We have investigated the stellar and wind properties of a sample of five late-type O dwarfs in order to address the weak wind problem. A grid of TLUSTY models was used to obtain the stellar parameters, and the wind parameters were determined by using
the CMFGEN code. We found that the spectra have mainly a photospheric origin. A weak wind signature is seen in CIV 1549, from where mass-loss rates consistent with previous CMFGEN results regarding O8-9V stars were obtained. A discrepancy of roughly 2 orders of magnitude is found between these mass-loss rates and the values predicted by theory (Mdot(Vink)), confirming a breakdown or a steepening of the modified wind momentum-luminosity relation at log L/Lsun < 5.2. We have estimated the carbon abundance for the stars of our sample and concluded that its uncertainty cannot cause the weak wind problem. Upper limits on Mdot were established for all objects using lines of different ions, namely, PV 1118,28, CIII 1176, NV 1239,43, Si IV 1394,03, and NIV 1718. All the values obtained are also in disagreement with theoretical predictions, bringing support to the reality of weak winds. Together with CIV 1549, the use of NV 1239,43 results in the lowest mass-loss rates: the upper limits indicate that Mdot must be less than about -1.0 dex Mdot(Vink). Regarding the other transitions, the upper limits still point to low rates: Mdot must be less than about $(-0.5 pm 0.2)$ dex Mdot(Vink). We have studied the behavior of the Halpha line with different mass-loss rates. We have also explored ways to fit the observed spectra with Mdot(Vink). By using large amounts of X-rays, we verified that few wind emissions take place, as in weak winds. However, unrealistic X-rays luminosities had to be used (log Lx/Lbol > -3.5) (abridged).
The ultraviolet spectra of all weak emission line central stars of planetary nebulae (WELS) with available IUE data is analyzed. We found that the WELS can be divided in three different groups regarding their UV: (1) Strong P-Cygni profiles (mainly i
n C IV 1549); (2) Weak P-Cygni features and (3) Absence of P-Cygni profiles. We have measured wind terminal velocities for all objects presenting P-Cygni profiles in N V 1238 and/or C IV 1549. The results obtained were compared to the UV data of the two prototype stars of the [WC]-PG 1159 class, namely, A30 and A78. They indicate that WELS are distinct from the [WC]-PG 1159 stars, in contrast to previous claims in the literature. In order to gain a better understanding about the WELS, we clearly need to determine their physical parameters and chemical abundances. First non LTE expanding atmosphere models (using the CMFGEN code) for the UV and optical spectra of the star Hen 2-12 are presented.
The ultraviolet spectra of all ``weak emission line central stars of planetary nebulae (WELS) with available IUE data are presented and discussed. We performed line identifications, equivalent width and flux measurements for several features in their
spectra. We found that the WELS can be divided in three different groups regarding their UV: i) Strong P-Cygni profiles (mainly in CIV 1549); ii) Weak P-Cygni features and iii) Absence of P-Cygni profiles. The last group encompasses stars with a featureless UV spectrum or with intense emission lines and a weak continuum, which are most likely of nebular origin. We have measured wind terminal velocities for all objects presenting P-Cygni profiles in N V 1238 and/or C IV 1549. The results obtained were compared to the UV data of the two prototype stars of the [WC]-PG 1159 class, namely, A30 and A78. For WELS presenting P-Cygnis, most of the terminal velocities fall in the range ~1000-1500 km/s, while [WC]-PG 1159 stars possess much higher values, of about 3000 km/s. The [WC]-PG1159 stars are characterized by intense, simultaneous P-Cygni emissions in the 1150-2000A interval of N V 1238, O V 1371 and C IV 1549. In contrast, we found that O V 1371 is very weak or absent in the WELS spectra. On the basis of the ultraviolet spectra alone, our findings indicate that [WC]-PG 1159 stars are distinct from the WELS, contrary to previous claims in the literature.
Aims: To investigate the spectra and the evolutionary stages of two peculiar emission-line stars: RJHA 49 and SS73 21. Methods: We used low and high resolution optical data. Line identifications and measurements were performed for several features in
their spectra. Results: For each object, we have derived the extinction and the excitation temperature from a set of [Fe II] lines, and the electron density from [N II] lines. For RJHA 49, no detailed spectroscopic study was done so far. Regarding SS73 21, our low resolution spectrum have confirmed the main characteristics found in previous works. On the other side, from our high resolution data, we have found that the Halpha line presents a double-peak, in contrast with the suggestion in the literature that it should reveal a P-Cygni profile. Surprisingly, we found a few He I transitions resembling P-Cygni profiles (e.g. He I 5876), directly suggesting that mass loss is active in SS73 21. We also discussed the nature of these two objects based on the data obtained. Although the evolutionary status of SS73 21 seems well established from previous studies (a proto-planetary nebula), the situation for RJHA 49 is not so clear mainly due to its unknown distance. However, from the strength of [N II] 5754 relative to [O I] 6300, the possibility of RJHA 49 being a LBV object is reduced, and a B[e]-supergiant or a proto-planetary nebula status is more plausible.
