Projected rotational velocities of WD1614+136 and WD1353+409 - implications for the rate of galactic Type Ia supernovae

The white dwarf stars WD1614+136 and WD1353+409 are not sufficiently massive to have formed through single star evolution. However, observations to date have not yet found any evidence for binarity. It has therefore been suggested that these stars are the result of a merger. In this paper we place an upper limit of approximately 50kms on the projected rotational velocities of both stars. This suggests that, if these stars are the results of a merger, efficient angular momentum loss with accompanying mass loss must have occurred. If the same process occurs following the merging of more massive white dwarf stars, the predicted rate of Type Ia supernovae due to merging white dwarfs may have been greatly over-estimated. Further observations to determine binarity in WD1614+136 and WD1353+409 are therefore encouraged.Comment: 3 pages. 1 figur

Farm Size and Land Use Changes in Vietnam Following Land Reforms

Over the last decade the Vietnamese government has instigated land reforms that recognise the household as the basic unit of production and allocate land use rights to households. Under the 1993 Land Law these rights can be transferred, exchanged, leased, inherited, and mortgaged. This Land Law provided the foundation for the development of a market for land use rights. During 2001, 400 farm households were surveyed in four provinces in Vietnam. Along with production and consumption data, evidence was sought of land accumulation and consolidation, land use changes, and attitudes to land reform issues. Analysis of the data shows that there is an active market for land use rights, but the level of activity varies considerably between provinces. Some individual households have acquired a large percentage of their land through buying or renting activities, as distinct from land that has been allocated or inherited. A more active market appears to be associated with opportunities for land use changes which lead to more profitable production activities. Lack of available land and in some cases labour, inadequate credit access, and reluctance to sell land use rights are identified as constraints to the land use right market, rather than transaction costs and the limit on land holdings.agricultural policy, land reform, land market, Vietnam, Agricultural and Food Policy, Land Economics/Use,

KIC 2856960: the impossible triple star

KIC 2856960 is a star in the Kepler field which was observed by Kepler for 4 years. It shows the primary and secondary eclipses of a close binary of 0.258d as well as complex dipping events that last for about 1.5d at a time and recur on a 204d period. The dips are thought to result when the close binary passes across the face of a third star. In this paper we present an attempt to model the dips. Despite the apparent simplicity of the system and strenuous efforts to find a solution, we find that we cannot match the dips with a triple star while satisfying Kepler's laws. The problem is that to match the dips the separation of the close binary has to be larger than possible relative to the outer orbit given the orbital periods. Quadruple star models can get round this problem but require the addition of a so-far undetected intermediate period of order 5 -- 20d that has be a near-perfect integer divisor of the outer 204d period. Although we have no good explanation for KIC 2856960, using the full set of Kepler data we are able to update several of its parameters. We also present a spectrum showing that KIC 2856960 is dominated by light from a K3- or K4-type star.Comment: 11 pages, 13 figures, accepted for publication in MNRAS August 21, 201

WD1953-011 - a magnetic white dwarf with peculiar field structure

We present H-alpha spectra of the magnetic white dwarf star WD1953-011 which confirm the presence of the broad Zeeman components corresponding to a field strength of about 500kG found by Maxted & Marsh (1999). We also find that the line profile is variable over a timescale of a day or less. The core of the H-alpha line also shows a narrow Zeeman triplet corresponding to a field strength of of about 100kG which appears to be almost constant in shape. These observations suggest that the magnetic field on WD1953-011 has a complex structure and that the star has a rotational period of hours or days which causes the observed variability of the spectra. We argue that neither an offset dipole model nor a double-dipole model are sufficient to explain our observations. Instead, we propose a two component model consisting of a high field region of magnetic field strength of about 500kG covering about 10% of the surface area of the star superimposed on an underlying dipolar field of mean field strength of about 70kG. Radial velocity measurements of the narrow Zeeman triplet show that the radial velocity is constant to within a few km/s so this star is unlikely to be a close binary.Comment: Accpeted for publication in MNRAS. 4 pages, 2 figure

KPD1930+2752 - a candidate Type Ia supernova progenitor

We present spectra of the pulsating sdB star KPD1930+2752 which confirm that this star is a binary. The radial velocities measured from the H-alpha and HeI6678 spectral lines vary sinusoidally with the same period (2h 17m) as the ellipsoidal variability seen by Billeres et al. (2000). The amplitude of the orbital motion (349.3+-2.7 km/s) combined with the canonical mass for sdB stars (0.5 solar masses) implies a total mass for the binary of 1.47+-0.01 solar masses The unseen companion star is almost certainly a white dwarf star. The binary will merge within about 200 million years due to gravitational wave radiation. The accretion of helium and other elements heavier than hydrogen onto the white dwarf which then exceeds the Chandrasekhar mass (1.4 solar masses) is a viable model for the cause of Type Ia supernovae. KPD1930+2752 is the first star to be discovered which is a good candidate for the progenitor of a Type Ia supernova of this type which will merge on an astrophysically interesting timescale.Comment: Accepted for publication in MNRAS. 4 pages, 2 figures. Added Institutio

