CD-25 11111B / IRAS 15462-2551 / PDS 144N
Scorpius
Circumstellar protoplanetary disk around an intermediate-mass YSO
RA 15:49:15.5
DEC -26:00:50.2
Vmag +14.2
Size 1" x 1"
Dist 475 light-years
Part I – Discovery, Confirmation, and Study
Earlier this year, I learned more about a type of object once only conjectured [0], then eventually found visible using the largest of telescopes, and now able to be spotted in 6-inch telescopes. However, while some of the brightest in the sky have been found in the last two decades, few amateur astronomers have laid eyes on them. I’m talking about protoplanetary disks, which are literally still-forming planetary systems.
My first encounter came in 2022 when I observed one in the Orion Nebula with a friend’s 36-inch and then my 16-inch [1]. But the situation with those is a bit extreme since they are having their disks ionized and dissipated largely by the intense ultraviolet radiation emitted from Theta Orionis C [2]. Thus, the only reason any of them are visible at that distance is because of external illumination and is the reason the term proplyd only refers to those that are “rendered visible through inclusion in our proximity to an H II region.” [3] What professional astronomers have been striving to find are those that might mimic more closely what our Sun and planetary system looked like around 4.6 billion years ago [4].
If we see such circumstellar disks face-on, they can display a variety of shapes, from concentric rings to swirling spirals. But if we see them edge-on, they look very different. The thick material along the equator of the disk blocks the starlight, so we can only see the parts of the disk above and below the midline, usually lit by scattered starlight.
One of the brightest known in visual wavelengths lies in extreme western Scorpius and goes by the designation PDS 144N. But for a full century, starting in the late 1800s, it was known as the faint star Córdoba Durchmusterung (the German word for “survey”) -25 11111 [5]. Only in 1992 was it found to be a double star (+13.1/+14.2 5.35”) and young stellar object (YSO) candidate [6]. That’s when the trio of Carballo, Wesselius, and Whittet matched up an object from the IRAS Point Source Catalogue with it while attempting to identify objects from that catalogue in the Scorpius-Centaurus association (Sco OB2) [7].
PDS 144 Scorpius.jpg
Location of PDS 144
This got it included as an object for spectroscopic follow-up in the Pico dos Dias Survey (PDS), which was a search for T Tauri stars at the Pico dos Dias Observatory in Brazil [8]. The second set of results from that survey was published in 1995 by Torres et al and they believed PDS 144S to be a “new and suspected Herbig Ae/Be star” since a feature common to such stars is the presence of infrared excess due to the thermal reradiation, usually explained by the presence of an accretion disk or an almost symmetric circumstellar halo. However, they were unable to take the spectra of PDN 144N (the “faint companion”) due to its faintness.
During an investigation of Herbig Ae/Be candidate stars in 2003 [9], Vieira et al found PDS 144S to be spectral type A5V and PDS 144N to be spectral type A2 IV. And in addition to the Ha emission signature of accretion, they found certain emission lines in the spectra of both stars that would indicate a photodissociation region around them.
In the summer of 2004, Marshall Perrin and colleagues used the 3-meter telescope at Lick Observatory to image PDS 144 [10]. They were performing an adaptive optics polarimetry survey of Herbig Ae/Be stars and on the night of July 5th, the seeing was excellent despite having to observe PDS 144 through an air mass of 2.25. The image they obtained – displayed in levels of polarized intensity – showed that while PDS 144S was point-like, PDS 144N was resolved at all wavelengths as a small nebula. In fact, in certain photometric bands, it appeared as an elliptical nebula bisected by a dark lane!
The idea that they might have found the brightest known edge-on circumstellar disk was so incredible that they quickly requested time on the twin Keck Observatories. And they got just such on the last days of August (though they did have to make a trade for observing time with one astronomer). Using the adoptive optics on the Keck II, they focused on the disk of PDS 144N by choosing exposure times to maximize signal-to-noise ratio, which left PDS 144S saturated in nearly all wavelengths.
apj411842f2_hr.jpg
This Ha velocity scan image from the Goddard Fabry–Pérot instrument on the ARC 3.5 m at Apache Point Observatory illustrates the radial velocity of the bipolar jets of the PDS 144 stars, and HH 689d and 689e. The three-color image is composed of blue: Ha - 114 ± 60 km s -1, green: Ha ± 60 km s -1, and red: Ha + 114 ± 60 km s -1. HH 689e lacks any blue component and is therefore pointed away from us while HH 689d lacks any red component and is therefore pointed toward us.
The image they got revealed a disk occulting PDS 144N – the first edge-on disk observed around a Herbig Ae star. In fact, we are viewing the disk only about 7° from perfectly edge-on. As for PDS 144S, it was found to be emitting an excess of infrared radiation, but the diffracted and scattered light of PDS 144S swamped any signal from a disk. However, PDS 144N was the only one that showed emission of polycyclic aromatic hydrocarbons (PAH), a class of organic compounds, in its spectra.
