Abstract by W. Marcolino
The radiatively driven wind theory provides a good agreement to measured
mass-loss rates for several O stars. However, low luminosity stars - of late spectral types - present rates much lower than expected, up to 2 orders of magnitude. Such huge discrepancy have serious consequences to stellar evolution theory and hydrodynamical models. In this work we present and discuss this so-called weak wind problem. We present results regarding five late-type O stars (O8-9V). The expanding atmosphere code CMFGEN was used to determine their stellar and wind properties. We discuss other mass-loss rate diagnostics besides the CIV 1549 line in the UV. We also show that although X-rays seem a nice way to recover the expected mass-loss rates and fit the observed spectra, the X-ray luminosity needed is in contrast with observations. Some perspectives and work in progress on the weak wind problem are discussed.
Background papers
Vink et al. 2000
Martins et al. 2005
Marcolino et al. 2009
Talk Slides
pdf
I wonder whether there would be any possibility to shed light on this issue using stellar evolutionary models. Adopting mass-loss rates that differ by two orders of magnitude should have some observable consequences. Has this been explored by anyone?
ReplyDeleteFor example, I could think of the observable consequences for the surface abundances. Higher mass loss rates will strip the envelope of the star faster, revealing the deeper layers of the star. These layers may, for example, be enhanced in nitrogen.
I would also expect some consequences for the evolutionary track (L, T, log g, ...) and the rotation rate (as the stellar wind also leads to angular momentum loss), probably to small to measure, but who knows.
Being optimistic: this could in principle provide a way to test the reality of weak winds independently of the diagnostics discussed here.
Being more realistic: stellar evolutionary models in these stars are plagued by other uncertainties, that may severely limit how conclusive such a test would be.
Hi all,
ReplyDeleteAida asked me to comment about the distances (specially for HD66788) and masses. Indeed, I was not so clear yesterday (sorry!).
- Distances: The methodology is the following: I actually adopted a luminosity typical for an O8-9V star (table 4 of Martins et al. 2005 - paper on O star parameters not weak winds). As I did not normalize the UV data, I used E(B-V) and the distance as free parameters to match the UV continuum. The values obtained for the distance and E(B-V) were compared to different works in the literature (with good agreement).
HD66788: The distance of 4.8kpc generated a bit of controversy during the talk. However, I do think this star is far away. The B-V in the GOS catalog is -0.07. The (Bo - Vo) of a typical O9V star is -0.27 (Martins et al. 2006). So, the E(B-V) is around 0.20. Remember that E(B-V) = (B-V)-(Bo-Vo), by definition. I found 0.22 from the UV fit. Furthermore, different references in the literature also quote large distances:
Katcheva et al. (2000): d=5.7kpc
Savage et al. (2001): d=4.3kpc and E(B-V)=0.22 (work on the ISM lines)
Reed (1993): d=5.1kpc (detail: title of the paper is "Distant OB stars in the Galaxy")
For the mass:
I first used TLUSTY to estimate Teff. Thus, with Teff and L (adopted), I computed the Radius and from Log g I computed the mass.
HD216532: The mass of this star was said to be too low (M=12Msun!). However, the log(g) is low in this object (=3.7). Also, its a relatively high rotator in the sample (vsini about 200km/s). Thus, we should interpret the mass of this object as an effective mass, i.e., somewhat attenuated by rotation (this is also valid for Zeta Oph in the paper). Also, the errors in logg are somewhat high (from the optical fit), so the mass also has a significant error. In any case, this parameter does not affect the mass-loss rate determinations.
cheers.
p.s. thanks for the great afternoon yesterday!
Hi Selma,
ReplyDeleteI agree 100%. I think such calculations could give some clues and/or provide a test to the existence of weak winds (meaning Mdot less than 1e-8Msun/yr). As far as I am concerned, nobody tested such models yet. A very nice suggestion made by someone yesterday is to look at old evolutionary tracks (with Mdot = 0) for stellar masses between 15-30Msun. This could give some hint, but recent calculations would be preferred.
