APOD: Stars and Dust in Corona Australis (2015 Jan 08)

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APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by APOD Robot » Thu Jan 08, 2015 6:09 am

Image Stars and Dust in Corona Australis

Explanation: Cosmic dust clouds and young, energetic stars inhabit this telescopic vista, less than 500 light-years away toward the northern boundary of Corona Australis, the Southern Crown. The dust clouds effectively block light from more distant background stars in the Milky Way. But the striking complex of reflection nebulae cataloged as NGC 6726, 6727, and IC 4812 produce a characteristic blue color as light from the region's young hot stars is reflected by the cosmic dust. The dust also obscures from view stars still in the process of formation. At the left, smaller yellowish nebula NGC 6729 bends around young variable star R Coronae Australis. Just below it, glowing arcs and loops shocked by outflows from embedded newborn stars are identified as Herbig-Haro objects. On the sky this field of view spans about 1 degree. That corresponds to almost 9 light-years at the estimated distance of the nearby star forming region.

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by Ann » Thu Jan 08, 2015 7:01 am

Oh, that's a lovely picture! Fantastic, dramatic, illustrative and very very blue! :D

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by starsurfer » Thu Jan 08, 2015 10:52 am

Oh wow, this is possibly the greatest or most photogenic star forming region in the sky! This is something I never get tired of seeing it and it's impossible to have a favourite image of NGC 6726, they're all so beautiful! I like the interesting composition of the full frame image, which is a 4 frame mosaic that also includes the globular cluster NGC 6723 in the neighbouring constellation of Sagittarius, it has dazzling star colours!

I think this is the first APOD for the CHART32 team and hopefully one of many! They could be considered to be the southern equivalent of Adam Block's observatory, both have 0.8 meter telescopes.

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by Joules » Thu Jan 08, 2015 2:18 pm

Wow!
I wonder if the Hale telescope could have matched this back in the day, had it been in the proper hemisphere.
Pretty good imagery for any telescope, far less a 32 incher.

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by Chris Peterson » Thu Jan 08, 2015 3:23 pm

Joules wrote:I wonder if the Hale telescope could have matched this back in the day, had it been in the proper hemisphere.
Pretty good imagery for any telescope, far less a 32 incher.
The size of the telescope hardly matters for images like this. A large scope will allow for a shorter total exposure time, but that's all. A convenience only. And if the diameter is too large (as with the Hale), it may be impossible to get the desired field of view in a single image.

This image would have been no different through a 10" scope, a 32" scope, or a 200" scope. Only the necessary exposure times to achieve any given degree of signal-to-noise would have been different. (Less than about 10" and the resolution would start being limited by the optics rather than the atmosphere.)

The real limitation "back in the day" would have been film, which can't come close to CCD sensors in terms of performance.
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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by Guest » Thu Jan 08, 2015 3:54 pm

Nice image. It reminds me a lot of the original poster from Star Wars.

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by JuanAustin » Thu Jan 08, 2015 4:25 pm

I am taking a trip to Sydney at the end of March and will be my first trip south of the equator. I would dearly love to do some stargazing while down under, does anyone have any recommendations to engage someone somewhere somehow? Thanks!
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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by ta152h0 » Thu Jan 08, 2015 5:49 pm

loiwer right, 3/4 way a double spike. two stars really close ?
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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by geckzilla » Thu Jan 08, 2015 6:25 pm

ta152h0 wrote:loiwer right, 3/4 way a double spike. two stars really close ?
Only apparently, it seems. The two have vastly different parallaxes. The one on the right, HD 176269, has a parallax of 23.39 mas. The one on the left, HD 176270, only has a parallax of 4.26 mas. So it would seem the one on the right is significantly closer to us.
http://simbad.u-strasbg.fr/simbad/sim-i ... =HD+176269
http://simbad.u-strasbg.fr/simbad/sim-i ... =HD+176270
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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by MarkBour » Thu Jan 08, 2015 9:30 pm