With a distance to PDS 144 that was “frustratingly uncertain”, Perrin and colleagues dutifully considered if they were in fact evolved post-main-sequence objects rather than young stars. This had to be done since protoplanetary nebula appear similar to pre-main-sequence A-type stars in several aspects, such as infrared excess from dust, the presence of PAH emission, and optical forbidden lines such as [S II]. In the end, they rejected such an interpretation because the morphology of PDS 144N more closely resembled edge-on disk systems and because both stars displayed an infrared excess and optical forbidden lines. They argued that “since their spectral types and presumably their masses differ, we would not expect them to both evolve off the main sequence simultaneously. It is much more plausible for the pair to be young stars approaching the main sequence together rather than departing it.”
PDS 144 with Three Telescopes.jpg
The best images ever taken of PDS 144
The question now was not simply could a circumstellar disk be imaged around PDS 144S but whether or not PDS 144 was a binary pair. Using the Hubble Space Telescope, two sets of images were taken almost 4 years apart [11]. The first was in 2006 and the second was in 2010. In the study, published in 2012 by Hornbeck et al, it was found that instead of an increased separation (which was expected per the proper motion reported in the UCAC3), their proper motions were nearly identical and consistent with the mean proper motion of the Upper Scorpius OB association (which is to the southwest). Using the mean distance estimate for Upper Sco of 475 light-years, the authors calculated that the dark lane of PDS 144N has a disk diameter of 106 AU and a disk height of 22 AU. They also noted that such a diameter was only slightly larger than that of our own solar system’s Kuiper Belt.
As for PDS 144S, by studying the jets it’s driving (which are closely aligned with those of PDS 144N), it would seem to have a disk around 17° from edge-on. And yet, the HST was unable to image any hint of a circumstellar disk! However, at such inclinations, disks are not typically visible in direct light imagery since the star, while reddened, may not be occulted by the disk. But using the Gemini Observatory, Perrin et al had already obtained data that hinted at the disk around PDS 144S being 22° from face-on [12].
In a 2017 study by Terada & Tokunaga, water ice was detected in the protoplanetary disk of PDS 144N but not that of PDS 144S [13]. This finding indicates that the “critical inclination angle to show the water ice absorption is larger in Herbig Ae disks than in low-mass YSOs…[and] could be due to the photodesorption of ice by the harsher far-ultraviolet radiation form the Herbig Ae stars.”
Part II – Observations:
Observationally, PDS 144N is simply the “secondary” of a 5.35”-wide double star. I found it for the first time myself early this year. With my 10-inch at 260x, it can be split from the “primary” when there’s good seeing that low in the sky. With my 16-inch, it’s a lot easier to see and at 440x the pair (PDS 144) is a wide, easy to split double star.
But while searching for PDS 144N might hardly seem worth it since it’s a faint point if light in the eyepiece, one must keep in mind that you are seeing the first confirmed Herbig Ae-Herbig Ae binary with one displaying a disk that can be spatially resolved and studied in professional telescopes. Plus, the two stars are among a small set of Herbig Ae stars known to drive jets associated with parsec-scale chains of HH knots. Incredible! I never thought I’d be able to see a protoplanetary disk with an intermediate-mass YSO being obscured at its center with my own eyes.
So, please “Give it a go and let us know” that you, too, are as excited as I am to have seen it!
[0] Nebula Hypothesis (https://en.wikipedia.org/wiki/Nebular_hypothesis)
[1] February 12, 2023 OotW (https://www.deepskyforum.com/showthr...-Orion-Nebulaj)
[2] Bally et al 1998 (https://ui.adsabs.harvard.edu/abs/19....293B/abstract)
[3] O’Dell 1998 (https://ui.adsabs.harvard.edu/abs/19....263O/abstract)
[4] Solar System’s age (https://www.astronomy.com/science/ho...r-system-form/)
[5] Thome 1892 (https://ui.adsabs.harvard.edu/abs/18......1T/abstract)
[6] Carballo, Wesselius, & Whittet 1992 (https://ui.adsabs.harvard.edu/abs/19....106C/abstract)
[7] Melnik & Dambis 2020 (https://ui.adsabs.harvard.edu/abs/20....112M/abstract)
[8] Torres et al 1995 (https://ui.adsabs.harvard.edu/abs/19...2146T/abstract)
[9] Vieira et al 2003 (https://ui.adsabs.harvard.edu/abs/20...2971V/abstract)
[10] Perrin et al 2006 (https://ui.adsabs.harvard.edu/abs/20...1272P/abstract)
[11] Hornbeck et al 2012 (https://ui.adsabs.harvard.edu/abs/20.....54H/abstract)
[12] Perrin et al poster 2006 (https://www.stsci.edu/~mperrin/pdfs/...sks_PDS144.pdf)
[13] Terada & Tokunaga 2017 (https://ui.adsabs.harvard.edu/abs/20....115T/abstract)
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