Disk Braking in young Stars: Probing Rotation in Chamaeleon i and Taurus-Auriga

Abstract

We present a comprehensive study of rotation, disk, and accretion signatures for 144 T Tauri stars in the young (~2 Myr old) Chamaeleon I and Taurus-Auriga star-forming regions based on multi-epoch high-resolution optical spectra from the Magellan Clay 6.5 m telescope supplemented by mid-infrared photometry from the Spitzer Space Telescope. In contrast to previous studies in the Orion Nebula Cluster and NGC 2264, we do not see a clear signature of disk braking in Tau-Aur and Cha I. We find that both accretors and non-accretors have similar distributions of vsin i. This result could be due to different initial conditions, insufficient time for disk braking, or a significant age spread within the regions. The rotational velocities in both regions show a clear mass dependence, with F-K stars rotating on average about twice as fast as M stars, consistent with results reported for other clusters of similar age. Similarly, we find the upper envelope of the observed values of specific angular momentum j varies as M 0.5 for our sample which spans a mass range of ~0.16-3 M ☉. This power law complements previous studies in Orion which estimated j M 0.25 for 2 Myr stars in the same mass regime, and a sharp decline in j with decreasing mass for older stars (~10 Myr) with M < 2 M ☉. Furthermore, the overall specific angular momentum of this ~10 Myr population is five times lower than that of non-accretors in our sample, and implies a stellar braking mechanism other than disk braking could be at work. For a subsample of 67 objects with mid-infrared photometry, we examine the connection between accretion signatures and dusty disks: in the vast majority of cases (63/67), the two properties correlate well, which suggests that the timescale of gas accretion is similar to the lifetime of inner disks.

Full text

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arXiv:0902.0001v1 [astro-ph.SR] 2 Feb 2009
Received 2008 October 27; accepted by ApJ 2009 January 29
Preprint typeset using LATEX style emulateapj v. 03/07/07
DISK-BRAKING IN YOUNG STARS: PROBING ROTATION IN CHAMAELEON I AND TAURUS-AURIGA
Duy Cuong Nguyen1, Ray Jayawardhana1, Marten H. van Kerkwijk1, Alexis Brandeker2, Alexander Scholz3,
Ivana Damjanov1
Received 2008 October 27; accepted by ApJ 2009 January 29
ABSTRACT
We present a comprehensive study of rotation, disk and accretion signatures for 144 T Tauri stars in
the young (∼2 Myr old) Chamaeleon I and Taurus-Auriga star forming regions based on multi-epoch
high-resolution optical spectra from the Magellan Clay 6.5 m telescope supplemented by mid-infrared
photometry from the Spitzer Space Telescope. In contrast to previous studies in the Orion Nebula
Cluster and NGC 2264, we do not see a clear signature of disk braking in Tau-Aur and Cha I. We
find that both accretors and non-accretors have similar distributions of v sin i. This result could
be due to different initial conditions, insufficient time for disk braking, or a significant age spread
within the regions. The rotational velocities in both regions show a clear mass dependence, with F–K
stars rotating on average about twice as fast as M stars, consistent with results reported for other
clusters of similar age. Similarly, we find the upper envelope of the observed values of specific angular
momentum j varies as M0.5for our sample which spans a mass range of ∼0.16M⊙to ∼3M⊙. This
power law complements previous studies in Orion which estimated j ∝ M0.25for ? 2Myr stars in
the same mass regime, and a sharp decline in j with decreasing mass for older stars (∼10Myr) with
M <2M⊙. Furthermore, the overall specific angular momentum of this ∼10Myr population is five
times lower than that of non-accretors in our sample, and implies a stellar braking mechanism other
than disk braking could be at work. For a subsample of 67 objects with mid-infrared photometry, we
examine the connection between accretion signatures and dusty disks: in the vast majority of cases
(63/67), the two properties correlate well, which suggests that the timescale of gas accretion is similar
to the lifetime of inner disks.
Subject headings: stars: pre–main sequence — stars: formation — stars: rotation — circumstellar
matter — accretion, accretion disks — stars: evolution — stars: statistics
1. INTRODUCTION
One of the major outstanding issues in star forma-
tion theory is the regulation of angular momentum in
young stars. The specific angular momentum of young
single stars at ∼ 1Myr is about four orders of magni-
tude lower than in molecular cloud cores, from which the
stars formed, indicating efficient rotational braking in the
early phases of stellar evolution (Bodenheimer 1995). In
this context, a large number of studies have explored the
connection between the presence of disks and rotation
(Herbst et al. 2007).
