Women labour supply and country specific institutions

In this thesis, I study the influence of country-specific institutions on the labour supply decision of prime-aged women. In chapter 1, I use reduced-form methods, to evaluate the impact of wide changes to the benefit system and childcare subsidies targeted at households with pre-school age children in France. I estimate strong responses to maternity leave type benefits, and a positive impact of childcare subsidies on mothers employment rates in the long-run. In chapter 2, I develop and estimate a static labour supply model with part-time wage equations as well as demand-side constraints. I can compare the elasticities estimates and predictions from tax reform simulations to those of models assuming a unique hourly productivity and the absence of any employment constraint. The structural model also enables me to clearly simulate the impact of each component of the wide policy reform studied in the first chapter. In chapter 3, I try to understand why the number of hours worked by British married women is lower than that of French married women. I find that in the presence of children, British mothers are far more responsive to financial incentives. Husbands’ earnings and their interaction with childcare prices seem to play an important role. Nevertheless, the fall in hours worked in the British households with children, despite facing lower taxes than in France, remains somewhat puzzling. It could be mainly attributed (in the framework used) to different preferences. In the final chapter, I study the impact of joint-taxation on the labour supply choices of married women with working husbands in France. I simulate a revenue-neutral reform that would cancel the tax penalty or gain associated with being married. I find that the overall labour supply of these women would increase by 1.2%

Age dependence of the Vega Phenomenon: Theory

In a separate paper (Decin et al 2003), we have re-examined the observations of IR excess obtained with the ISO satellite and discussed the ages of stars with excess. The amount of dust (measured by the luminosity fraction \fdust=L_\mathrm{IR}/L_{\star}) seen around main-sequence stars of different ages shows several interesting trends. To discuss these results in the context of a physical model, we develop in this paper an analytical model for the dust production in Vega-type systems. Previously it has been claimed that a powerlaw slope of about -2 in the diagram plotting amount of dust versus time could be explained by a simple collisional cascade. We show that such a cascade in fact results in a powerlaw \fdust\propto t^{-1} if the dust removal processes are dominated by collisions. A powerlaw \fdust\propto t^{-2} only results when the dust removal processes become dominated by Pointing-Robertson drag. This may be the case in the Kuiper Belt of our own solar system, but it is certainly not the case in any of the observed disks. A steeper slope can, however, be created by including continuous stirring into the models. We show that the existence of both young and old Vega-like systems with large amounts of dust (\fdust\simeq 10^{-3}) can be explained qualitatively by Kuiper-Belt-like structures with \emph{delayed stirring}. Finally, the absence of young stars with intermediate amounts of dust may be due to the fact that stirring due to planet formation may not be active in young low-mass disks. The considerations in this paper support the picture of simultaneous stirring and dust production proposed by Kenyon and Bromley (2002).Comment: 26 pages, 3 figures, accepted for Publication in Ap

A Vega--like disk associated with the planetary system of rho (1) CNC

Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Identification of a nearby stellar association in the Hipparcos catalog: implications for recent, local star formation

The TW Hydrae Association (~55 pc from Earth) is the nearest known region of recent star formation. Based primarily on the Hipparcos catalog, we have now identified a group of 9 or 10 co-moving star systems at a common distance (~45 pc) from Earth that appear to comprise another, somewhat older, association (``the Tucanae Association''). Together with ages and motions recently determined for some nearby field stars, the existence of the Tucanae and TW Hydrae Associations suggests that the Sun is now close to a region that was the site of substantial star formation only 10-40 million years ago. The TW Hydrae Association represents a final chapter in the local star formation history.Comment: 5 pages incl figs and table

The properties of the high-mass star formation region IRAS22475+5939

IRAS22475+5939 has been well researched by previous astronomers. But we still get some new characteristics about it, using the first observations in lines of CO J=2-1,13CO J=2-1,13CO J=3-2 by the KOSMA 3 m telescope. The mapping of the intensity ratio of 13CO J=3-2 and 13CO J=2-1 shows the distribution of the temperature with two peaks, which don't coincide with IRAS22475+5939 source and the center of the HII region, but at the edge of the HII region. The overlays of the Spitzer IRAC 8um and CO contours indicate that they are associated with each other and the strongest polycyclic aromatic hydrocarbons (PAHs) emission is at the position of IRAS22475+5939 source. While the IRAS LRS spectrum at 7-23 um and the PHT-s spectrum at 2-12 um of IRAS22475+5939 source also exhibit strong PAHs emission characters at the main PAH bands. The diversity of PAH family should be responsible for the plateaus of PAHs emission in the PHT-s spectrum and the IRAS-LRS spectrum. An analysis and modeling in infrared bands suggest that IRAS22475+5939 is more likely to be a Class I YSO. Where this is the case, the star is likely to have a temperature T_{EFF} \sim 9995.8 K, mass \sim15.34 M_(sun), luminosity \sim 1.54*10^4 L_(sun) and age \sim 1.54*10^4 yr. The model shows that the circumstellar disc emission is important for the wavelength between 1 and 10 um, otherwise, envelope fluxes for lambda >10 um. The bipolar outflow is confirmed in the molecular cloud. The excited star of the HII region has the chance to be the driving source of the outflow. The high resolution is required.Comment: 12 pages, 20 figures and 5 tables, Accepted for publication in RA

