It is currently Wed, 17-10-18, 20:54 GMT

All times are UTC




Post new topic Reply to topic  [ 8 posts ] 
Author Message
PostPosted: Tue, 27-09-16, 19:00 GMT 
Offline
Site Admin
User avatar

Joined: Fri, 31-08-07, 7:01 GMT
Posts: 4579
Location: Hamburg, Germany
Thanks to Andrew's great scripting work, we now have a new stars.dat file with almost 2 million Gaia stars for celestia.Sci. First of all, it is of interest how these stars are distributed over the Milky Way neighborhood. For this purpose, I prepared two figures of the Milky Way region around our Sun (marked red as selection):
[Click on image by all means and then hit the browser's fullscreen key (F11?)]
Attachment:
milky_gaia2.jpg
milky_gaia2.jpg [ 123.63 KiB | Viewed 2596 times ]

or here in a different view:
[Click on image by all means and then hit the browser's fullscreen key (F11?)]
Attachment:
milky_gaia1.jpg
milky_gaia1.jpg [ 74.29 KiB | Viewed 2596 times ]


Fridger

_________________
Image


Top
 Profile  
 
PostPosted: Tue, 27-09-16, 19:47 GMT 
Offline
User avatar

Joined: Mon, 07-01-08, 13:30 GMT
Posts: 350
Location: Switzerland
One question raised in the thread in the Lounge is whether it is possible to identify the spiral structure of the Milky Way from the Gaia dataset. I have tried to do this by marking the OB stars: massive, short-lived stars that do not live long enough to move very far from their birthplaces, which should be concentrated in the spiral arms. With the dataset as it stands, this is not possible due to a large number of stars that are misidentified as OB-stars based on their B-V colour index. I am working on further updates to the dataset that may help correct this.

Watch this space!


Top
 Profile  
 
PostPosted: Tue, 27-09-16, 20:24 GMT 
Offline
Site Admin
User avatar

Joined: Fri, 31-08-07, 7:01 GMT
Posts: 4579
Location: Hamburg, Germany
ajtribick wrote:
One question raised in the thread in the Lounge is whether it is possible to identify the spiral structure of the Milky Way from the Gaia dataset. I have tried to do this by marking the OB stars: massive, short-lived stars that do not live long enough to move very far from their birthplaces, which should be concentrated in the spiral arms. With the dataset as it stands, this is not possible due to a large number of stars that are misidentified as OB-stars based on their B-V colour index. I am working on further updates to the dataset that may help correct this.

Watch this space!


About two years ago I did quite a lot of research work to infer the MW spiral structure and extent from new kinds of data.
++++++++++++++++++++++++++++++++++
The point was to combine the new data on Massive Young Stellar Objects (MYSOs) and compact / ultra-compact HII regions with a complete set of published ATNF pulsar data, which largely increases the evidence for the arm structure from the RMS = Red MSX Source data alone.
++++++++++++++++++++++++++++++++++

I actually communicated several times with the RMS lead scientist Prof. J.S. Urquhart, who was so kind as to also send me their "hot" numerical data! Chris (ElChristou) helped actively on the graphical side.

Essential Literature:
===============
http://arxiv.org/pdf/1310.4758v1 (published in MNRAS 437, 1791–1807 (2014), link
http://mnras.oxfordjournals.org/content ... l.pdf+html
http://www.ast.leeds.ac.uk/RMS/
http://www.sci-news.com/astronomy/scien ... 01649.html

The RMS reference about MYSOs and compact / ultra-compact HII regions represents the cleanest recent evidence for a four principal arm structure of the MW (Norma, Sagittarius, Perseus and Scutum-Centaurus arms). Moreover, it provides lots of data tracing the so-called Outer arm in the region of present interest (II nd quadrant) (You remember that the Spitzer telescope claimed in 2008 to see only 2 principal arms...)

Here is a summary image from the above RMS paper:
[click on image by all means for a much bigger size!]
Attachment:
rms_MW.jpg
rms_MW.jpg [ 102.47 KiB | Viewed 2583 times ]


Soon more details...
Watch this space... ;-)

The MW template in celestia.Sci takes these new astronomical measurements into account!

_________________
Image


Top
 Profile  
 
PostPosted: Wed, 28-09-16, 17:37 GMT 
Offline
Site Admin
User avatar

Joined: Fri, 31-08-07, 7:01 GMT
Posts: 4579
Location: Hamburg, Germany
Perhaps these excerpts from an interview of Prof. Melvin Hoare (Univ. Leeds/UK) make the importance of MYSOs versus YSOs in the tracing task of the MW arms more transparent. Moreover, arguments are given below why Spitzer was missing two MW arms in 2008!

Prof. M. Hoare wrote:
The astronomers behind the new study used several radio telescopes in Australia, USA and China to individually observe about 1650 massive stars that had been identified by the RMS Survey. From their observations, the distances and luminosities of the massive stars were calculated, revealing a distribution across four spiral arms.
...
NASA’s Spitzer Space Telescope, on the other hand, scoured the Galaxy for infrared light emitted by stars. It was announced in 2008 that Spitzer had found about 110 million stars, but only evidence of two spiral arms.