Disk braking is defined here as a process that provides
rotational braking based on magnetic coupling between
the star and the disk. One possible theoretical scenario
for disk braking is ‘disk-locking’, originally proposed for
T Tauri stars by Camenzind (1990), Koenigl (1991), and
Shu et al. (1994). In that case, the magnetic connection
between the star and the disk produces a torque onto the
star, transfering angular momentum to the disk (presum-
ably, from where it is eventually removed by, e.g., mag-
netically driven winds). An alternative scenario for disk
braking is stellar winds powered by accretion, as recently
modeled by Matt & Pudritz (2005). For a more detailed
overview of the theoretical work on disk braking, see for
Electronic address: nguyen@astro.utoronto.ca
1Department of Astronomy & Astrophysics, University of
Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
2Department of Astronomy, Stockholm Observatory, SE-106 91
Stockholm, Sweden
3SUPA, School of Physics & Astronomy, University of St. An-
drews, North Haugh, St. Andrews, KY16 9SS, United Kingdom
example the review by Matt & Pudritz (2008).
If disk braking is at work, we expect to observe three
kinds of stars: slow rotators with disks, slow rotators
without disks, and fast rotators without disks. This dis-
tribution corresponds to the following evolutionary se-
quence: while stars are coupled to their disks, they will
rotate slowly; once stars lose their disks, they will con-
tinue to rotate slowly for some time but gradually spin-up
as they contract towards the main sequence, with some
stars eventually becoming fast rotators. Thus, rapidly
rotating stars with disks should not exist in an ideal disk
braking scenario.
Observationally, the evidence for disk braking is con-
fusing. Some photometric studies found a correla-
tion between rotational properties and near-infrared
color excess suggestive of disks (e.g. Edwards et al.
1993; Herbst et al. 2002), whereas others have not (e.g.
Stassun et al. 1999; Makidon et al. 2004). The photo-
metric monitoring program of Lamm et al. (2005) ob-
served disk braking in ∼2–3Myr NGC 2264, but with the
effect less pronounced for low mass stars. Recent studies
using Spitzer mid-infrared observations of the ∼ 1Myr
Orion Nebula Cluster (ONC) and NGC 2264 support
a disk-rotation connection: stars with longer rotation
periods were found to be more likely than those with
short periods to have IR excesses (Rebull et al. 2006;
Cieza & Baliber 2007). However, the mid-infrared study
by Cieza & Baliber (2006) of IC 348 did not find the
preferential distribution of rotation with disk presence.
While both near-infrared and mid-infrared signatures
indicate the presence of a dusty disk, they do not prove

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2Nguyen et al.
the coupling between star and disk as required by the
disk braking scenario. To demonstrate a direct link be-
tween the inner disk and the central star, a better diag-
nostic for disk braking may be ongoing accretion. For
strongly active accretors, rotation periods are difficult to
determine since period measurements rely on the pres-
ence of stable starspot regions; therefore, period samples
may be biased towards weakly accreting stars. In some
respects, it is advantageous to use projected rotational
velocity (v sin i) instead of rotation periods.
A recent spectroscopic study of disk accretion in low-
mass young stars by Jayawardhana et al. (2006) found
evidence of a possible accretion-rotation connection in
the η Cha (∼ 6Myr) and TWA (∼ 8Myr) associations.
All accretors in their sample of 41 stars were slow ro-
tators, with v sin i ? 20 km s−1, whereas the non-
accretors showed a large span in rotational velocities, up
to 50 km s−1. However, given the small number of accre-
tors, they caution that those results should be checked
with larger samples. A larger study of solar-like mass
stars in NGC 2264 by Fallscheer & Herbst (2006) found
disk braking signatures in using UV excess indicative of
accretion. For a review of recent observational studies on
rotation and angular momentum evolution of young stel-
lar objects and brown dwarfs, see Herbst et al. (2007).
As part of a comprehensive, multi-epoch spectroscopic
survey, we present here a study of rotation and disk-
braking at ages of ∼ 2Myr in the star forming regions
Taurus-Auriga and Chamaeleon I. This study comprises
144 stars, which significantly enlarges the previously
available sample of spectroscopic data in those two re-
gions (see the summary by Rebull et al. (2004) for cur-
rently available rotational data). From the spectra, we
extract v sin i, and, as accretion indicators, the full width
of Hα at 10% of the peak (hereafter, Hα 10% width) and
Ca II fluxes. We investigate the distribution of v sini,
estimate angular momentum values, and test for the sig-
nature of disk braking.