By the way ... I recall a few times where people (particularly those less experienced, I think) used to criticize the way distances and spans were described in an APOD description. I really like the wording on this one:
On the sky this field of view spans about 1 degree. That corresponds to almost 9 light-years at the estimated distance of the nearby star forming region.
By stating it in that order, there seems to be no room left for confusion, even for novices such as myself.
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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by Nitpicker » Thu Jan 08, 2015 9:35 pm

JuanAustin wrote:I am taking a trip to Sydney at the end of March and will be my first trip south of the equator. I would dearly love to do some stargazing while down under, does anyone have any recommendations to engage someone somewhere somehow? Thanks!
Sydney is a city of four million which loves its bright lights, so there will be some driving involved for dark skies. Here are a couple of informative links:

http://en.sydney.com/Astronomy2009_p3487.aspx

http://astronomy.org.au/amateur/amateur ... australia/

I might recommend you contact one of the amateur societies around Sydney.

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by Ann » Thu Jan 08, 2015 11:53 pm

geckzilla wrote:
ta152h0 wrote:loiwer right, 3/4 way a double spike. two stars really close ?
Only apparently, it seems. The two have vastly different parallaxes. The one on the right, HD 176269, has a parallax of 23.39 mas. The one on the left, HD 176270, only has a parallax of 4.26 mas. So it would seem the one on the right is significantly closer to us.
http://simbad.u-strasbg.fr/simbad/sim-i ... =HD+176269
http://simbad.u-strasbg.fr/simbad/sim-i ... =HD+176270
I'd say we are most definitely talking about a double star, for the following reasons:

1) It would be absolutely extremely unlikely for two stars of such similar colors and magnitudes to be so close together in the sky if they were not physically related. Given the fact that both stars are also blue, which is a much rarer color among 6th to 7th magnitude stars than yellow, makes it even more unlikely that they are not related.

2) The simple fact that these two stars are blue means that they are also (fairly) massive. Massive stars are particularly likely to form binaries. Even though this paper refers to much more massive stars than HD 176270 and HD 176269, it still suggests that binarity is a quite common phenomenon among early A- and late B-type stars like HD 176270 and HD 176269.

3) If these stars are really a (very wide) pair, but extremely close together on the sky, the Hipparcos parallaxes measured by the Hipparcos satellite are likely to go haywire. In other words, they are not reliable at all. Therefore, the Hipparcos parallax of HD 176269 on the right is 23.39 ± 6.47 mas. That is a high degree of uncertainty. The Hipparcos parallax of HD 176270 on the left is 4.26 ± 4.38 mas. So if we are talking about signal to noise here, the noise is worse than the signal. The proper motion in RA is equally uncertain for HD 176270, 6.03 ± 7.44 mas. For HD 176269, it is 16.05 ± 10.63 mas. So Hipparcos parallaxes and proper motion measurements are pretty useless here.

4) Finally, HD 176269 and 176270 are the brightest stars of an active but low-mass region of star formation. That the most massive stars formed in such a region of star formation would be early A- and late B-type stars like HD 176269 and 176270 is exactly what we should expect.

HD 176270 and 176269 are definitely a pair.

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by Nitpicker » Fri Jan 09, 2015 1:28 am

Ann wrote: I'd say we are most definitely talking about a double star, for the following reasons:

1) It would be absolutely extremely unlikely for two stars of such similar colors and magnitudes to be so close together in the sky if they were not physically related. Given the fact that both stars are also blue, which is a much rarer color among 6th to 7th magnitude stars than yellow, makes it even more unlikely that they are not related.

2) The simple fact that these two stars are blue means that they are also (fairly) massive. Massive stars are particularly likely to form binaries. Even though this paper refers to much more massive stars than HD 176270 and HD 176269, it still suggests that binarity is a quite common phenomenon among early A- and late B-type stars like HD 176270 and HD 176269.