++++++++++++++++++++++
“Spitzer only sees much cooler, lower mass stars – stars like our Sun – which are much more numerous than the massive stars that we were targeting.”

Massive stars are much less common than their lower mass counterparts because they only live for a short time – about *10 million years*. The shorter lifetimes of massive stars means that they are only found in the arms in which they formed, which could explain the discrepancy in the number of galactic arms that different research teams have claimed.
+++++++++++++++++++++++

Since lower mass stars live much longer than massive stars and thus rotate around our Galaxy many times, they have time enough for spreading out in the MW disc. The gravitational pull in the two stellar arms that Spitzer revealed is enough to pile up the majority of stars in those arms, but not in the other two. However, the gas is compressed enough in all four arms to lead to massive star (MYSO) formation.


Fridger

_________________
Image


Top
 Profile  
 
PostPosted: Tue, 29-11-16, 20:40 GMT 
Offline
User avatar

Joined: Mon, 07-01-08, 13:30 GMT
Posts: 350
Location: Switzerland
Apologies for the lack of recent updates.

The generation of the star database now includes information from infrared magnitudes to estimate the properties of the stars with unknown spectral types.

As a result the "infamous" star I posted in the other thread comes out as an F6, which is a lot closer to the actual spectral type (F3V) than the previous estimate of K0. Adding the relevant star names to starnames.dat, it now looks like this...

Attachment:
File comment: View from the vicinity of Boyajian's Star
boyajians_star.jpg
boyajians_star.jpg [ 116.98 KiB | Viewed 2387 times ]


The bright star in the background is Deneb.

I can't see any alien megastructures, I guess the light curve must have been caused by something else ;)


Top
 Profile  
 
PostPosted: Mon, 30-04-18, 19:33 GMT 
Offline
User avatar

Joined: Mon, 07-01-08, 13:30 GMT
Posts: 350
Location: Switzerland
Hi everyone, apologies for a long absence. I'm hoping that real life stuff is now calmed down enough to contribute here on a more regular basis, and it would be nice to finish my unfinished projects. :)

It will be interesting to process this in the light of DR2.

On the one hand, DR2 contains effective temperature values, which will eliminate one of the slowest bits of the DR1 processing code.

On the other hand, the number of stars in DR2 is too huge, I suspect this will need some fairly substantial reworkings of the internals (e.g. incorporate something like sqlite so as to avoid having to load the entire star db in memory). Perhaps it makes sense as a first step to select the subset of stars with TYC and HIP ids (will have to investigate the query functionality to see if it's possible to extract that selection) and process those.

One other thing that might be a useful improvement is to change the parallax conversion from the naïve distance = 1/parallax to the statistical approach suggested in this paper, using the suggested exponential distribution. The third paper in that series recommends a length scale of 1.35 kpc for end-of-mission Gaia data and 0.11 kpc for the TGAS subset (which would probably be the appropriate value here). Data from (X)HIP would also be necessary because bright stars are not present in the Gaia data, presumably that would require its own length scale for inverting the parallaxes.


Top
 Profile  
 
PostPosted: Mon, 30-04-18, 20:25 GMT 
Offline
Site Admin
User avatar

Joined: Fri, 31-08-07, 7:01 GMT
Posts: 4579
Location: Hamburg, Germany
Oh, welcome back, Andrew...and that at a very good moment!

i) DR2 and what to do with so many stars? ;-) Perhaps we could at least get rid of
these nasty DR1 stripes...?

ii) I am just half way through a git-conversion and migration of the SVN celestia.Sci repository to GitHub.

In fact, a more careful reading of the GitHub regulations revealed that faculty and other
academics may apply for a unlimited free private repository as well. This I did
in form of a well-reading page about myself, the .Sci team and the celestia.Sci project.
Here is the answer from the evaluation committee:

GitHub wrote:
Hey t00fri, we have some awesome news

We've upgraded you to a plan with unlimited free private repositories, which will be free for the next two years. After that, you'll get an email saying that your coupon is expiring. You can reapply for another coupon if you still have academic status. We don't have any collaboration limits, so any group projects you may encounter can be hosted via your account.


So this is handy: our collaboration will presumably be considerably intensified via a GitHub repository, while initially, we have the useful remaining privacy for completing a few more things.

And best of all: the transition from private to public is just one click ;-)

Cheers,
Fridger

_________________
Image


Top
 Profile  
 
PostPosted: Thu, 03-05-18, 12:12 GMT 
Offline

Joined: Thu, 05-12-13, 20:17 GMT
Posts: 73
KIC 8462852

Gaia DR1 2081900940499099136

Parallaxes (mas): 2.55

Gaia DR2 2081900940499099136

Parallaxes (mas): 2.2185
mag : 11.762624
teff_val (K) : 5899.25
radius_val (sunRad) : 1.65
lum_val (solLum) : 2.963

http://www.brucegary.net/ts6/

Image


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group