Strange Quarks

The light curves of the two recent bright southern novae, V462 Lup and V572 Vel, have developed somewhat since I last wrote about them, the first peaking at magnitude 5.2, now 11.1, and the second peaking at 4.8, now 10.3.

T CrB remains quiet and foreboding, low in the early NW evening sky, taunting us with its pre-eruption ellipsoidal variations.

Writing this update reminded me of a long-standing plan to write a nova distance calculator plug-in for VStar, based upon the rate of decline of a nova.

I recently carried out photometry of the short period pulsating variable star SX Phoenicis.

This star has a very short period of 79 minutes over which it ranges between around magnitude 6.8 and 7.5! Contrast this with a Cepheid variable whose period is measured in many days or weeks and Miras with pulsation periods of many months.

The light curve above was derived from photometry of more than 400 images, using median stacking to group images close in time in order to reduce the scatter between datapoints. Almost two cycles were captured here. There are not many observations of SX Phoenicis in the AAVSO International Database, so I will very likely contribute more.

SX Phoenicis is the prototype (the first example) of a class of variable stars which has two pulsation modes, one with a period of around 79 minutes (as shown in my light curve above), the other around 62 minutes. The temperature of the star varies between 7,230 and 8,170 degrees Kelvin at minimum and maximum brightness, smallest and largest radius, respectively.

The origin of this kind of star is unclear but one possibility is that it comes from the merger of two stars, creating a single star that is more luminous and more blue than expected of the older galactic halo population in which it resides, a so-called blue straggler.

I gave a talk on June 5 2024 to ASSA about the imminent nova T Corona Borealis.

One of the things I showed (reproducing a result from Brad Schaefer’s 2023 paper) was the similarity between the light curves around the eruptions of 1866 and 1946. The VStar plot below shows visual band data for the two eruptions in which the difference between the two eruption peaks has been added to the times of the 1866 observations.

I also showed the similarity between observations leading up to the 1946 eruption (the rise and dip) and recent observations:

and the two overlaid:

How much time remains before the next eruption is uncertain, but the signs are that it’s only weeks or in the worst case, a few months away.

T Corona Borealis — T CrB for short — is one of ten known recurrent novae. At a distance of around 3000 light years, it was first discovered as a nova eruption in 1866 by John Birmingham. Outbursts occur approximately once every 80 years. It may have been observed in 1217 and in 1787 as well.

T CrB is expected to explode again this year reaching naked eye visibility, around magnitude 2 or 3. The last outburst was in 1946. As always when we talk about astronomical objects, these events happened long ago, 3000 years ago in this case, with the evidence only expected to reach us this year due to the speed of light.

This is likely to be the brightest nova of a generation, certainly of the known recurrent novae. It will quickly rise from a visual band magnitude of 9 or 10 within a day or two to become visible to the naked for a few days, remaining a binocular object for a week or thereabouts, then returning to its pre-eruption magnitude within a month or so.

Exactly when it will brighten is uncertain and is discussed by nova expert Brad Schaefer and others in this AAVSO article, but the prediction is 2024.4 +/- 0.3, so May or June, but it could be earlier or later.

There was a pre-eruption dip in the light curve, 1.1 years before the 1946 outburst. A similar dip happened in March 2023 as shown in the last two years of T CrB observations in V and B bands, more prominent in the B band.

T CrB is located low in the north-eastern sky from Adelaide starting in the late evening. This Stellarium screenshot shows the circumstance for Apr 8 at midnight when T CrB is around 15 degrees above the horizon. Waiting an hour or two will help make observing easier with T CrB culminating at around 29 degrees above the horizon, but the region is viewable from around 11:30pm with a clear NE horizon.

This unprocessed, untracked image was taken with my DSLR (Canon 1100D, 100mm lens, f2.0, ISO 100, 10 secs) on Apr 7 at 2am, so it’s a little further rotated anti-clockwise than the Stellarium view above. The red arrow points to where T CrB will become visible and the green arrow points to alpha CrB (Alphecca). This shows the bright stars of the constellation of Corona Borealis.