2. TARGET SELECTION AND OBSERVATIONS
We used 572 high-resolution optical spectra of 144
members in the ∼2Myr old Chamaeleon I and Taurus-
Auriga star forming regions obtained with the echelle
spectrograph MIKE (Bernstein et al. 2003) on the Mag-
ellan Clay 6.5 meter telescope at the Las Campanas Ob-
servatory, Chile. The data were collected on 15 nights
during four observing runs between 2006 February and
2006 December. We complemented our optical spec-
tra with infrared measurements from the InfraRed Ar-
ray Camera (IRAC; Fazio et al. 2004) aboard the Spitzer
Space Telescope.For Cha I, we used the results of
Damjanov et al. (2007), and for Tau-Aur we analyzed
publicly available images obtained between 2004 Septem-
ber and 2007 March, using the methods described in de-
tail by Damjanov et al. (2007). Our results are listed in
tables 1 & 2.
Our sample consists of a magnitude-limited subset (R
≤ 17.6 for Cha I; R ≤ 13.4 for Tau-Aur) of targets from
Leinert et al. (1993), Ghez et al. (1993), Simon et al.
(1995), Kohler & Leinert (1998), Brice˜ no et al. (2002),
and Luhman (2004a,b). To isolate the possible influence
of binarity on disk braking in this study, we excluded
from our sample unresolved wide binaries and double-
lined spectroscopic binaries. Our targets span the spec-
tral type range from F2 to M5 based on published classi-
fications. In addition, we observed a sample of 25 slowly
rotating velocity standard stars selected from the list of
Nidever et al. (2002); these cover the same spectral range
as our targets. We determined the spectral type for 13
targets without prior classification by fitting their spec-
tra against those of the standard stars, and identifying
the best fits.
MIKE is a slit-fed double echelle spectrograph with
blue and red arms. For this study, we used only the
red spectra, which cover the range of 4900–9300˚ A in
34 spectral orders. The 0.35′′slit was used with no bin-
ning to obtain the highest possible spectral resolution,
R ∼ 60000. The pixel scale was 0.14′′pixel−1in the
spatial direction, and approximately 0.024˚ Apixel−1at
6500˚ A in the spectral direction.
tial direction of the projected slit is wavelength depen-
dent, and not aligned with the CCD columns. To extract
these slanted spectra, we used customized routines run-
ning in the ESO-MIDAS environment (described in detail
in Brandeker et al., in preparation). Integration times
were chosen such that we obtained signal-to-noise ratios
(S/N)> 30 per spectral resolution element at 6500˚ A;
they typically ranged from 60 to 1200 seconds depend-
ing on seeing.
In MIKE, the spa-
3. ANALYSIS
3.1. Accretion Signatures
The Hα equivalent width (EW) has long been used
to distinguish accreting or classical T Tauri stars (EW
> 10˚ A) from non-accreting or weak-line T Tauri stars
(EW < 10˚ A). In accretors, in the context of the mag-
netospheric accretion scenario, the Hα emission arises
largely from the gas falling in from the inner disk edge
onto the star. In non-accretors, chromospheric activity
is the main source of Hα emission, and is thus gener-
ally weaker. The Hα profiles of accretors also tend to be
much broader than those of non-accretors due to the high
velocity of the infalling gas and Stark broadening. (The
latter is expected to be important in Hα, since the line
optical depths are high; see Muzerolle et al. (2001) for
further discussion.) Asymmetry in the Hα profile of ac-
cretors is also commonly observed as a result of viewing
geometry, absorption by a wind component, or both.
Since the Hα EW depends on the spectral type,
White & Basri (2003) proposed to use as a more robust
accretion diagnostic the Hα 10% width. By comparing
this measurement with veiling in their stellar spectra,
they found that a Hα 10% width larger than270 km s−1
reliably indicates accretion. A less conservative accretion
cutoff of 200 km s−1was adopted by Jayawardhana et al.
(2003) for their study of young very low mass objects,
based on empirical results and physical reasoning; how-
ever, they cautioned that it should be used in combina-
tion with additional diagnostics whenever possible. In
later studies, it was found that Hα 10% width not only
appears to be a good qualitative indicator of accretion
but also correlates with the mass accretion rates derived
by other means: the 200 km s−1threshold corresponds to
a mass accretion rate of ∼10−11M⊙yr−1(Natta et al.
2004).
