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Darrrius
2006-Dec-01, 02:21 PM
Hi

I wonder if anyone could answer this for me. I was just wondering, if it is known, how many new stars are formed in the milky way on a yearly basis?

Also, at what point can we say - "yes this is a new star" rather than a collection of dust and gas.

antoniseb
2006-Dec-01, 02:27 PM
At the moment, we can't say exactly, but it looks like it is about three solar masses of star are formed each year, which might be about seven stars on average.

However, as you alluded to, there is no observable instant when you say, this object has just this moment changed from a starless collapsing core to a new enshrouded star. There is also a little fuzziness in our definition of what a star is. Do you count the brown dwarfs? That might double your star formation rate, or more.

Darrrius
2006-Dec-01, 02:42 PM
Hmm, I guess the definition issue is more important than I first thought. Thanks for your answer though, thought provoking.

I was heading in the direction of asking why we do not see Supernovae in our galaxy on a fairly regular basis... got me wondering how many stars are "born" each year, and in turn how many "die" as supernovae each year. I'm aware that there probably isnt much of a pattern between the two.

Is it only a certain type (or mass) of star thay will go supernovae when it "dies".

I guess I should do some research on Supernovae, I'd be grateful if anyone could reccomend a book or resource...

Many Thanks

antoniseb
2006-Dec-01, 02:47 PM
I guess I should do some research on Supernovae, I'd be grateful if anyone could reccomend a book or resource...

These days, it might be enough to start by looking at Wikipedia, and following all the links. There will be some places where you'll find some disagreement, but you'll quickly figure out a model that works well enough for you.

Now, if you had asked: How many years are there per star formed in the Milky Way with enough mass to eventually become a Supernova, the answer would have been somewhere between 50 and 200 years per star. The formation rate of these matches the death rate.

Darrrius
2006-Dec-01, 03:14 PM
Ahh - thanks that has helped my understanding. Now, off to Wiki land i go!

Tim Thompson
2006-Dec-01, 04:15 PM
I was heading in the direction of asking why we do not see Supernovae in our galaxy on a fairly regular basis ...
That's the easy one. We miss most of the SNe that go off in our own galaxy because they are shrouded behind dense clouds of dust. Many years ago I was part of a group proposing to monitor the Milky Way at radio wavelengths, using the NASA DSN antennae (http://deepspace.jpl.nasa.gov/dsn/), specifically to search for hidden SNe, or other hidden transient & variable sources, but NASA did not fund the project. However, we do know from radio observations, that we have missed many SNe which have gone off in the starburst galaxy M82 (http://www.seds.org/messier/m/m082.html), but deeply enshrouded in dust.

The star formation rate in the Milky way (or any other galaxy) can only be determined by comparing a model for the galaxy with observations. In the case of the Milky Way that number is currently somewhere around 2-4 solar masses per year, but cannot be pinned down more precisely than that (i.e., Nab & Ostriker, 2006 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006MNRAS.366..899N&db_key=AST&d ata_type=HTML&format=&high=4366fa465114639)). But star formation rates are quite variable with time, and the Milky Way has experienced episodes of more vigorous star formation in the relatively recent past (i.e., de la Fuente Marcos & de la Fuente Marcos, 2004 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004NewA....9..475D&db_key=AST&d ata_type=HTML&format=&high=4366fa465114639)). And also keep in mind that stars form preferentially in parts of the galaxy where the gas & dust are more dense, along the edges of spiral arms (i.e., Luna & Carrasco, 2005 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006ApJ...641..938L&db_key=AST&d ata_type=HTML&format=&high=4366fa465127786)).

Nereid
2006-Dec-01, 04:30 PM
Given your past project Tim, you might be a particularly good person to ask the following questions:

1) Suppose 2MASS, IRAS, and Spitzer* were to monitor the sky, with complete coverage to the limit of their respective wavebands, once a month, how many SNe would they likely detect, in a year?

2) What would the numbers be, if the monitoring were limited to only Local Group galaxies?

3) Ditto, for the galaxies brightest in the optical (the RC3 catalogue + the recently discovered nearby dwarfs + 'in plane' bright galaxies such as Maffei 1 and 2)?

4) What limits (bounds) can we put on the (per century) rate of SNe in starburst galaxies such as M82 and the Antennae?

I'm actually more interested in the 'algorithm(s)' that you would use to get the answers than the answers themselves, and how well constrained the key parameters in those algorithms are.

