Affiliation
American Association of Variable Star Observers (AAVSO)
Wed, 02/24/2021 - 22:19
AAVSO Alert Notice 731 announces a campaign on the T Tau star TW Hya. Please see the notice for details and observing instructions.
There are threads for this campaign under the following forums:
- Campaigns and Observation Reports: https://www.aavso.org/hsttess-observations-tw-hya-37-42-2021-01
- Young Stellar Objects (YSO): https://www.aavso.org/hsttess-observations-tw-hya-37-42-2021
Please subscribe to these threads if you are participating in the campaign so you can be updated by the astronomer and by HQ. Join in the discussion or ask questions there!
Many thanks, and Good observing,
Elizabeth O. Waagen, AAVSO HQ
Causes of this star's variability are grouped in three bullets in Alert Notice 731 as follows:
- Are stellar variations due to enhanced accretion events or long-lived flares, which case the star to become bluer, or
- are they due to variable circumstellar extinction, in which case the star will become redder, or
- are they due to obscuration of the photosphere by optically-thick irregularities in the circumstellar disk, in which case the color should not change?
Can someone explain why the second bullet causes the star to become redder but the third bullet does not? Typically blue light is attenuated and scattered more than red by optically thick material. Therefore, I don't understand why the third bullet doesn't also cause reddening.
Also the first bullet talks about the star becoming bluer due to enhanced accretion events. Doesn't this imply that the star get bluer when it brightens. In the 2nd bullet doesn't this imply that the star get redder when it dims or stating the obverse it is also bluer when brighter. In other words for both causes the sign of the correlation of color vs brightness to be the same for both causes, specifically, bluer when brighter for both the first and second bullets?.
Brad Walter.
Hi Brad,
I wasn't particularly clear there. I'm thinking of the situation where part of the star is blocked by something opaque. We see large dips in some T Tauri stars that have very small color changes, suggesting total absorption in all bands covering some fraction of the photosphere. If the optical depth is low at I and high at B, as in normal extinction, you get a color change. Think of an eclipse, but by, say, an opaque protuberance sticking out above the disk, or by the edge of a warped disk.
Fred Walter
Got it re opaque blockage. However looking at the first two bullets,
- Are stellar variations due to enhanced accretion events or long-lived flares, which case the star to become bluer, or
- are they due to variable circumstellar extinction, in which case the star will become redder
These both should be bluer when brighter. If you are doing two color photometry, say B & V what Characteristics do you use to distinguish between them?
Brad Walter, WBY
Sorry about the…
Hi Brad,
Sorry about the long delay in responding - I don't frequent this forum.
Accretion has the colors of a 10,000K black body; extinction results in a different color change, with 1 mag of visual absorption giving a color change E(B-V) about 0.3 mag. We also see a difference in the spectrum: accretion will enhance the blue continuum (absorption lines appear to weaken - an effect called veiling), and emission lines will strengthen. Changes in extinction alone will not affect the equivalent widths of the emission or absorption lines.
Fred
AAVSO Alert Notice 733 gives the schedule of HST observing times for the T Tau star TW Hya. The first observations are scheduled for 2021 March 09 UT. Please see the notice for details and observing instructions. This campaign was announced in Alert Notice 731.
There are threads for this campaign under the following forums:
- Campaigns and Observation Reports: https://www.aavso.org/hsttess-observations-tw-hya-37-42-2021-01
- Young Stellar Objects (YSO): https://www.aavso.org/hsttess-observations-tw-hya-37-42-2021
Please subscribe to these threads if you are participating in the campaign so you can be updated by the astronomer and by HQ. Join in the discussion or ask questions there!
Many thanks, and Good observing,
Elizabeth O. Waagen, AAVSO HQ
Hi all,
Thanks again to those of you who have been collecting and reporting data. Over the past month TW Hya has varied by over 1 mag at B and lesser amounts at VRI, on timescales of a few days.
The final HST schedule is now out. There will be 12 observations in all. The remaining observation times (in UT), through April 8, are:
6: Start time: 03 Apr 2021 04:11:15 End time: 03 Apr 2021 05:04:17
7: Start time: 04 Apr 2021 07:11:10 End time: 04 Apr 2021 08:04:12
8: Start time: 05 Apr 2021 03:49:55 End time: 05 Apr 2021 04:42:57
9: Start time: 05 Apr 2021 13:21:38 End time: 05 Apr 2021 14:14:40
10: Start time: 06 Apr 2021 13:10:56 End time: 06 Apr 2021 14:03:58
11: Start time: 07 Apr 2021 13:00:14 End time: 07 Apr 2021 13:53:16
12: Start time: 08 Apr 2021 12:52:27 End time: 08 Apr 2021 13:45:29
Depending on your longitude, if you can get observations during the times of the HST observations, they will be most valuable. But all observations are important, since the TESS observations ended on April 2. BVRI (or ri) filter photometry is most useful.
Thanks, and clear skies,
Fred
AAVSO Alert Notice 737 gives the dates/times of the final set of upcoming HST observations of TW Hya. Please see the Notice for details and observing instructions.
There are threads for this campaign under the following forums:
- Campaigns and Observation Reports: https://www.aavso.org/hsttess-observations-tw-hya-37-42-2021-01
- Young Stellar Objects (YSO): https://www.aavso.org/hsttess-observations-tw-hya-37-42-2021
Please subscribe to these threads if you are participating in the campaign so you can be updated by the astronomer and by HQ. Join in the discussion or ask questions there!
Many thanks, and Good observing,
Elizabeth O. Waagen, AAVSO HQ