3) If these stars are really a (very wide) pair, but extremely close together on the sky, the Hipparcos parallaxes measured by the Hipparcos satellite are likely to go haywire. In other words, they are not reliable at all. Therefore, the Hipparcos parallax of HD 176269 on the right is 23.39 ± 6.47 mas. That is a high degree of uncertainty. The Hipparcos parallax of HD 176270 on the left is 4.26 ± 4.38 mas. So if we are talking about signal to noise here, the noise is worse than the signal. The proper motion in RA is equally uncertain for HD 176270, 6.03 ± 7.44 mas. For HD 176269, it is 16.05 ± 10.63 mas. So Hipparcos parallaxes and proper motion measurements are pretty useless here.

4) Finally, HD 176269 and 176270 are the brightest stars of an active but low-mass region of star formation. That the most massive stars formed in such a region of star formation would be early A- and late B-type stars like HD 176269 and 176270 is exactly what we should expect.

HD 176270 and 176269 are definitely a pair.

Ann
Nope, sorry, it is not signal and noise, it is measurement and error. It means that the parallax of HD 176270 is between 0 and 8.64 mas, and the parallax of HD 176269 is between 16.92 and 29.86 mas. So, the pair form a double star, but only an otherwise unrelated, optical double.

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by ta152h0 » Fri Jan 09, 2015 1:44 am

do binary stars exhibit two spikes ? and such spikes, if they exist can reveal a distance in two dimensions ?
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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by geckzilla » Fri Jan 09, 2015 1:48 am

Even if you take the maximum uncertainty for the two they don't come close to one another. 16.92 and 13.47 mas. Their proper motions are also divergent. A color coincidence does not a double make.
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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by geckzilla » Fri Jan 09, 2015 1:54 am

ta152h0 wrote:do binary stars exhibit two spikes ? and such spikes, if they exist can reveal a distance in two dimensions ?
If they are far enough apart for the two points to occupy two different points on the sensor, then yes. Many binaries are not separated enough but the existence of close plural systems can also be inferred by looking at their light curves over time. Don't ask me for specifics on that last bit. That's in the "I wish I knew more about it" category for me.
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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by Ann » Fri Jan 09, 2015 1:58 am

I agree that "signal to noise" was a poor choice of words. However, to me there is no question whatsoever that these two stars are a pair, and that the Hipparcos measurements are in error.

Several years ago, I checked about a thousand blue stars listed in Sky Catalog 2000.0, Volume 1, versus the Hipparcos Cataloge. I found several iffy Hipparcos Catalogue parallaxes. The iffiness was particularly noticeable for stars in clusters, which are very close to one another. Stars that obviously belonged to the same cluster could have quite different parallaxes.

The spectral classes for HD 176269 and HD 176270 are listed as B7/B8V and B9.5IV:, which is very similar. Their magnitudes are 6.690 ± 0.007 and 6.399 ± 0.010, respectively. Their Johnson B-V colors are 0.004 ± 0.016 and 0.020 ± 0.010, respectively. This is extremely similar. Please note that the parallax for HD 176269, 23.39 ± 6.47 mas, puts it at a distance of 139 ± 39 light-years from the Earth, and gives it a luminosity of 3.1 ± 1.7 solar. Is that a likely luminosity for a star of spectral class B7/B8V? Yes, it is true that very young stars are considerably fainter than more mature main sequence stars, but a luminosity of 3.1 solar is still ridiculously low for a main sequence B-type star.

In any case, if HD 176269 is so close that its luminosity is no more than 3.1 solar, it has to be very young indeed. It must be practically newly hatched from a region of star formation. Where is that region of star formation that is located only between a hundred and two hundred light-years from the Earth right in front of the far more distant Corona Australis star forming region?

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by geckzilla » Fri Jan 09, 2015 2:18 am

Might you have looked at older, more iffy values? How long ago did you do this, exactly? These values are referenced from a 2007 reduction which is supposed to be an improvement over the 1997 catalog.
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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by Ann » Fri Jan 09, 2015 2:31 am

I looked at the values of an older catalog.

But there is another thing. Again consider the color indexes of HD 176269 and 176270. Not only are the colors very similar, but they are also a little redder than we would expect for stars of their spectral classes. But that is fine if they are immersed in a star forming region, where we expect to see a lot of dust. The Corona Australis star forming region is clearly dusty, and HD 176269 and 176270 appear to be affected by the same amount of dust reddening.

