From jop at astro.columbia.edu Fri Jun 14 22:17:53 2019 From: jop at astro.columbia.edu (Joe Patterson) Date: Fri, 14 Jun 2019 22:17:53 -0400 Subject: (cba:news) AM CVn and HP Lib... and new stars for June Message-ID: <0048670a-4adc-7853-865d-56e11a5386bf@astro.columbia.edu> Hi CBAers, Old and new campaigns..... 1. AM CVn. Great coverage all around, and I've used it to establish a unique cycle count for the orbital signal, going back to 1978. The period is slowly decreasing... and we can definitely quit now. 2. HP Lib. Another winner. No long-term ephemeris (yet), but a clear detection of the orbital signal, despite its meager amplitude (0.005 mag peak-to-trough). Amazing how crowd-sourcing brings out the really weak signals. Still a worthy target for southerners (~ another month?) , but getting borderline for northerners. FOR BOTH OF THESE STARS (AM and HP): if you've been observing these stars, can you study your data for presence/absence of night-to-night variability, and send me the result? My time-series analysis is maximally sensitive if I subtract the mean light (and trends). And because different people observe with different filters, airmasses, and comparison stars, there's no straightforward way to compare them. You can, though, with your own data. I'm basically interested in just one thing: how much night-to-night variability do these two stars have? Not within the time series, but *night-to-night*. It's very low, but HOW low? Because both stars are very blue, they will appear slightly fainter at higher airmass. I estimate a likely 0.08 mag/airmass correction (if you're inclined to apply it). BTW this is a correction to the *differential* magnitude (it would be zero if the comparison star were equally blue). This correction is not worth fussing over, if you observe with under fairly repeatable circumstances (airmass and comparison star). 3. CR BOO. Time to quit on this one. Unlike the other two helium variables, this one varies a lot - fast, slow, periodic, nonperiodic - and it's a project to figure it out. It has never done us the courtesy of staying put for a few weeks on end. And new stars (first installment). 4. MAXI J1820+070 = ASASSN-18ey, the great X-ray nova of 2018. Back at 18th mag now... but there's a good chance of seeing the orbital signal now, since the X-rays are gone. We know the period is *near* 17 hours (because of the superhumps), but now the star is probably ripe for measurement of the actual Porb. 5. V339 DEL. A recent nova ripe for Porb measurement. 6. V1974 CYGNI. One of our favorites. Dubbed "the nova of the century" in 1992. Let's make it surrender some secrets *this* century, too. This star has three clocks: orbit, positive superhump, negative superhump. Other good seasonal stars in a day or two. As usual, an optimum strategy for these stars is: pick a favorite, and follow as long as airmass and sky conditions tolerate. joe Also, since most of you observe (mostly) unfiltered, can you take an hour to observe the star through a V filter and establish a calibrated V brightness. No time variability needed, just one number! ____________________________________________________________ Center for Backyard Astrophysics (CBA) mailing lists https://cbastro.org/communications/mailing-lists/ From jop at astro.columbia.edu Tue Jun 18 11:01:40 2019 From: jop at astro.columbia.edu (Joe Patterson) Date: Tue, 18 Jun 2019 11:01:40 -0400 Subject: (cba:news) comments re HP Lib... and AM CVn Message-ID: Hi CBAers, These stars - one celebrated, and the other deserving to be - are really unique among CVs. Their orbital periods are very short of course (17 and 18 minutes), they have dominant superhumps, and the spectra sre basically pure helium. But the property I find most remarkable, and never commented on, is the lack of variability in the light curve - apart from the orbital and superhump periods. The "flickering" never exceeds 0.02 mag, and night-to-night variations never exceed ~0.05 mag. The underlying reason for such constancy is a puzzle, but not yet one with a convincing explanation. I thought I would append one long night of HP Lib light curve, courtesy of Gordon Myers, to illustrate what the raw light curve looks like. You can see the effect of atmospheric extinction (actually differential extinction, arising from the bluer color - and thus greater extinction - of HP Lib). To analyze the light curve(s) over weeks, as we do, we (I) have to correct for this extinction - although sometimes by simply *deleting* high-airmass observations, since data quality sometimes degrades quickly there. Anyway, you can see the 0.05 mag superhumps, and the extinction. With a lot of data spread around the world, you can dig out the orbital signal as well, although it's ~7X weaker (and thus invisible, except in a power spectrum). Gordon and Berto have been carrying most of the water on this one, with LONG time series. Great evening target in April-May-June. joe ____________________________________________________________ Center for Backyard Astrophysics (CBA) mailing lists https://cbastro.org/communications/mailing-lists/ From jp42 at columbia.edu Tue Jun 18 11:22:00 2019 From: jp42 at columbia.edu (Joe Patterson) Date: Tue, 18 Jun 2019 11:22:00 -0400 Subject: (cba:news) Fwd: comments re HP Lib... and AM CVn In-Reply-To: References: Message-ID: <90920112-bd0f-b837-3b61-c58f131e7ab2@columbia.edu> oops, now it's attached. Hi CBAers, These stars - one celebrated, and the other deserving to be - are really unique among CVs. Their orbital periods are very short of course (17 and 18 minutes), they have dominant superhumps, and the spectra sre basically pure helium. But the property I find most remarkable, and never commented on, is the lack of variability in the light curve - apart from the orbital and superhump periods. The "flickering" never exceeds 0.02 mag, and night-to-night variations never exceed ~0.05 mag. The underlying reason for such constancy is a puzzle, but not yet one with a convincing explanation. I thought I would append one long night of HP Lib light curve, courtesy of Gordon Myers, to illustrate what the raw light curve looks like. You can see the effect of atmospheric extinction (actually differential extinction, arising from the bluer color - and thus greater extinction - of HP Lib). To analyze the light curve(s) over weeks, as we do, we (I) have to correct for this extinction - although sometimes by simply *deleting* high-airmass observations, since data quality sometimes degrades quickly there. Anyway, you can see the 0.05 mag superhumps, and the extinction. With a lot of data spread around the world, you can dig out the orbital signal as well, although it's ~7X weaker (and thus invisible, except in a power spectrum). Gordon and Berto have been carrying most of the water on this one, with LONG time series. Great evening target in April-May-June. joe ____________________________________________________________ Center for Backyard Astrophysics (CBA) mailing lists https://cbastro.org/communications/mailing-lists/ ____________________________________________________________ Center for Backyard Astrophysics (CBA) mailing lists https://cbastro.org/communications/mailing-lists/ From jop at astro.columbia.edu Tue Jun 18 15:34:50 2019 From: jop at astro.columbia.edu (Joe Patterson) Date: Tue, 18 Jun 2019 15:34:50 -0400 Subject: (cba:news) the missing attachment, I hope... Message-ID: <76704e6f-e6fd-9c8e-181c-6a0a9374157d@astro.columbia.edu> Several crises erupted this morning here, so here's a third try... joe ____________________________________________________________ Center for Backyard Astrophysics (CBA) mailing lists https://cbastro.org/communications/mailing-lists/