A Predicted
Great Dimming of T Tauri: Has it Begun?
By Tracy L. Beck
Accepted for Publication in
the Astronomical Journal
DOI: 10.3847/1538-3881/ad9a88
Abstract:
The optical star in the T
Tauri triple system is the prototype of young sun-like stars in our galaxy.
This complex and dynamic system has evidence for misaligned disks and outflows,
and molecular material in a circumbinary ring that obscures the southern
infrared binary, T Tau South. Observations by members of the American
Association of Variable Star Observers (AAVSO) show that T Tau North, the
optical star, has dimmed by up to ~2 magnitudes in the visual over the course
of the past decade. The dimming across the B, V, R and I bands has a color
character typical of changes in ISM extinction, suggesting an increase in
obscuration along the line of sight to T Tau North. Material associated with
the circumbinary ring around T Tau South has been predicted to occult the
optical star via wide-scale orbital motion of the system. Through analysis of
the geometrical configuration and motion of dust structures in the system, it
seems that a great dimming of T Tau North by line-of-sight material associated
with the T Tau South binary has, in fact, begun. Based on the extent and motion
of the circumbinary ring material associated with the southern binary, T Tau
North will likely experience dimming events for decades to come and may
disappear from the optical sky as the densest mid-plane region of the ring
traverses our line of sight.
Introduction:
T
Tauri (or T Tau) was first identified as an interesting astronomical source in
the 1850's because of 
its variable flux, and the nearby nebula which
would also change in brightness ("Hind's variable nebula", Hind 1864;
see Figure 1). In 1949, Alfred Joy used
T Tauri as the prototype of a new class of stars, the "T Tauris",
typified by their emission line spectra and variable nature (Joy 1949). Since that time, it has been established that
T Tauri and the other defining members of this class are in fact sun-like stars
in the early stages of forming. The T
Tauris are young stars, less than about 2 solar masses and younger than about 3
million years old.
The
optical wide-field view of T Tau shown in Figure 1 (right) spans about 2.5
arc-minutes on the sky, or about 3600 astronomical units (AU).
In
1980, T Tauri was found to be a double star.
This new companion called "T Tau South" was detected only in
infrared light; it is not seen in the optical.
In projected separation on the sky, T Tau South is approximately 100 AU
to the south of T Tau North, the optical star.
In
1997, T Tau South ( T Tau S) was itself found to be
two stars (Sa and Sb), making the T Tauri system a young triple star (see
Figure 2). The T Tau S binary has an
average separation of 12 AU, and a binary orbital period of 27 years. This southern binary is slowly orbiting T Tau
North (T Tau N), in a wide-scale orbit that will take thousands of years to
complete. Figure 2 shows a zoomed in
view of the three stars in the T Tau triple system. T Tau N, the optical star, is brightest in
this image but both stars in the T Tau S binary vary significantly in their
infrared flux output.
The
narrow infrared view shown in Figure 2 spans about 1.5 arc-seconds on
the sky, or about 200 AU.
The
Tau South binary is optically obscured and seen only in infrared and longer
wavelengths. This is because it is observed
through approximately 20 magnitudes of extinction from a circumbinary ring of
gas and dust that is viewed nearly edge-on.
T Tau S is closer to us; it is foreground
to T Tau N.
Ultraviolet
imaging and spectroscopy from the Hubble Space Telescope has revealed the
circumbinary ring around T Tauri South as a silhouette edge-on disk nebula - a
dark lane of no emission seen with lobes of light on either side (see Figure
3). T Tau S is in the foreground, T Tau N is thought
to be at least 300-400AU behind T Tau S (Beck et al. 2020).
New Results:
Extensive
optical stellar brightness measurements are available through the database of
the American Association of Variable Star Observers (AAVSO; www.aavso.org). Thousands
of visible light measurements were collected from 1970 to the present for T Tau
(see Figure 4). From 1970 to ~2016, T
Tau varied by ~0.5 magnitudes around an average brightness level of about 10.2
magnitude. From ~2017 to the present,
the optical light from T Tau has dimmed significantly and abruptly, by up to 2
magnitudes. Although T Tau is known as a
historical variable star, this significant and rapid drop in optical brightness
of T Tau North hasn’t been seen in over a century.