Incidence and survival of remnant disks around main-sequence stars

We present photometric ISO 60 and 170um measurements, complemented by some IRAS data at 60um, of a sample of 84 nearby main-sequence stars of spectral class A, F, G and K in order to determine the incidence of dust disks around such main-sequence stars. Of the stars younger than 400 Myr one in two has a disk; for the older stars this is true for only one in ten. We conclude that most stars arrive on the main sequence surrounded by a disk; this disk then decays in about 400 Myr. Because (i) the dust particles disappear and must be replenished on a much shorter time scale and (ii) the collision of planetesimals is a good source of new dust, we suggest that the rapid decay of the disks is caused by the destruction and escape of planetesimals. We suggest that the dissipation of the disk is related to the heavy bombardment phase in our Solar System. Whether all stars arrive on the main sequence surrounded by a disk cannot be established: some very young stars do not have a disk. And not all stars destroy their disk in a similar way: some stars as old as the Sun still have significant disks.Comment: 16 pages, 9 figures, Astron & Astrophys. in pres

Spatial Separation of the 3.29 micron Emission Feature and Associated 2 micron Continuum in NGC 7023

We present a new 0.9" resolution 3.29 micron narrowband image of the reflection nebula NGC 7023. We find that the 3.29 micron IEF in NGC 7023 is brightest in narrow filaments NW of the illuminating star. These filaments have been seen in images of K', molecular hydrogen emission lines, the 6.2 and 11.3 micron IEFs, and HCO+. We also detect 3.29 micron emission faintly but distinctly between the filaments and the star. The 3.29 micron image is in contrast to narrowband images at 2.09, 2.14, and 2.18 micron, which show an extended emission peak midway between the filaments and the star, and much fainter emission near the filaments. The [2.18]-[3.29] color shows a wide variation, ranging from 3.4-3.6 mag at the 2 micron continuum peak to 5.5 mag in the filaments. We observe [2.18]-[3.29] to increase smoothly with increasing distance from the star, up until the filament, suggesting that the main difference between the spatial distributions of the 2 micron continuum and the the 3.29 micron emission is related to the incident stellar flux. Our result suggests that the 3.29 micron IEF carriers are likely to be distinct from, but related to, the 2 micron continuum emitters. Our finding also imply that, in NGC 7023, the 2 micron continuum emitters are mainly associated with HI, while the 3.29 micron IEF carriers are primarily found in warm molecular hydrogen, but that both can survive in HI or molecular hydrogen. (abridged)Comment: to appear in ApJ, including 1 table and 8 figures, high resolution figures available at http://www.ast.cam.ac.uk/~jin/n7023

The chemical composition of the circumstellar envelopes around yellow hypergiant stars

The yellow hypergiant stars (YHGs) are extremely luminous and massive objects whose general properties are poorly known. Only two of this kind of star show massive circumstellar envelopes, IRC+10420 and AFGL2343. We aim to study the chemistry of the circumstellar envelopes around these two sources, by comparison with well known AGB stars and protoplanetary nebulae. We also estimate the abundances of the observed molecular species. We have performed single-dish observations of different transitions for twelve molecular species. We have compared the ratio of the intensities of the molecular transitions and of the estimated abundances in AFGL2343 and IRC+10420 with those in O-rich and C-rich AGB stars and protoplanetary nebulae. Both YHGs, AFGL2343, and IRC+10420, have been found to have an O-rich chemistry similar to that in O-rich AGB stars, though for AFGL2343 the emission of most molecules compared with 13CO lines is relatively weak. Clear differences with the other evolved sources appear when we compare the line intensity corrected for distance and the profile widths which are, respectively, very intense and very wide in YHGs. The abundances obtained for IRC+10420 agree with those found in AGB stars, but in general those found in AFGL2343, except for 13CO, are too low. This apparently low molecular abundance in AFGL2343 could be due to the fact that these molecules are present only in an inner region of the shell where the mass is relatively low.Comment: 14 pages, 12 figure