*Of course, Spitzer is not (purely) a survey instrument; what I'm getting at is threshhold detectability by wavelength, in the NIR and FIR.

StupendousMan
2006-Dec-03, 01:02 AM
1) Suppose 2MASS, IRAS, and Spitzer* were to monitor the sky, with complete coverage to the limit of their respective wavebands, once a month, how many SNe would they likely detect, in a year?



That's a tough question to answer for several reasons. First, one would want to cover each area of the sky more frequently than once a month, in order to observe each supernova candidate a number of times (5 or 10) at decent signal to noise to confirm it as a supernova. There are very few instruments which can cover the entire sky in less than one month to a depth that is appropriate for this task, especially once you want to go beyond the visible.

Second, there's the question -- how deep do you go? There's a tradeoff in exposure time: exposing longer will go deeper and find more supernovae in a given area, but will prevent you from covering as much area. It takes a detailed calculation to figure out what strategy provides the largest number of supernovae for a particular instrument.

Third, there's the question of how you sift through the data to detect the supernovae and confirm them. And there are probably important factors which I can forgetting now.

In any case, the absolutely crucial question is: to what distance can your survey detect supernovae? The number you observe will depend on that distance in a very strong fashion.




2) What would the numbers be, if the monitoring were limited to only Local Group galaxies?


The important members of our Local Group are the Milky Way, M31, and maybe M33. The number of supernovae we've observed in these galaxies in the past is very small: fewer than 10 in the MW over the past millenium, 1 in M31 over the past 121 years, none in M33 over the past hundred years or so. It's likely that we (in theory) detect only a fraction of the events in each galaxy, due to obscuration, but I doubt that the fraction is less than, oh, 25 percent or so. That still leaves a rate of on the order of one to three per century.



3) Ditto, for the galaxies brightest in the optical (the RC3 catalogue + the recently discovered nearby dwarfs + 'in plane' bright galaxies such as Maffei 1 and 2)?


Well, that's basically the list of galaxies monitored by several of the nearby SN search teams, such as the Lick Observatory Supernova Search. They are all purely optical searches. One of them (LOSS) finds about 80 per years, and another (the Puckett team) finds about 30. The total number of SNe closer than z=0.03 found last year was 73.

You can find a table of events at

http://stupendous.rit.edu/richmond/sne/sn.list

which may be convenient for computing statistics.



4) What limits (bounds) can we put on the (per century) rate of SNe in starburst galaxies such as M82 and the Antennae?

I'm actually more interested in the 'algorithm(s)' that you would use to get the answers than the answers themselves, and how well constrained the key parameters in those algorithms are.


Well, I guess you might consider reading this particular reference, although it's certainly very old by now. It does a fair job, I think, of describing some of the "algorithms", as you put it.

The Supernova Rate in Starburst Galaxies (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1998PASP..110..553R&db_key=AST&d ata_type=HTML&format=&high=44c573e6c328857)

Tim Thompson
2006-Dec-06, 05:05 PM
Given your past project Tim, you might be a particularly good person to ask the following questions: ...
Sorry for the slow response, I just get a bit busy now & then. I really don't know how to give good answers to your questions. They are all tied together, and require better knowledge of the supernova rate than we have. The idea I was working on (named Argus by Woody Sullivan (http://www.astro.washington.edu/woody/) & Mike Klein) was intended to help to measure the SN rate in the Milky Way by finding the hidden SNe that we overlook. There has not been any such program since then, to the best of my knowledge. So we really still do not have any good idea what the SN rate is, either here in the Milky Way, or anywhere else.

There are some recent papers of interest along these lines. Sharon, et al., 2006a (http://arxiv.org/abs/astro-ph/0611920) and Sharon, et al., 2006b (http://arxiv.org/abs/astro-ph/0610228) describe an ongoing project to measure the SN-Ia rate, in both low & high redshift galaxies. Schanne, et al., 2006 (http://arxiv.org/abs/astro-ph/0609566) argue that the observed SN-Ia rate in the galactic center is much lower than the 0.5 SN/century mark set by observation of high energy positrons.

In peculiar places like M82 (http://seds.lpl.arizona.edu/messier/m/m082.html), for instance, where star formation comes in bursts, so the SNe must come in bursts. There must be several per century at least, maybe more, to generate the tremendous galactic winds (http://www.spitzer.caltech.edu/Media/releases/ssc2006-09/index.shtml). Even detailed studies of the radio SNR in M82 (i.e., McDonald, et al., 2001 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001MNRAS.322..100M&db_key=AST&d ata_type=HTML&format=)) are unable to come up with a SN rate.