For this study, we use the Hα 10% width as one ac-
cretion diagnostic, which we computed as follows. First,

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Disk-Braking in Young Stars3
we estimated the continuum level at Hα by linearly in-
terpolating between flux measurements in the range of
500 km s−1to 1000 km s−1on either side of the line.
Next, the maximum flux level of Hα emission was mea-
sured with respect to this continuum level. Finally, the
crossing points of the Hα emission with the 10% flux
level are identified, and the width was measured. The
results for our targets are listed in tables 1 & 2; we note
that for some objects, the measurements were uncertain,
e.g. because of absorption components or double-peaked
profiles with one peak close to 10% of the height of the
main peak. Note, however, this is not critical: all these
sources are clearly accretors.
To obtain mass accretion rates, we use the Ca II-λ8662
emission fluxes (FCa I I) which have been shown to be a
more robust quantitative indicator of accretion than Hα
10% width (Nguyen et al. 2008, submitted to ApJL).
We derived the fluxes from the observed emission equiva-
lent widths. To determine the widths, we integrated the
emission above the continuum level. For emission pro-
files attenuated by a broad absorption feature, we used
the median flux within 0.2˚ A of the absorption minima
as an approximate continuum level for integration, sim-
ilar to what was done by Muzerolle et al. (1998).
infer the emission fluxes from the equivalent widths, we
must know the underlying photospheric continuum flux.
We used the continuum flux predicted by the PHOENIX
synthetic spectra for a specified Teffand surface gravity.
We inferred Tefffrom our spectral types, and assumed a
surface gravity of logg = 4.0 (cgs units). For five targets
shared by Mohanty et al. (2005), our results were lower
by 0.05 to 0.41 dex. We ignored veiling, which may lead
to an underestimate of line fluxes.
To
3.2. Projected Rotational Velocity
The projected rotational velocity v sin i of each target
was determined by fitting the target spectra against sets
of artificially broadened template spectra derived from
one of the observed slowly rotating standard stars. For
each target, we initially selected the standard star closest
in spectral type as a template. To broaden the templates,
we convolved the original template spectra with the an-
alytical rotational broadening function of Gray (2005)
assuming a limb darkening factor of 0.65.
Our routine to estimate v sin i of a target consists of
four steps. First, we fitted the target spectra with tem-
plate spectra broadened from 0 to 200 km s−1in steps of
10 km s−1, and recorded the v sin i value of the best fit
for each echelle order. Second, we refined our search to
projected rotational velocities within 10 km s−1of the
first-pass results in steps of 1 km s−1, and revised our
estimates accordingly. Third, we computed weighted av-
erages over the echelle orders, after removing outliers us-
ing a standard Tukey filter, i.e. values lying 1.5 times the
interquartile range below the first quartile and above the
third quartile were discarded (cf. Hoaglin et al. (2000);
for a Gaussian distribution, this filter corresponds to re-
moving data points beyond 2.7 σ). Fourth, we calculated
the weighted average across epochs and used it as a pro-
visional v sin i estimate of the target.
To finalize our v sin i estimates, we checked the pro-
visional results using different templates and found that
the variation in estimates was typically an order of mag-
nitude larger than the weighted standard error of individ-
Fig.1.— The 8µm excess vs.
of the peak (Hα 10% width) for 13 Cha I and 54 Tau-Aur mem-
bers. Suspected accretors and non-accretors based on Hα emission
are denoted by solid and hollow symbols, respectively. The Hα
10% width error bars do not correspond to the measurement un-
certainty, but to the scatter in our multi-epoch data. There is a
clear separation of disk candidates above (and the non-disk candi-
dates below) [3.6] − [8.0] = 0.5 illustrated by the dashed line, and
a delineation between accretors and non-accretors at the cutoff
of 200 km s−1adopted originally by Jayawardhana et al. (2003).
Note that some non-accretors appear above the cutoff because of
the additional broadening due to rapid rotation, see Fig. 3.
the full width of Hα at 10%
ual estimates. Therefore, for each target, we calculated
two additional v sin i estimates using the next two closest
standard stars by spectral type, and adopted as v sin i
the estimate from the best-fit template, and as uncer-
tainty, the standard deviation of the estimates between
different templates. The results are listed in Tables 1 &
2. We considered the potential influence of veiling on
our v sin i estimates: strong mass accretors will have
strong veiling which could affect the v sin i estimates.
However, we found no correlation between accretion sig-
natures and rotational velocities when comparing these
values for individual stars over time.