Danny's suggestion to check the C/N abundances is also very good. I estimated values for C but did not look to N in detail in the paper. Unfortunately the uncertainty is usually big.
Regarding Walborn statements about weak winds. Indeed different types of weak winds exist (morphology). For example, SMC stars have weaker winds than Milky Way stars.
ReplyDeleteWhat I would like to emphasize is the weak wind "problem". When you see the word "problem", it means a discrepancy with the predictions made by the radiatively driven wind theory.
Aida, Myron,
ReplyDeleteFrom the GOS Catalog, HD66788 is a multiple system. Although I could constraint Teff and logg reasonably well from the optical spectrum, I saw that Halpha is not perfectly symmetric. This might be a signature of one of the companions, for example.
If logg (from wings of Balmer lines) and Teff (HeI/II lines) are affected by the companions, the mass could be also affected. However, this is difficult to be confirmed/quantified and actually I did not see any evidence of odd profiles for these specific diagnostics. In any case, in my opinion, the mass we have is not exceptionally large.
The 4 distinct categories of "weak winds":
ReplyDelete1. Late O dwarfs with NORMAL wind profiles for their spectral types (the subject of this talk).
The wind profiles are strongly correlated with the optical spectral types (and hence, basic stellar parameters) of normal OB stars. In particular, all normal O8-O9 V stars have similar, weak C IV wind profiles, which interpolate smoothly in the declining profile strength sequence from O6 (the latest type for which the wind absorption is black) through B1, in which the C IV lines are purely photospheric. (See the IUE O Atlas.) It is misleading to compare O8-O9 V UV spectra to O5 V with its much stronger normal wind profiles, giving the impression that the former are anomalous. They are not. This should really be called the Strong Wind Model Problem (SWMP).
2. Relatively rare early O dwarfs with ABNORMALLY weak wind profiles for their spectral types, such as Theta1 Ori C and others in the IUE Atlas and my 2006 May STScI Symposium paper. These are invariably located in very young regions and may have "underdeveloped" winds because they are still on or near the ZAMS, although that remains to be investigated quantitatively, and it is high time that it were...
3. Weak wind profiles and especially low terminal velocities due to low metallicity, as in the SMC. Of course this and one or both of the preceding categories may occur simultaneously, but it is important to recognize the distinction, which sometimes appears to be lost in the literature.
4. I recently learned that specialists in X-rays from late-B stars call them "weak-wind stars". Well, indeed they are, but who needs more distinct meanings of the same term?!
The derived masses (except for one case that also has a much larger distance than the others) are too low for the spectral types by about a factor of two. Could this be related to the mass-loss-rate problem?
ReplyDeleteThe most interesting idea I've heard on this subject came up near the end of the discussion of this stimulating talk and the interactions from different perspectives. I noted that the "weak-wind" problem begins after spectral type O6, precisely where the C IV wind profiles begin to weaken with advancing spectral type in the main-sequence spectra. Thereafter the divergence from the predictions increases along a smooth curve toward the latest O types. I asked Wagner what model deficiency might produce such an effect. He (and/or Danny?) mentioned an idea (from Michel Cure?) that the model analyses for the later types may be progressively missing more and more of the mass in the winds with decreasing Teff. Nino noted that a problem with the radiative-transfer analysis might produce such an effect. This sounds very promising to me, and entirely consistent with the empirical phenomenology.
Of course, my last remark applies to the case that the problem is with the analysis of the observations, and that the purely theoretical predictions are correct. So far as I can see, the opposite is equally possible at the present time. I suggest that the way forward is not to analyze a larger O8-9 V sample; I guarantee you that the results will be the same, because the spectra are. Rather, it will be more useful to analyze representatives of the full HRD, including late-O giants and supergiants. They certainly have stronger winds than the dwarfs, at comparable Teff. Then we may have a better idea whether the current observational results for the late-O dwarfs are likely to be correct, or not.
ReplyDelete