Personally, I would like to see more work along these lines. We really don't have a good handle on supernovae; the rates are not well known, and models of type-II SN still don't explode in acceptable fashion.

Nereid
2006-Dec-28, 01:10 AM
Sorry for the slow response, I just get a bit busy now & then. I really don't know how to give good answers to your questions. They are all tied together, and require better knowledge of the supernova rate than we have. The idea I was working on (named Argus by Woody Sullivan (http://www.astro.washington.edu/woody/) & Mike Klein) was intended to help to measure the SN rate in the Milky Way by finding the hidden SNe that we overlook. There has not been any such program since then, to the best of my knowledge. So we really still do not have any good idea what the SN rate is, either here in the Milky Way, or anywhere else.

There are some recent papers of interest along these lines. Sharon, et al., 2006a (http://arxiv.org/abs/astro-ph/0611920) and Sharon, et al., 2006b (http://arxiv.org/abs/astro-ph/0610228) describe an ongoing project to measure the SN-Ia rate, in both low & high redshift galaxies. Schanne, et al., 2006 (http://arxiv.org/abs/astro-ph/0609566) argue that the observed SN-Ia rate in the galactic center is much lower than the 0.5 SN/century mark set by observation of high energy positrons.

In peculiar places like M82 (http://seds.lpl.arizona.edu/messier/m/m082.html), for instance, where star formation comes in bursts, so the SNe must come in bursts. There must be several per century at least, maybe more, to generate the tremendous galactic winds (http://www.spitzer.caltech.edu/Media/releases/ssc2006-09/index.shtml). Even detailed studies of the radio SNR in M82 (i.e., McDonald, et al., 2001 (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001MNRAS.322..100M&db_key=AST&d ata_type=HTML&format=)) are unable to come up with a SN rate.

Personally, I would like to see more work along these lines. We really don't have a good handle on supernovae; the rates are not well known, and models of type-II SN still don't explode in acceptable fashion.Many (belated) thanks!

All papers are interesting reads, and I found Sullivan et al. (astro-ph/0605455) (http://arxiv.org/abs/astro-ph/0605455) (referenced in the Sharon et al. low-z paper) very good too.

Clearly, there's great scope for a much better understanding of supernovae.

However, the question which motivated my post (and your unsuccessful proposal) remains open - how does the rate of supernovae vary by waveband, for bright, nearby galaxies? Especially the rate in starburst galaxies, for which the optical may be one of the worst bands in which to try to get a (contemporary) handle on SN rates.

I wonder - how many SN did 2MASS, COBE, and IRAS detect? Of those detected, how many were also detected in other wavebands, and what (if anything) can we learn about extinction in starburst galaxies?

Or could x-ray, gamma, or radio surveys be used to get a handle on this?

Amber Robot
2006-Dec-28, 04:15 PM
At the moment, we can't say exactly, but it looks like it is about three solar masses of star are formed each year, which might be about seven stars on average.

And the types of stars that end up as supernovae are made in fewer number than those that don't.

StupendousMan
2006-Dec-28, 04:23 PM
.... how does the rate of supernovae vary by waveband, for bright, nearby galaxies? Especially the rate in starburst galaxies, for which the optical may be one of the worst bands in which to try to get a (contemporary) handle on SN rates.


There is very little data to address your question. Most of the dedicated supernova searches in nearby galaxies take unfiltered pictures, in order to reach as deep as possible. We can't compare the rate detected by survey X, using the B-band filter, against that of survey Y, using the V-band filter.

Moreover, only a couple of search teams discover a statistically significant number of events each year. That also makes it difficult to compare different searches. The different methods they use to detect the new supernovae (exactly how one compares a new picture to an old one) make it even _more_ difficult.

Back in the old days of photographic astronomy, most supernovae were discovered on blue-sensitive plates. In theory, one might compare the rates from those searches to the rates from more recent, CCD-based searches, which tend to be more sensitive in the red. In fact, some people have already thought of doing this. I suggest you look up papers written by the Cappellaro, Turatto, Tsvetkov, and collaborators, back in the late nineteen-eighties and early nineteen-nineties.



I wonder - how many SN did 2MASS, COBE, and IRAS detect? Of those detected, how many were also detected in other wavebands, and what (if anything) can we learn about extinction in starburst galaxies?

