Yolanda Gomez

Compact protoplanetary disks around the stars of a young binary system

Planet formation is believed to occur in the disks of gas and dust that surround young solar-type stars. Most stars, however, form in multiple systems, where the presence of a close companion could affect the structure of the disk and perhaps interfere with planet formation. It has been difficult to investigate this because of the resolution needed. Here we report interferometric observations (at a wavelength of 7 mm) of the core of the star-forming region L1551. We have achieved a linear resolution of seven astronomical units (less than the diameter of Jupiters orbit). The core of L1551 contains two distinct disks, with a separation of 45 AU; these appear to be associated with a binary system. Both disks are spatially resolved, with semi-major axes of about 10 AU, which is about a factor of ten smaller than disks around isolated stars. The disk masses are of order 0.05 solar masses, which could be enough to form planetary systems like our own.

Water-maser emission from a planetary nebula with a magnetized torus

A star like the Sun becomes a planetary nebula towards the end of its life, when the envelope ejected during the earlier giant phase becomes photoionized as the surface of the remnant star reaches a temperature of ∼30,000 K. The spherical symmetry of the giant phase is lost in the transition to a planetary nebula, when non-spherical shells and powerful jets develop. Molecules that were present in the giant envelope are progressively destroyed by the radiation. The water-vapour masers that are typical of the giant envelopes therefore are not expected to persist in planetary nebulae. Here we report the detection of water-maser emission from the planetary nebula K3-35. The masers are in a magnetized torus with a radius of about 85 astronomical units and are also found at the surprisingly large distance of about 5,000 astronomical units from the star, in the tips of bipolar lobes of gas. The precessing jets from K3-35 are probably involved in the excitation of the distant masers, although their existence is nevertheless puzzling. We infer that K3-35 is being observed at the very moment of its transformation from a giant star to a planetary nebula.

A Survey for Water Maser Emission toward Planetary Nebulae: New Detection in IRAS 17347–3139

We report on a water maser survey toward a sample of 27 planetary nebulae (PNe) using the Robledo de Chavela and Medicina single-dish antennas, as well as the Very Large Array (VLA). Two detections have been obtained: the already known water maser emission in K3-35, and a new cluster of masers in IRAS 17347-3139. This low rate of detections is compatible with the short lifetime of water molecules in PNe (~100 yr). The water maser cluster at IRAS 17347-3139 are distributed on a ellipse of size 02 × 01, spatially associated with compact 1.3 cm continuum emission (simultaneously observed with the VLA). From archive VLA continuum data at 4.9, 8.4, and 14.9 GHz, a spectral index α = 0.76 ± 0.03 (Sν ∝ να) is derived for this radio source, which is consistent with either a partially optically thick ionized region or an ionized wind. However, the latter scenario can be ruled out by mass-loss considerations, thus indicating that this source is probably a young PN. The spatial distribution and the radial velocities of the water masers are suggestive of a rotating and expanding maser ring, tracing the innermost regions of a torus formed at the end of the asymptotic giant branch phase. Given that the 1.3 cm continuum emission peak is located near one of the tips of the major axis of the ellipse of masers, we speculate on a possible binary nature of IRAS 17347-3139, where the radio continuum emission could belong to one of the components and the water masers would be associated with a companion.

Imaging MWC 349 from 7 Millimeters to 90 Centimeters

MWC 349A is the brightest radio continuum star in the centimeter domain. The thermal radio continuum emission is believed to originate in an ionized bipolar flow that photoevaporates from the surfaces of a neutral Keplerian disk. In this work we present high angular resolution observations taken with the Very Large Array (VLA) at wavelengths from 7 mm to 90 cm that allow the study of structures over 2 orders of magnitude in size in this object. The 7 mm image shows an intermediate equatorial region ~004 wide, with no free-free emission, that could be the neutral photoevaporating disk around MWC 349A. Combining these data with archival observations at 1.3, 2, 3.6, 6, and 20 cm, we estimate that the flux increases with frequency as ν0.67±0.03 and the angular size decreases with frequency as ν-0.74±0.03, confirming the presence of a biconical thermal wind that expands at constant velocity. We also report the marginal detection of MWC 349A at 90 cm. At the wavelengths of 3.6, 6, and 20 cm, we image and model the interaction zone between the winds of MWC 349A and MWC 349B, supporting the physical association of these components. Finally, by comparing 6 cm data taken in 1982 and 1996 we find evidence of variability in MWC 349A that is interpreted as a decrease of ~2% in the mass-loss rate over the time interval of the observations.

Steroid metabolism in granulosa and theca interna cells from preovulatory follicles of domestic hen (Gallus domesticus)

