 that show it to be a second single white dwarf with an infrared excess. As a first approximation, the excess around GD 362, which amounts to about 3% of the total stellar luminosity, can be explained by emission from a passive, flat, opaque dust disk that lies within the Roche radius of the white dwarf. The dust may have been produced by the tidal disruption of a large parent body such as an asteroid. Accretion from this circumstellar disk could account for the remarkably high abundance of metals in the star's photosphere.Comment: 12 pages, 2 figures. ApJ Letters, in pres

The shaping effect of collimated fast outflows in the Egg nebula

We present high angular resolution observations of the HC$_3$N J=5--4 line from the Egg nebula, which is the archetype of protoplanetary nebulae. We find that the HC$_{\rm 3}$N emission in the approaching and receding portion of the envelope traces a clumpy hollow shell, similar to that seen in normal carbon rich envelopes. Near the systemic velocity, the hollow shell is fragmented into several large blobs or arcs with missing portions correspond spatially to locations of previously reported high--velocity outlows in the Egg nebula. This provides direct evidence for the disruption of the slowly--expanding envelope ejected during the AGB phase by the collimated fast outflows initiated during the transition to the protoplanetary nebula phase. We also find that the intersection of fast molecular outflows previously suggested as the location of the central post-AGB star is significantly offset from the center of the hollow shell. From modelling the HC$_3$N distribution we could reproduce qualitatively the spatial kinematics of the HC$_3$N J=5--4 emission using a HC$_3$N shell with two pairs of cavities cleared by the collimated high velocity outflows along the polar direction and in the equatorial plane. We infer a relatively high abundance of HC$_3$N/H$_2$ $\sim$3x10$^{-6}$ for an estimated mass--loss rate of 3x10$^{-5}$ M$_\odot$ yr$^{-1}$ in the HC$_3$N shell. The high abundance of HC$_3$N and the presence of some weaker J=5--4 emission in the vicinity of the central post-AGB star suggest an unusually efficient formation of this molecule in the Egg nebula.Comment: 22 pages, 6 figures, submitted to the Astrophysical Journa

Where Are The M Dwarf Disks Older Than 10 Million Years?

We present 11.7-micron observations of nine late-type dwarfs obtained at the Keck I 10-meter telescope in December 2002 and April 2003. Our targets were selected for their youth or apparent IRAS 12-micron excess. For all nine sources, excess infrared emission is not detected. We find that stellar wind drag can dominate the circumstellar grain removal and plausibly explain the dearth of M Dwarf systems older than 10 Myr with currently detected infrared excesses. We predict M dwarfs possess fractional infrared excess on the order of L_{IR}/L_{*}\sim10^{-6} and this may be detectable with future efforts.Comment: 24 pages, 2 figures, accepted to Ap

The Mid-Infrared Emitting Dust Around AB Aur

Using the Keck I telescope, we have obtained 11.7 micron and 18.7 micron images of the circumstellar dust emission from AB Aur, a Herbig Ae star. We find that AB Aur is probably resolved at 18.7 micron with an angular diameter of 1.2" at a surface brightness of 3.5 Jy/arcsec^2. Most of the dust mass detected at millimeter wavelengths does not contribute to the 18.7 micron emission, which is plausibly explained if the system possesses a relatively cold, massive disk. We find that models with an optically thick, geometrically thin disk, surrounded by an optically thin spherical envelope fit the data somewhat better than flared disk models.Comment: ApJ in press, 4 color figure

Перспективи розвитку аграрного туризму в Карпатському регіоні

To date, the activities of agritourism enterprises is an important element in the study of socio-economic relations at local, regional and national levels. Agritourism has become an alternative form of effective functioning and on-farm diversification. The development of this form of entrepreneurship can play a crucial role in the revival of the economy of rural areas in the Carpathian region, where the prospects, the tourism industry was and still is one of the best in Ukraine. The use of agro-tourist potential in the modern conditions of decentralization of management will lead to socio-economic development of the region by increasing revenues of local budgets, increase of investment attractiveness, increase of employment, especially in rural mostly rural areas, landscape conservation or positive change, rational use of natural and cultural-historical heritage. Agrarian tourism is a real alternative for today's unemployment and mass migration of the rural population. This kind of entrepreneurial activity can create a new labor market, become a source of income for the rural dweller and increase the general standard of living in the village. The purpose of this article is to present the specifics of agrarian tourism as a form of entrepreneurial activity in the village in the Carpathian region. The paper deals with the actual issues of research of the specificity of agro-tourism, the principles of its development, the introduction of tourism services in rural areas with a view to diversifying socio-economic activity, raising the level of well-being and prestige of rural labor. To achieve the goal, many years of research experience in this field were analyzed, abstract-logical and statistical methods of analysis and cognition were used.На сьогоднішній день діяльність агротуристичних підприємств є важливим елементом дослідження соціально-економічних відносин на місцевому, регіональному та національному рівнях. Агротуризм став альтернативною формою ефективного функціонування та диверсифікації фермерських господарств. Розвиток даної форми підприємництва може відіграти вирішальну роль у відродженні економіки сільських територій в Карпатському регіоні, де перспективи агротуристичної індустрії були і залишаються одними з найкращих в Україні. Використання агротуристичного потенціалу в сучасних умовах децентралізації управління призведе до соціально-економічного розвитку регіону за рахунок збільшення дохідної частини місцевих бюджетів, підвищення інвестиційної привабливості, збільшення зайнятості населення, особливо у сільських здебільшого депресивних районах, збереженню ландшафту чи його позитивній зміні, раціональному використанню природної та культурно-історичної спадщини

Other Kuiper Belts

When a main sequence star evolves into a red giant and its Kuiper Belt Object's (KBO's) reach a temperature of about 170 K, the dust released during the rapid ice-sublimation of these cometary bodies may lead to a detectable infrared excess at 25 microns, depending upon the mass of the KBO's. Analysis of IRAS data for 66 first ascent red giants with 200 L(Sun) < L < 300 L(Sun) within 150 pc of the Sun provides an upper limit to the mass in KBO's at 45 AU orbital radius that is usually less than about 0.1 M(Earth). With improved infrared data, we may detect systems of KBO's around first ascent red giants that are analogs to our Solar System's KBO's.Comment: 18 pages, 4 figures, accepted by Ap