4. RESULTS
4.1. Accretion and Disk Presence
To examine the correlation between disk presence and
accretion, we show in Fig. 1 the 8µm excess against Hα
10% widths for those targets for which both measure-
ments are available. (The error bars on the Hα 10%
width refer to the standard deviation of the estimates
over epochs.) Out of the 67 objects in this subsample,
22 show evidence of both accretion and disk presence (cf.
upper right regions in Fig. 1), implying that gas from the
inner disk is still being channeled onto the star, and 41
objects have neither infrared excess nor signs of accre-
tion.Thus, in nearly all cases (63/67), the accretion
signature is well correlated with disk presence.
Of the four exceptions, the three non-accretors with

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4Nguyen et al.
infrared excess, all in Cha I, are CHXR 20, Hn 18,
and ISO 52. For these objects, accretion rates may
have dropped below measurable levels in Hα 10% width
even though the disks persist.
of Hn 18 is detectable and indicates a negligible accre-
tion rate of 4.4 × 10−11M⊙yr−1. Also, accretion may
be variable on short timescales (Nguyen et al.
submitted to ApJL). For CHXR 20, Ca II-λ8662 emis-
sion was undetected at one epoch, and is present at two
other epochs with a suggested small accretion rate of
2.6×10−10M⊙yr−1. The only accretor without infrared
excess, LkCa 21, was observed on a single epoch with a
Hα 10% width of 277km s−1; this value includes a contri-
bution from rotational broadening of 46km s−1. The net
10% width is below the threshold for accretors originally
set out by (White & Basri 2003). In addition, Ca II-
λ8662 emission was not observed in LkCa 21 implying
that it likely is not an accretor.
The Ca II-λ8662 flux
2008,
4.2. Stellar Mass and Rotational Velocity
Rotational velocity is known to vary as a function
of stellar mass in young stars, likely because the effi-
ciency of angular momentum removal depends on mag-
netic activity, which in turn depends on stellar mass (cf.
Scholz et al. 2007). To probe the rotation-mass depen-
dence, we show the projected rotational velocity as a
function of spectral type in Fig. 2. Here late-K spectral
type corresponds to ∼ 1M⊙ (Baraffe et al. 1998). The
results are similar to what was found previously (e.g.
Scholz et al. 2007; Rebull et al. 2002). Higher mass stars
tend to have faster projected rotational velocity overall
than their lower mass counterparts.
To examine this rotation-mass trend further, we di-
vided our targets into two mass bins consisting of F–K
type stars, and M type stars. In Fig. 3, we show boxplots
of v sin i for the two mass bins: the horizontal lines in-
side the rectangles indicate the median values. Clearly,
in both Cha I and Tau-Aur, the median v sin i for the
higher mass bins, 26 km s−1and 24 km s−1, are signifi-
cantly faster than those of the lower mass bins, both at
11 km s−1.
To get a quantitative sense of the difference in rota-
tional velocity between the high and low mass stars, we
applied the Kolmogorov-Smirnov (K-S) test. This analy-
sis shows there is a probability of only ∼0.5% for Cha I,
and ∼0.1% for Tau-Aur that the v sin i for the two mass
bins were drawn from the same distribution.
When interpreting this finding, one should take into
account that the stars in our sample assuming an age of
∼2Myr span roughly a range of 1–4R⊙in stellar radii.
To gauge the contribution of stellar radius on v sin i, we
evaluated the specific angular momentum in our sample
as follows. First, we converted spectral type to effec-
tive temperature by looking up and interpolating values
from Sherry, Walter & Wolk (2004).
the effective temperature to obtain estimates of mass
M, radius R, and moment of inertia I from the models
of D’Antona & Mazzitelli (1997). Third, we combined
these values with our v sin i estimates to compute the
projected specific angular momentum using the relation
j sin i = (v sin i)I/MR. In Fig. 4, we show j sin i as a
function of stellar mass. From the best linear fit to the
upper envelope of the datapoints, we find by eye that
j ∝ M0.5. Indeed, there is an increase in specific angular
Second, we used
Fig. 2.— Projected rotational velocities v sin i as a function
of spectral type. Suspected accretors and non-accretors based on
Hα emission are denoted by solid and hollow symbols, respectively.
The v sin i errors represent the combined uncertainty between re-
sults using different template spectra, and over different epochs.
The dashed line represents the median v sin i for bins covering on
either side two spectral subtypes. The overall appearance of this
plot is comparable to v sin i distribution in other young clusters:
projected rotational velocity tends to increase with stellar mass.