But what if HD 176269 is located much closer to us than HD 176270, far away from the dust of the Corona Australis star forming region? Then where does the dust come from that reddens HD 176269 by apparently exactly the same amount as HD 176270? Are we to assume that there is a long narrow column of dust that stretches between us and HD 176269 and abruptly comes to an end at the star, so that it does not add more dust reddening to HD 176270?

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by geckzilla » Fri Jan 09, 2015 2:54 am

I think you have a good argument that the young stars both formed recently in the same stellar nursery even though that is not explicitly the point you are trying to make. I still unconvinced that they are gravitationally bound.
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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by Ann » Fri Jan 09, 2015 2:56 am

geckzilla wrote:I think you have a good argument that the young stars both formed recently in the same stellar nursery even though that is not explicitly the point you are trying to make. I still unconvinced that they are gravitationally bound.
They may not be. Their separation may be greater than it looks from our vantage point. But to me it is absolutely obvious that they are not as widely separated as their Hipparcos parallaxes suggest.

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by geckzilla » Fri Jan 09, 2015 3:09 am

Well, it may be deserved, but from this outsider's viewpoint it is also worth noting that you appear to have an ax to grind with the Hipparcos catalog which makes you prone to wanting it to be wrong.

You may be interested in a couple of paragraphs from this paper specifically devoted to these two stars. There are uncertainties about them because no one has given them a really good look more recently other than that 2007 Hipparcos reduction (at least as far as I can find).
A.11. CrA-11 (HD 176269, HR 7169)

CrA-11, more commonly known as HD 176269, is an optically detected PMS star with a spectral type of B8V that was first observed by Knacke et al. (1973) as HR 7169 to be a possible member of the young stellar association near R CrA (~12' to the SW). Another B8V star ~12'' away, HR 7170 (HD 176270), was also listed as a possible member and we include it in Table 7. These two sources were detected, but not spatially resolved, by the IRAS (IRAS 18; Wilking et al. 1992) and EINSTEIN satellites (CrAPMS 10; Walter et al. 1997).

HD 176269 was observed in the near-infrared by Glass & Penston (1975) and is referred to in their Table 1 as source l; HD 176270 was also observed, and is listed as source k. Later, Patten (1998) does not classify the HD 176269/HD 176270 pair (which he calls source R05) as a likely association member: he finds a spectral type of B9V for the source and detects a ROSAT X-ray counterpart, however the source does not fulfill enough criteria for him to consider it an association member. Note that these stars are far enough afield so that they were not included as part of the Forbrich & Preibisch (2007) Chandra study.

HD 176269 was also detected with ISOCAM by O99, and is referred to as ISO-CrA 110 in their survey. ISO-CrA 110 was found to have a mid-infrared excess based on those observations so it is considered one of their YSO candidates. HD 176270 is referred to as ISO-CrA 111 in the same study, and it was not found to have a mid-infrared excess.

These studies are consistent with the Spitzer observations; we classify HD 176269 as a Class III based on the shape of its SED. Although we do not classify HD 176270 as a YSO candidate, it very likely is also a Class III based on its colors and SED shape (see Table 7). The SEDs of the two stars look similar out to 8 μm. However, at 24 μm, HD 176269 is much brighter than HD 176270. Neither source is detected at 70 μm.
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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by Ann » Fri Jan 09, 2015 5:14 am

geckzilla wrote:Well, it may be deserved, but from this outsider's viewpoint it is also worth noting that you appear to have an ax to grind with the Hipparcos catalog which makes you prone to wanting it to be wrong.

You may be interested in a couple of paragraphs from this paper specifically devoted to these two stars. There are uncertainties about them because no one has given them a really good look more recently other than that 2007 Hipparcos reduction (at least as far as I can find).
A.11. CrA-11 (HD 176269, HR 7169)

CrA-11, more commonly known as HD 176269, is an optically detected PMS star with a spectral type of B8V that was first observed by Knacke et al. (1973) as HR 7169 to be a possible member of the young stellar association near R CrA (~12' to the SW). Another B8V star ~12'' away, HR 7170 (HD 176270), was also listed as a possible member and we include it in Table 7. These two sources were detected, but not spatially resolved, by the IRAS (IRAS 18; Wilking et al. 1992) and EINSTEIN satellites (CrAPMS 10; Walter et al. 1997).

