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trinitree88
2009-Sep-01, 03:54 PM
The authors, Neilson et al find that a survey of Cepheids in the Large Magellanic Cloud, finds most of them with circumstellar envelopes..(CSE).
This additional infrared signature causes their brightness to be overestimated, affecting distance inferences. As Cepheids are a major step on estimating the distance scale of the Universe, understanding this accurately helps. It seems the rates of mass loss are larger than previously estimated because of the newly reduced masses of the Cepheids in question, based on reductions of their luminosities. This is not related straightforwardly to their periods, as other Cepheid properties often are.
Further studies should clarify this, as usual.

SEE:http://arxiv.org/PS_cache/arxiv/pdf/0908/0908.4591v1.pdf

Glom
2009-Sep-01, 09:54 PM
So Cepheids are dimmer than first thought? So that means the universe is smaller than first thought?

Well, well. It's a small universe after all.

Messier Tidy Upper
2009-Sep-02, 02:48 AM
So the Cepehids are surrounded by dust but that makes them actually *less* luminous?

I would have thought the reverse would be true - that because their light was being obscured by dust the Cepehids would actually more luminous than we thought instead?

Unless the dusty circumstellar disks are somehow contributing extra light (& I'm not sure how they'd do that) then I don't see how they get the "less luminous" conclusion.

ngc3314
2009-Sep-02, 12:36 PM
Keep in mind that the paper is solely concerned with the infrared properties of the Cepheids (I don't think these envelopes contribute significant optical reddening, as extended as they are). At long IR wavelengths, the emission from the grains in the envelopes contributes additional emission (originally absorbed at shorter wavelengths and reradiated) beyond what would attribute to the stelar photosphere as extrapolated form shorter wavelengths. This may affect a technique which is promising for improving Cepheid distances - going into the IR, where both dust reddening and the amplitude of variation are reduced. There was a claim early on that the amplitude was so small that mean luminosities based on single random-phase measurements in the IR would be as good as a proper cycle-averaged mean in the optical.

If all Cepheids are affected on average to the same extent, existing distance estimates aren't affected (since calibration is against stars at otherwise estimated distances, not directly against calculated properties). On the other hand, if these dusty outflows are functions of metal abundance or are nonspherical and have preferential orientations, then there is an issue to be assessed.

mantiss
2009-Sep-02, 04:32 PM
The authors, Neilson et al find that a survey of Cepheids in the Large Magellanic Cloud, finds most of them with circumstellar envelopes..(CSE).

I wonder about those in our own galaxy. I'm sure that such envelopes would have been studied by now, so is the discrepancy only for the LMC?

trinitree88
2009-Sep-02, 04:49 PM
I wonder about those in our own galaxy. I'm sure that such envelopes would have been studied by now, so is the discrepancy only for the LMC?

The paper was only the LMC. I'm sure there'll be more. This is similar...SEE:http://www3.interscience.wiley.com/journal/118762044/abstract

Messier Tidy Upper
2009-Sep-05, 01:58 PM
Keep in mind that the paper is solely concerned with the infrared properties of the Cepheids (I don't think these envelopes contribute significant optical reddening, as extended as they are). At long IR wavelengths, the emission from the grains in the envelopes contributes additional emission (originally absorbed at shorter wavelengths and reradiated) beyond what would attribute to the stellar photosphere as extrapolated form shorter wavelengths. ....

Aha. Now it starts to make sense. Thanks.

So to be clear on this they're /your'e saying the size / magnitude of a Cepheid is confused in Infra-red because the circumstellar dust makes them look larger / brighter than they are? Is that right?

trinitree88
2009-Sep-09, 04:02 PM
Aha. Now it starts to make sense. Thanks.

So to be clear on this they're /your'e saying the size / magnitude of a Cepheid is confused in Infra-red because the circumstellar dust makes them look larger / brighter than they are? Is that right?

Steve. Yes. pete