The capability of granulosa and theca interna cells, from preovulatory follicles of the domestic hen, to metabolize steroid precursors was evaluated. Granulosa and theca interna cells were isolated from ovarian preovulatory follicles at three different developmental stages: F1, F3 and F5. Tritiated pregnenolone (P5), progesterone (P4), dehydroepiandrosterone (DHEA), androstenedione (A4) and testosterone (T) were employed as precursors and their metabolic products were evaluated. The major metabolite of P5 by granulosa cells was P4, but we also observed low amounts of 5beta-pregnandione. DHEA metabolism by granulosa cells yielded mainly A4, and minute quantities of 5beta-androstan-3,17-dione (5beta-dione) were detected. The only significant metabolite obtained in granulosa cells from A4 was 5beta-dione, whereas T was only transformed into A4. On the other hand, P5 metabolism by theca interna cells yielded A4 as the main product, also P4, 17alpha-OHP4, 17alpha-OHP5, 5beta-pregnandione, and DHEA, were found. When DHEA was the precursor A4 was produced in higher amounts than 5beta-dione. A4 was mainly transformed into 5beta-dione. In similar conditions, T was transformed into A4. These results show that granulosa cells have enzymatic activities of 3beta-hydroxysteroid dehydrogenase/5-4 isomerase (3beta-HSD from P5 and DHEA), 17beta-hydroxysteroid dehydrogenase (17beta-HSD from T) and 5beta-reductase (from P5, DHEA and A4). Whereas theca interna cells have enzymatic activities of cytochrome P450c17 (from P5 and P4), 3beta-HSD (from P5 and DHEA), 17beta-HSD (from T) and 5beta-reductase (from P4, DHEA and A4). These data support the concept that theca interna cells have the ability to synthesize androgens from progestins produced in granulosa cells. In addition, since theca interna cells did not show the capacity to aromatize androgens suggests that interaction between theca interna and theca externa cells occurs in vivo, thus confirming the three cell model for estrogen production. Furthermore, the fact that other metabolites were produced both in granulosa and theca interna cells, but in a different extent, suggests that complex mechanisms are participating in the regulation of steroid synthesis in avian ovary follicles.

Water maser detections in southern candidate post-AGB stars and planetary nebulae

Aims. We study the incidence and characteristics of water masers in the envelopes of stars in the post-AGB and PN evolutionary stages. Methods. We used the 64-m antenna in Parkes (Australia) to search for water maser emission at 22 GHz, towards a sample of 74 sources with IRAS colours characteristic of post-AGB stars and PNe, at declination < −32 o . In our sample, 39% of the sources are PNe or PNe candidates, and 50% are post-AGB stars or post-AGB candidates. Results. We detected four new water masers, all of them in optically obscured sources: three are in PNe candidates (IRAS 12405−6219, IRAS 15103−5754, and IRAS 16333−4807) and one is in a post-AGB candidate (IRAS 13500−6106). The PN candidate IRAS 15103−5754 has water fountain characteristics and could be the first PN of this class found. Conclusions. We confirm the trend suggested in Paper I that water maser emission during the post-AGB phase is more likely to be present in obscured sources with massive envelopes than in objects with optical counterpart. We propose an evolutionary scenario for water masers in the post-AGB and PNe stages, in which “water fountain” masers could develop during the post-AGB and early PN stages. Later PNe would exhibit lower velocity maser emission, both along jets and close to the central objects, with only the central masers remaining in more evolved PNe.

Kinematics of the H2O masers at the centre of the planetary nebula K3–35

We have studied the kinematics traced by the water masers located at the centre of the planetary nebula (PN) K3–35, using data from previous Very Large Array (VLA) observations. An analysis of the spatial distribution and line-of-sight velocities of the maser spots allows us to identify typical patterns of a rotating and expanding ring in the position–velocity diagrams, according to our kinematical model. We find that the distribution of the masers is compatible with tracing a circular ring with a � 0.021-arcsec (� 100-au) radius, observed with an inclination angle of 55 ◦ with respect to the line of sight. We derive expansion and rotation velocities of 1.4 and 3.1 km s −1 , respectively. The orientation of the ring, projected on the plane of the sky, at position angle (PA) � 158 ◦ , is almost orthogonal to the direction of the innermost region of the jet observed in K3–35, suggesting the presence of a disc or torus that may be related to the collimation of the outflow.

Detection of the Angular Expansion Rate and Determination of the Distance of the Planetary Nebula NGC 6302

Using VLA data from epochs separated by 2.75 yr, we measured the angular expansion of the planetary nebula NGC 6302 to be 1.75±0.3 milliarcseconds yr −1 . Adopting an expansion velocity of 13 km s −1 for the ionized gas and a simple model for the structure and dynamics of the nebula, we estimate a distance of 1.6±0.6 kpc for NGC 6302

The distance to NGC 6302

New 6 cm continuum and radio recombination line VLA observations as well as CO observations are presented of the bright PN NGC 6302. The H110-alpha line is significantly broader than the H76-alpha line, an effect which is attributed to electron pressure broadening. An electron density of 25,000/cu cm is derived, a value in good agreement with previous optical estimates. Adopting this value and a geometric model for the nebula, a distance of 2.2 + or - 1.1 kpc is derived. An attempt is made to measure the angular expansion in the nebula using 6 cm continuum observations and the technique of Masson (1986). The failure to measure any expansion implies a lower limit of 0.7 + or - 0.3 kpc for the distance. An IRAS luminosity of 13,000 solar, an ionized nebular mass of about 0.2 solar, a neutral hydrogen mass of about 0.05 solar, and a molecular hydrogen mass of about 0.5 solar are derived. The luminosity and total mass are among the largest known for PNs. 49 refs.

A precessing jet model for the PN K 3 − 35: simulated radio-continuum emission

The bipolar morphology of the planetary nebula (PN) K 3 - 35 observed in radio-continuum images was modelled with 3D hydrodynamic simulations with the adaptive grid code YGUAZU-A. We find that the observed morphology of this PN can be reproduced considering a precessing jet evolving in a dense AGB circumstellar medium, given by a mass-loss rate M csm = 5 x 10 -5 M ⊙ yr -1 and a terminal velocity v w = 10 km s -1 . Synthetic thermal radio-continuum maps were generated from numerical results for several frequencies. Comparing the maps and the total fluxes obtained from the simulations with the observational results, we find that a model of precessing dense jets, where each jet injects material into the surrounding CSM at a rate M j = 2.8 x 10 -4 M ⊙ yr -1 (equivalent to a density of 8 x 10 4 cm -3 ), a velocity of 1500 km s -1 , a precession period of 100 yr and a semi-aperture precession angle of 20° agrees well with the observations.