HD 176269 was observed in the near-infrared by Glass & Penston (1975) and is referred to in their Table 1 as source l; HD 176270 was also observed, and is listed as source k. Later, Patten (1998) does not classify the HD 176269/HD 176270 pair (which he calls source R05) as a likely association member: he finds a spectral type of B9V for the source and detects a ROSAT X-ray counterpart, however the source does not fulfill enough criteria for him to consider it an association member. Note that these stars are far enough afield so that they were not included as part of the Forbrich & Preibisch (2007) Chandra study.

HD 176269 was also detected with ISOCAM by O99, and is referred to as ISO-CrA 110 in their survey. ISO-CrA 110 was found to have a mid-infrared excess based on those observations so it is considered one of their YSO candidates. HD 176270 is referred to as ISO-CrA 111 in the same study, and it was not found to have a mid-infrared excess.

These studies are consistent with the Spitzer observations; we classify HD 176269 as a Class III based on the shape of its SED. Although we do not classify HD 176270 as a YSO candidate, it very likely is also a Class III based on its colors and SED shape (see Table 7). The SEDs of the two stars look similar out to 8 μm. However, at 24 μm, HD 176269 is much brighter than HD 176270. Neither source is detected at 70 μm.
Thanks for that info, Geck. As to what I can conclude from that science-speak, there is nothing there to really contradict the possibility that these two stars are gravitationally bound.

But let me get back to Hipparcos. I don't think it can always be trusted, but that doesn't mean that I think it is usually wrong. I don't.

But let's look at the Hipparcos parallaxes for the Trapezium cluster in the Orion Nebula. These are the Hipparcos parallaxes for the four brightest stars as referred to by Simbad database:

Theta 1 A Orionis: -52.82 [33.69] mas
Theta 1 B Orionis: (Simbad gives no parallax)
Theta 1 C Orionis: -13.00 [5.05] mas
Theta 1 D Orionis: -27.24 [8.21] mas

Note that all the three parallaxes given are certifiably wrong, because they are all negative. Negative parallaxes are impossible. An infinitely distant object would have a parallax of zero in a flat universe.

Not only are the three parallaxes all negative, but they also differ a lot among themselves.

I certainly realize that the Trapezium must be an almost impossibly difficult place to measure parallaxes. The area is tiny and crowded, the stars are orbiting very, very fast around their common center of gravity, and chaotically swirling clouds of dust and glowing ionized gas must make it absolute incredibly hard to measure the parallaxes of the stars correctly. But for all of that, the Trapezium is a good example of the "iffiness" I see in the Hipparcos data sometimes.

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by starsurfer » Fri Jan 09, 2015 9:55 am

The description says the image spans 1 degree but actually the field of view of the image is less than half a degree, somewhere between 20-30 arcminutes.

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Re: APOD: Stars and Dust in Corona Australis (2015 Jan 08)

Post by DavidLeodis » Sat Jan 10, 2015 12:40 am

I'm clearly rubbish at spotting things in some APOD images as I cannot definitely see two presumably separate blue reflection nebulae NGC 6726 and 6727 that are marked in the labelled version of the wider field-of-view image as NGC 6726-7 that is brought up through the 'NGC 6726, 6727, and IC 4812' link in the explanation to the APOD (and also through the CHART32 website link in the credit). It all looks like only one nebula to me :!:

In the labelled image it marks Bernes 157 of which in the CHART32 website it states "Bernes 157, a boomerang -shaped dark nebula partly seen in the bottom left corner is 520 light years distant. It stretches around the Corona Australis Nebula like a huge, draping black scarf". Even knowing that I still don't see it and if it had not been mentioned I would never have given the area marked as Bernes 157 a second glance! :?