The economics of land fragmentation in the north of Vietnam

Land fragmentation, where a single farm has a number of parcels of land, is a common feature of agriculture in many countries, especially in developing countries. In Vietnam, land fragmentation is common, especially in the north. For the whole country, there are about 75 million parcels of land, an average of seven to eight plots per farm household. Such fragmentation can be seen to have negative and positive benefits for farm households and the community generally. Comparative statics analysis and analysis of survey data have led to the conclusion that small-sized farms are likely to be more fragmented, and that fragmentation had a negative impact on crop productivity and increased family labour use and other money expenses. Policies which allow the appropriate opportunity cost of labour to be reflected at the farm level may provide appropriate incentives to trigger farm size change and land consolidation. Policies which tip the benefits in favour of fewer and larger plots, such as strong and effective research and development, an active extension system and strong administrative management, may also lead to land consolidation.development economics, land fragmentation, land use and tenure, production economics, Resource /Energy Economics and Policy,

The economics of land fragmentation in the north of Vietnam

Land fragmentation, in which a single farm household operates more than one separate piece of land, is significant issue in Vietnamese agriculture, especially in the North. For the whole country, there are about 75 million plots of land, an average of 7-8 plots per farm household. Such fragmentation can be seen to have negative and positive benefits for farm households and the community generally. The negative impacts can be reduced mechanisation, higher cost, loss of land due to boundaries, increased negative externalities, and more limited application of new technologies. On the other hand, land fragmentation may have some benefits to farmers such as spreading output risk, seasonal labour use, and crop diversification. Comparative static analysis and analysis of survey data have led to the conclusion that small sized farms are likely to be more fragmented and the number of plots held by a household is not a significant determinant of yield and output risk spreading but is a significant factor in crop diversification. Policies which allow the appropriate opportunity cost of labour to be reflected at the farm level may provide appropriate incentives to trigger farm size change and land consolidation. Policies which tip the benefits in favour of fewer and larger plots such as strong and effective research and development, an active extension system and strong administrative management may also lead to land consolidation and thus allow some of the benefits which will accrue to the economy more generally to be obtained by farmers.Land Economics/Use,

The population of hot subdwarf stars studied with Gaia II. The Gaia DR2 catalogue of hot subluminous stars

Based on data from the ESA Gaia Data Release 2 (DR2) and several ground-based, multi-band photometry surveys we compiled an all-sky catalogue of $39\,800$ hot subluminous star candidates selected in Gaia DR2 by means of colour, absolute magnitude and reduced proper motion cuts. We expect the majority of the candidates to be hot subdwarf stars of spectral type B and O, followed by blue horizontal branch stars of late B-type (HBB), hot post-AGB stars, and central stars of planetary nebulae. The contamination by cooler stars should be about $10\%$. The catalogue is magnitude limited to Gaia $G<19\,{\rm mag}$ and covers the whole sky. Except within the Galactic plane and LMC/SMC regions, we expect the catalogue to be almost complete up to about $1.5\,{\rm kpc}$. The main purpose of this catalogue is to serve as input target list for the large-scale photometric and spectroscopic surveys which are ongoing or scheduled to start in the coming years. In the long run, securing a statistically significant sample of spectroscopically confirmed hot subluminous stars is key to advance towards a more detailed understanding of the latest stages of stellar evolution for single and binary stars.Comment: 13 pages, A&A, accepte

Evidence for bimodal orbital separations of white dwarf-red dwarf binary stars

We present the results of a radial velocity survey of 20 white dwarf plus M dwarf binaries selected as a follow up to a \textit{Hubble Space Telescope} study that aimed to spatially resolve suspected binaries. Our candidates are taken from the list of targets that were spatially unresolved with \textit{Hubble}. We have determined the orbital periods for 16 of these compact binary candidates. The period distribution ranges from 0.14 to 9.16\,d and peaks near 0.6\,d. The original sample therefore contains two sets of binaries, wide orbits ($\approx100-1000$\,au) and close orbits ($\lesssim1-10$\,au), with no systems found in the $\approx10-100$\,au range. This observational evidence confirms the bimodal distribution predicted by population models and is also similar to results obtained in previous studies. We find no binary periods in the months to years range, supporting the post common envelope evolution scenario. One of our targets, WD\,1504+546, was discovered to be an eclipsing binary with a period of 0.93\,d