Guess What I Was Doing New Year’s Eve?

It was clear here on New Year’s Eve and I decided to try something a bit different.  I installed the 150 line/mm grating in the spectrograph and tried to acquire a spectrum of a newly-discovered transient object called ASAS-SN14mv.  It appears to be a dwarf nova in outburst and since I gather lots of photometric data on cataclysmic variables, and dwarf novae in particular, it made sense to try.  The object was at about 12th magnitude so I knew it would be a challenge.  Figure #1 is the resulting spectrum from the combination of three 1800-second exposures.

 

Note the absence of hydrogen lines.  In fact, when compared to the spectrum of an A0V star (like Vega) the spectrum is fairly similar except for this absence.  Outbursts of dwarf novae originate in the accretion disk surrounding the white dwarf star in a very close double-star system.  During the early stages of the outburst the disk reaches a temperature similar to the photosphere of a star like Vega, around 10,000 K, so not surprising that it’s continuum shape would mimic that of Vega.  Prior to outburst (in quiescence) these objects typically have obvious emission lines of hydrogen and helium.  But the light from the outburst overwhelms the brightness of the emission lines and, conversely, usually show broad lines of hydrogen absorption, sometimes with small emission peaks in their centers.  At the time these data were acquired ASAS-SN14mv was likely very close to it’s maximum brightness (around 12th magnitude).  The spectrum shows hints at a number of lines other than the two mentioned in the caption, but being a fairly novice spectroscopist I’m hesitant to say for sure what they are.  But comparing to A-type stars (objects with similar surface temperatures) I could guess that Fe II λ4926 is present,  as well as badly subtracted sodium night sky lines at λ5683-5688, mercury at λ5460, and another sodium doublet at λ5890-5896.  In fact the biggest challenge with the extraction of these spectra from the 2-d images was to get accurate subtractions of night sky lines.  Figure 2 shows what one of the spectra looked like before being extracted.  Figure 3 is a spectrum of the night skies here.  Pretty dreadful!

 

 

 

The fact is that some of these lines might actually be useful for wavelength calibration since my only reference lamp, at present is Neon – and there are no bright lines blueward of about 5400 Angstroms. Note that the values in the Figure 3 annotations are based on a similar plot that appears on Christian Buil’s site HERE. Christian’s site is a wealth of information for anyone interested in the topic of astronomical spectroscopy and is very highly recommended!  Clearly he’s dealing with the same bright sky problems that I do and achieves great results!

I had to be at the studio early the morning of the eclipse, so these were taken from a location near the studio (on Gable street) which had a pretty good western horizon – at least for an industrial area lit up like a maximum security prison yard.  The pictures were taken with a Nikon D5100, with a 300mm telephoto.  The ISO changed depending on the light level, from ISO100 to 4000.

Nova Delphini was discovered on the 14th of August, 2013 by Japanese amateur Koichi Itagak.  Eventually designated V339 Delphihi by the International Astronomical Union, Nova Delphini became an easy naked-eye object throughout the end of August and one of the 30 apparently brightest in recorded history.  It’s location in the northern mid-summer skies made it a favorite target for stargazers throughout the late summer and early fall of 2013.  It also appeared just as amateur astronomers have begun to engage in regular spectrographic monitoring as numerous relatively low-cost but very capable spectrometers have become available from commercial vendors, most notably from Shelyak Instruments in France.  It has become, by far, the single most spectrogaphically-observed object in history as amateurs from around the world contributed over a thousand high quality spectra to the ARAS (or Astronomical Ring for Access to Spectroscopy) Nova Delphini database, available here.

I had previously signed on to another campaign (the WR 134, 135, and 137 campaign coordinated by Noel Richardson and Tony Moffat at U Montreal) so I did not start regularly monitoring Nova Delphini until the WR stars disappeared behind the large maple tree in my back yard.  Once I could no longer acquire high signal-to-noise data from the WR stars I switched to monitoring this object, and ultimately contributed 20 or so spectra to the ARAS effort.

Novae, like other members of the cataclysmic variable star class, derive from binary star systems comprised of a collapsed white dwarf star (think something the size of earth but the mass of the sun) and a red dwarf star (something a third the mass and half the diameter of the sun).  The two stars are very close to one-another, so close that the much stronger gravitational pull of the white dwarf primary star is sucking material from the outer atmosphere of the red dwarf secondary.  That mass, comprised of almost pure hydrogen, must obey the laws of energy and angular momentum conservation and so settles into a disk of material, called an accretion disk, which surrounds the white dwarf primary.  Eventually the disk material manages to radiate away enough energy that it finally settles onto the surface of the white dwarf (a fairly violent process in itself) and ultimately forms a very thin (some inches thick) atmosphere of pure hydrogen.  Once enough Hydrogen has accumulated the temperature and pressure at the bottom of this very thin shell become sufficient to trigger the fusion of Hydrogen into Helium.  When that happens the shell experiences a thermonuclear runaway – and essentially becomes a massive Hydrogen bomb, blasting itself into space at thousands of kilometers per second and taking with it anything else that might have been near the surface of the white dwarf.  The spent fuel remnants from the white dwarf’s previous life as a normal star, including Helium, Carbon, Nitrogen, Oxygen, Silicon and Iron, anything that might have been near the surface, is also ejected at very high velocities into interstellar space.  When all of this occurs the total luminosity of the system can increase by a factor of more than 10,000 times.  And thus we see it as a nova.