Or could x-ray, gamma, or radio surveys be used to get a handle on this?

There may have been supernovae detected during the course of the 2MASS survey, but I'm not aware of them. At least the 2MASS survey had the spatial resolution to pick out SNe from their parent galaxies. COBE's resolution was several degrees, way too poor to detect SNe. IRAS lies in a middle ground; it _might_ have been able to detect some nearby supernovae. However, I suspect that the delays between observing and reducing the data probably prevented any discoveries from being followed up in time.

Radio astronomers have detected several outbursts which correspond to supernovae previously detected in the optical, and on a few occasions, supernovae which had not yet been noticed in the optical. In most cases, however, the radio emission rises from interactions of the ejecta with circumstellar material, and does not become very luminous until weeks or months after the explosion. Again, this makes it hard to confirm the event.

The current crop of gamma-ray satellites are looking for brief bursts of high-energy radiation. They have detected several GRBs which are associated with supernovae. It is possible that they may have seen bursts of X-rays from ordinary supernovae, but again, I'm not aware of any; you could search through IAU Circulars and GRB mailing lists.

For the most part, supernovae are still discovered in the optical. That situation is not likely to change in the near future. There may be some events discovered in the near-IR -- at 1 to 2 microns, perhaps -- but that won't make a big change in the properties of the detected population, I would guess.

Nereid
2006-Dec-31, 06:51 PM
There is very little data to address your question. Most of the dedicated supernova searches in nearby galaxies take unfiltered pictures, in order to reach as deep as possible. We can't compare the rate detected by survey X, using the B-band filter, against that of survey Y, using the V-band filter.

Moreover, only a couple of search teams discover a statistically significant number of events each year. That also makes it difficult to compare different searches. The different methods they use to detect the new supernovae (exactly how one compares a new picture to an old one) make it even _more_ difficult.

Back in the old days of photographic astronomy, most supernovae were discovered on blue-sensitive plates. In theory, one might compare the rates from those searches to the rates from more recent, CCD-based searches, which tend to be more sensitive in the red. In fact, some people have already thought of doing this. I suggest you look up papers written by the Cappellaro, Turatto, Tsvetkov, and collaborators, back in the late nineteen-eighties and early nineteen-nineties.



There may have been supernovae detected during the course of the 2MASS survey, but I'm not aware of them. At least the 2MASS survey had the spatial resolution to pick out SNe from their parent galaxies. COBE's resolution was several degrees, way too poor to detect SNe. IRAS lies in a middle ground; it _might_ have been able to detect some nearby supernovae. However, I suspect that the delays between observing and reducing the data probably prevented any discoveries from being followed up in time.

Radio astronomers have detected several outbursts which correspond to supernovae previously detected in the optical, and on a few occasions, supernovae which had not yet been noticed in the optical. In most cases, however, the radio emission rises from interactions of the ejecta with circumstellar material, and does not become very luminous until weeks or months after the explosion. Again, this makes it hard to confirm the event.

The current crop of gamma-ray satellites are looking for brief bursts of high-energy radiation. They have detected several GRBs which are associated with supernovae. It is possible that they may have seen bursts of X-rays from ordinary supernovae, but again, I'm not aware of any; you could search through IAU Circulars and GRB mailing lists.

For the most part, supernovae are still discovered in the optical. That situation is not likely to change in the near future. There may be some events discovered in the near-IR -- at 1 to 2 microns, perhaps -- but that won't make a big change in the properties of the detected population, I would guess.Thanks.

While the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) (http://www.cfht.hawaii.edu/Science/CFHLS/) could yield some useful data (it includes a five broadband coverage in at least some part of the survey - u*, g', r', i', z'), it would yield a lot more if the same J, H, Ks filter set as 2MASS used were to be added.

Closer to home, I wonder how helpful it would be if some of the serious amateur SN searches (e.g. Puckett (http://www.cometwatch.com/)) were to change search strategies and detectors/filters, to concentrate on starburst galaxies and the NIR?

Of course, when the LSST (http://www.lsst.org/lsst_home.shtml) and Pan-STARRS (http://pan-starrs.ifa.hawaii.edu/public/) come on stream, a small change in the survey programme(s), to cover the NIR, would likely advance this area of SN research tremendously!

(not to mention that PHA detection might be improved too, or at least better characterised ...)

I'll go dig up some papers by Cappellaro, Turatto, Tsvetkov, and collaborators; thanks for the lead.