Fig. 3.— Boxplots of v sin i for Cha I and Tau-Aur grouped into
two mass bins. Clearly, for both regions, the rotational velocities
of high mass stars is faster than their lower mass counterparts by
a factor of 2–2.5. The central rectangles span the first quartile to
the third quartiles with the segment inside indicating the median
values, and “whiskers” above and below the box show the locations
of the minima and maxima after applying a Tukey filter; statistical
outliers and suspected outliers are shown as filled dots and hollow
dots, respectively.

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Disk-Braking in Young Stars5
Fig. 4.— Specific angular momentum j as a function of stel-
lar mass computed assuming an age of 2Myr from the models
of D’Antona & Mazzitelli (1997).
accretors based on Hα emission are denoted by solid and hollow
symbols, respectively. Targets from Cha I are represented by tri-
angles, and those from Tau-Aur are drawn as squares. The dashed
line represents the median v sin i for bins spanning logM/M⊙±0.1.
The dotted line is a linear fit by eye to the upper bound of the data
and has a slope of 0.5.
Suspected accretors and non-
momentum with increasing stellar mass.
4.3. Accretion and Rotational Velocity
To check for a connection between accretion and rota-
tion, in Fig. 5 & 6, we show v sin i as a function of Hα
10% width and of 8µm excess for our targets. In addi-
tion, the figures show both the intrinsic contribution of
rotation to the line widths, and the separation between
accretors and non-accretors.
We compared the distribution of v sin i for accre-
tors and non-accretors using a number of K-S tests.
To account for the rotation-mass dependence (see §4.2),
we carried out these tests for the two mass bins (F–K
type and M type) separately. The probability that the
v sin i of accretors and non-accretors were drawn from
the same distribution in Cha I is 6% for the high-mass
targets, and 50% for the low-mass ones. The probabil-
ities for high- and low-mass targets in Tau-Aur are 8%
and 10%, respectively. For the entire sample, the cor-
responding probabilities for high- and low-mass targets
is 30% and 7%, respectively. Thus, any connection be-
tween accretion and projected rotational velocity is at
best marginally significant. The v sin i distributions are
shown in Fig. 7.
Since the presence of dusty disks is strongly correlated
with accretion in our targets, it is not surprising there is,
for high and low mass stars in Tau-Aur respectively, a 9%
and 13% probability that the v sin i for stars with and
without disks were drawn from the same distribution.
It would appear that the presence of ongoing accretion
or a disk has no significant effect on the rotation in our
sample. This is contrary to the standard disk braking
Fig. 5.— Projected rotational velocity v sin i vs. Hα 10% width
for a sample of T Tauri stars in the Chamaeleon I and Taurus-
Auriga star forming regions. Suspected accretors and non-accretors
based on Hα emission are denoted by solid and hollow symbols,
respectively. The Hα 10% width error bars do not correspond
to the measurement uncertainty, but to the scatter in our multi-
epoch data. The v sin i errors represent the combined uncertainty
between results using different template spectra, and over different
epochs. The intrinsic contribution of rotation to line width is shown
by the dashed line. The adopted boundary between accretors and
non-accretors is shown by the dotted line.
scenario, as outlined in §1.
One particular reason for the negative test results is
the presence of a significant number of rapidly rotat-
ing accretors, as seen in Fig. 5. Based on Spitzer data,
Rebull et al. (2006) find that the fraction of stars with
disks is very low for rotation periods P < 1.8d (see their
Fig. 3). For a radius of 1R⊙and an average sini, this
period corresponds to a projected rotational velocity of
22 km s−1. This value scales linearly with stellar radius.
In our sample, we see 5–10 objects rotating faster than
this threshold, where the exact number depends on the
inclinations and stellar radii. This type of objects is not
expected in the evolutionary sequence for the standard
disk braking scenario described in §1.
Previous studies drew conclusions about the disk-
braking scenario based on rotation periods from pho-
tometric data, while we use projected rotational veloc-
ity. To check whether this makes a difference, we ran
Monte Carlo simulations based on published data from
previous photometric studies, e.g. Stassun et al. (1999),
Herbst et al. (2002) surveys in the ONC. In the simu-
lations, rotation periods were converted to v sin i by
selecting random viewing angles and using uniformly
distributed stellar radii of 1–4R⊙.
Herbst et al. (2002), where there was previous indica-
tion of disk-braking, we found probabilities of <1% that
diskless stars have the same distribution of v sin i as disk
harboring stars, hence we recovered their evidence for
disk braking. Furthermore, for data from Stassun et al.
In the case of