Multiple
brightness measurements in different color bands reported by observers of the AAVSO
show that this recent dimming of T Tau N has a “redder when faint”
character. This dimming and simultaneous
reddening can be explained by an increase in dusty material along the line of
sight to T Tau N. Small astrophysical
dust grains scatter ultraviolet and blue optical light more efficiently than
red and infrared light. Hence dimming
events caused by intervening dust are accompanied by such a reddening in the
color of the star, as seen for T Tau.
Figure 4: An animated light
curve of over 22,000 visual magnitude brightness measurements of T Tauri from
1970 through 2024.
What is causing the recent dimming of T Tau N?
A future optical dimming of T Tau N was first
speculated in 2003 (Walter et al. 2003), and predicted to occur soon by Flores
et al (2020) and Beck et al. (2020). A
closer look at the system geometry and the orbital motion of this triple star
presents a clear case for the cause of this new dimming event (See Figure 5).
Figure 5: An animation that
qualitatively represents the geometry and orbits in the T Tau triple system,
and the future orbital motion. As the
close binary goes through more than one orbit, the wider orbital motion of T
Tau S relative to T Tau N also progresses.
The circumbinary ring of gas and dust around the T Tau S binary occults
the optical star, T Tau N, causing it to dim.
(Sizes of the stars and the close orbit are exaggerated for clarity).
Over
the next several decades, the T Tau S binary will move toward the west on the
sky in the wide orbit relative to T Tau N.
As this happens, the extended regions of the circumbinary ring that obscures
the T Tau S binary are starting to occult T Tau N. This will dim the total optical flux from T
Tau that we see here on earth. The
beginning of this process is the likely cause of the recent 2 magnitude
decrease in brightness seen for T Tau N since 2016 (See Figure 4).
What can we expect in the future?
In
the future, T Tau S and its circumbinary ring will continue to traverse our
line of sight through wide-scale orbital motion, and will hence continue to dim
the brightness of T Tau N.
How
long will this last? And how faint will T Tau N get?
The
answers to these interesting questions are not yet known in detail because they
depend very significantly on the overall size, structure and density of the
outer circumbinary ring around T Tau S. Yet,
some simple estimates and predictions can be made.
As
T Tau S moves relative to T Tau N the wide orbit, it has been found to traverse
an average of 100 milli-arcseconds (0.1 arc second or about 14 AU) on the sky in a
decade (Schaefer et al. 2020). From the
UV image of T Tau presented in Figure 3, the extent of the circumbinary ring in
the direction of the motion of T Tau S is approximately 1 arc second or
140AU. Hence, at the rate of apparent
motion of T Tau S relative to T Tau N, we can expect dimming events from the
circumbinary ring to last on the order of ~100 years.
T
Tau S is obscured to optical invisibility by ~20 magnitudes of gas and dust in
the circumbinary ring. T Tau S is viewed
only through the foreground material in the ring; the background material is
behind the T Tau S binary. Hence, the
full line of sight through the front and the back of the ring could have as
much as 40 magnitudes of gas and dust.
As T Tau S and the extended circumbinary ring continues along the
relative motion in the wide orbit, we could be viewing T Tau N through the full
extent of this 40 magnitudes of gas and dust. Alternatively, it’s possible that we will be
viewing T Tau N through lower density material in the outer region of the ring. Considering the 10.2 average magnitude of T
Tau N before the dimming events, if the obscuring material in the outer region
of the ring is more than ~15 magnitudes, then T Tau N will temporarily disappear
from the optical night sky.
Why is this important?
There
has never before been an occurrence where a young star system has completely
disappeared from the optical night sky because of orbital motion and occultation
of gas and dust disks. Several young stars (or their companion stars) have
experienced dimming events from material in their own circumstellar disks or in
their environments. But these events
have never resulted in complete optical disappearance of the system.
It will be unprecedented if T Tau N does dim
to the point of optical invisibility, making this prototype of the T Tauri
class of young stars observable only at infrared or longer wavelengths.
Over
the next century, monitoring of the absorption features in the spectra of T Tau
N as the circumbinary ring passes through our line of sight provides the
exciting opportunity to study the composition of the gas and icy dust grains in
these outer regions of the ring. With
the ~100AU projected distance relative to T Tau S, we will be able to study the
composition and chemistry of potential planetesimal forming material around
this triple star at a similar formation distance as the Kuiper belt in our own
solar system.
It
is going to be fascinating to watch the T Tauri triple star system in the
coming decades!
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