Here is an AAVSO finder chart, with stars only down to magnitude 5. You’ll need to rotate it slightly clockwise to match the views above.

The comparison star marked 22 is the magnitude 2.2 star Alpheccca (alpha CrB). Izar and Arcturus (epsilon and alpha Bootis) do not appear in this finder chart, and are at upper left of the constellation Corona Borealis in this orientation. T CrB may approach Alpheccca in brightness.

I’ll be looking out for T CrB whenever I can stay up late enough or get up early enough until the outburst happens, using just the unaided eye in the first instance. Once visible in outburst, I’ll make estimates with 7×50 binoculars and time-permitting, DSLR images for subsequent photometry, submitting both to the AAVSO International Database.

Messages will also be posted on the AAVSO nova forum when the T CrB outburst happens.

I took wide field DSLR images of PNV J17261813-3809354, now Nova Sco 2024 or V1723 Sco, on February 11, 12, 15, and 17 at around 5:30am Adelaide time (ACDT) with subsequent calibration and photometry yielding observations submitted to the AAVSO International Database (AID).

The nova is marked on this image (click to expand):

Trailing is becoming apparent on this 10 second exposure, visual band image (calibrated and median stacked from a subset of images), as is the deliberate defocussing to spread the light over multiple elements of the DSLR’s Bayer array.

Here is the light curve as of February 17:

After around 9 days, there are only 116 visual band observations from observers around the world, 49 from DSLRs, 27 from some other imager, and 40 from visual observing (binoculars, telescope).

The nova seems to have peaked at around magnitude 6.8 or 6.9 and as of the time of writing (February 17) is dimmer than magnitude 8 and is steadily declining.

My 4 visual band DSLR observations are shown in purple, with the one under the cross hairs at magnitude 8.1 in close agreement re: time and magnitude with a visual observation made by Andrew Pearce (the discoverer). I have also submitted blue and red band observations, not shown above.

My imaging gear is fairly minimal, as shown below:

You can’t choose the time or sky conditions. Here are hand-held iPhone 13 images of the some of the pre-dawn skies, before and after observation on Feb 15 and Feb 11:

I recently gave a talk via ASSA@Home about making the connection between variable star observations (from the AAVSO International Database and Kepler mission) with the processes in the star systems themselves.

I gave examples of pulsating variables (Chi Cyg, RS Pup, RR Lyr, T Umi), an Algol type eclipsing binary (ASAS J035812+1629.7), the enigmatic luminous blue variable eta Car, and the recurrent nova RS Oph, showed videos linking light curves with stellar processes, inspected time series observations, created phase plots and carried out analyses such as period search and time-frequency analysis using VStar.

While not as polished and more ad hoc and exploratory than last year’s RS Oph recurrent nova eruption talk for ASSA, it was fun and seemed to be appreciated, despite less than best audio and video at times. It’s also not easy watching yourself give a talk. 🙂

As of July 24 2020, there are more than 100 Nova Ret 2020 observations, most visual, with a handful of CCD submissions. Mine (visual) are shown in purple.

Most observers are from Brazil (10), then Bolivia and Australia (5 each), with one from Italy and South Africa.

The southerly declination of the target and comparison stars in Reticulum (almost -55^o degrees) makes northern hemisphere observations difficult.

A linear fit shows an overall decline of around 0.17 magnitudes per day, but there’s a lot of spread in the data and novae are unpredictable.

I was humbled recently to be a recipient of the 2019 Merit Award from the American Association of Variable Star Observers (AAVSO), having previously received The Director’s Award in 2011 and the Bill Bradfield Award from ASSA in 2012.

Receiving the Merit Award at Melbourne University from AAVSO Director, Stella Kafka, Dec 10 2019

Patrick Wills also received the AAVSO Merit Award this year for his work on the AAVSO Variable Star Index, a database that VStar also uses, including the web services Patrick has created.