In the days and weeks immediately following the outburst rapid changes in the temperature and density of the expanding shell of gas lead to dynamic and complex changes in the appearance of the nova’s spectrum.  Additionally, unlike a supernova, the primary star that was the source of the explosion is still there, strongly radiating in the ultraviolet and x-ray region and thus ionizing material from the inside of the rapidly expanding shell.  Within hours following the announcement of the nova’s discovery members of the ARAS forum began around-the-clock monitoring of Nova Delphini 2013, acquiring high-quality spectra from over three dozen locations around the world.

Immediately recognizing the value these spectra would represent towards a better understanding of novae in general Dr Steven Shore, a world-renown expert on the topic, began a series of communications helping to guide the ARAS effort.  The website has a series of pages of Steven’s explanations of what the spectra were indicating and what observers might expect to see next based on observations of past novae.  Nicely illustrated with dozens of the ARAS spectra these pages are an invaluable and unique resource for amateur spectroscopists wanting to better understand the data they’ve gathered.  Check it out HERE.

Anyway, shown in Figure 1 is a combined plot of all of the Nova Del spectra acquired at the Beverly Hills Observatory.

 

 

 

After many false starts this year I finally got back out to the observatories and did some real observing!  And the target of the first nights of photometry for 2014 is the eclipsing nova-like (NL) star UX Ursa Majoris.  Discovered in 1933 by S. Beljawski, UX UMa was first thought to be a typical eclipsing binary star.  But once astronomers started observing the eclipses it was found the orbital period, as determined by the time between eclipses, was much shorter than most eclipsing systems.  UX UMa’s “period”, at just 0.1967 days (=4.7 hours), would normally indicate that it was a member of the W UMa subclass, which are stars orbiting so closely that they’ve almost merged, sharing a common “envelope” or outer atmosphere.  One observable consequence of such closely orbiting stars is that both stars are gravitationally distorted into ellipsoids so that the effective surface area of the stars as seen from our vantage point is always changing.  Thus there is no section of the light curve where the sum of the light we see from the system is constant for any period of time.   Visual observations of UX UMa showed it to have a fairly flat light curve except for the eclipses, so it obviously was not a W UMa type eclipsing binary.

By the middle of the last century photometric and spectroscopic observations had revealed that UX UMa was, in fact, an eclipsing nova-like system.  For a really good description of this star and a quick synopsis of the nova-like class check out this page from the American Association of Variable Star Observers (AAVSO).

In the past month or so I’ve been able to get three nights of data on UX UMa; May 18/19, May 25/26, and June 15/16.  The light curves for each are shown, below.  The plots show the brightness (increasing in the vertical axis) against time (increasing along the horizontal axis) for UX UMa (the red dots) and a “check” star (the blue dots), each as compared to a common “comparison” star.   Both the comparison and check stars are thought to be of constant brightness, so the blue curve, if everything was perfect, should be a perfectly straight line.  There are many many reasons why that is not so, but the scatter in the blue curve is an indication of how well I’ve managed to measure the brightness of the variable star.  In fact the scatter in the points in the blue curve help determine the “error” in the measurements of the variable star (the red dots).  The small vertical spikes that you see on the red dots is an indication of the error in each measurement.

Note that you can click on each of the plots to see them in full resolution.

 

 

 

There is a lot going on in the light curve of UX UMa!  The most obvious feature in the light curve is, of course, the eclipse, where the total light from the system is diminished by more than a full magnitude.  The eclipses are not symmetrical – a reflection of the fact that three different significant sources of light, each with it’s own characteristics (the accretion disk, the white dwarf, and the “hot spot” caused by the stream of material being sucked off of the red dwarf star impacting the outer edge of the accretion disk) are eclipsed.  Even outside of eclipse there is some interesting flickering going on that hints at being periodic.   And there is, perhaps counter intuitively, a gradual brightening towards when the red dwarf star passes behind the accretion disk.  The binary actually appears brighter due to the fact that the side of the red dwarf facing the white dwarf is being heated (something called the reflection effect, though it’s not a reflection) by radiation from the white dwarf and accretion disk,  thus making it’s surface much brighter on that side.  Got it?  That’s a fine run-on sentence!

So the earth was “buzzed” by asteroid 2005 YU55 the other night.  A roughly 1000-foot long bit of cosmic debris passing within the moon’s orbit at a distance of 202,000 miles.   At the time that I was trying to catch the object it’s motion against the background stars was almost 500 arcseconds a minute, moving towards the east-northeast.  Since the apparent east-towards-west motion of the stars across the sky due to the earth’s almost 24-hour rotation period (called diurnal motion) is about 897.5 arcseconds per minute, that meant that as far as tracking the asteroid was concerned it would appear to be moving from east to west at somewhat less than half the speed of diurnal motion.  Well, the CGE mounts I use for the telescopes are not able to track something like that.  So that meant I had to set the telescope at a location a few minutes ahead of the asteroid and wait for it to enter the field of view.  Since the telescope would be driven at a speed necessary to keep the star field steady (compensate for the diurnal motion) that meant the object would enter the field of view from the west and track towards the east through the field of view.

My initial attempts to locate 2005 YU55 were failures.  I’d forgotten that the positions I’d downloaded were geocentric positions; that is, where the object would appear against the background sky as seen by an imaginary viewer located at the earth’s center.  In almost any other instance that would get me close enough to assure the object was in the camera’s 8.5-arcminute field of view.  Not this time!  Because the object was so close the location of the observer on the earth’s surface is an important consideration.  In this case the difference in apparent position of 2005 YU55 as seen by an observer on the earth’s surface (the topocentric coordinates) could be different from the geocentric coordinates by a degree or more.  Doh!  So I quickly converted a few of the coordinates to topocentric coordinates (thank you again Jean Meeus!) and viola!