International Journal of Astronomy and Astrophysics, 2019, 9, 321-334
http://www.scirp.org/journal/ijaa
ISSN Online: 2161-4725
ISSN Print: 2161-4717

Optical Spectroscopic Monitoring Observationsof a T Tauri Star V409 Tau
Hinako Akimoto, Yoichi Itoh
Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo, Hyogo, Japan

How to cite this paper: Akimoto, H. and
Itoh, Y. (2019) Optical Spectroscopic Monitoring Observations of a T Tauri Star V409
Tau. International Journal of Astronomy
and Astrophysics, 9, 321-334.
https://doi.org/10.4236/ijaa.2019.93023
Received: July 24, 2019
Accepted: September 16, 2019
Published: September 19, 2019
Copyright © 2019 by author(s) and
Scientific Research Publishing Inc.
This work is licensed under the Creative
Commons Attribution International
License (CC BY 4.0).
http://creativecommons.org/licenses/by/4.0/
Open Access

Abstract
We report the results of optical spectroscopic monitoring observations of a T
Tauri star, V409 Tau. A previous photometric study indicated that this star
experienced dimming events due to the obscuration of light from the central
star with a distorted circumstellar disk. We conducted medium-resolution (R~10,000) spectroscopic observations with 2-m Nayuta telescope at Nishi-Harima Astronomical Observatory. Spectra were obtained in 18 nights
between November 2015 and March 2016. Several absorption lines such as Ca
I and Li, and the Hα emission line were confirmed in the spectra. The Ic-band
magnitudes of V409 Tau changed by approximately 1 magnitude during theobservation epoch. The equivalent widths of the five absorption lines areroughly constant despite changes in the Ic-band magnitudes. We conclude
that the light variation of the star is caused by the obscuration of light fromthe central star with a distorted circumstellar disk, based on the relationshipbetween the equivalent widths of the absorption lines and the Ic-band magnitudes. The blue component of the Hα emission line was dominant during theobservation epoch, and an inverse P Cygni profile was observed in eight ofthe spectra. The time-variable inverse P Cygni profile of the Hα emission line
indicates unsteady mass accretion from the circumstellar disk to the central
star.

Keywords
Star Formation, Pre-Main Sequence Stars, T Tauri Stars

1. Introduction
Young stellar objects (YSOs) are generally variable stars. Some objects show periodic variability, while others exhibit irregular variability. Some objects also exhibit episodic variations. For example, FU Orionis outbursts represent the most
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H. Akimoto, Y. Itohextreme case of variability in these objects [1]. Most members of this class haveundergone outbursts of 4 - 6 magnitude in optical brightness [2] [3]. In [4], theauthors proposed the movement of cool spots on the stellar surface due to stellar
rotation, unsteady accretion of circumstellar material onto the star, and obscuration of light from the photosphere by a distorted circumstellar disk as causes ofthe light variation of YSOs.
Thus far, numerous photometric studies on the variability of YSOs have been
conducted. In [5], the photometric monitoring of 24 YSOs was reported, and theauthors observed one or two (quasi-) sinusoidal curve(s) with a period of 1.2 to12 days in the optical light curves of 20 objects. They suggested spot movementon the stellar surface due to stellar rotation as the cause of the light variation.
Also, numerous studies have also been reported on the variability of YSOs observed by spectroscopy. In [6], spectroscopic monitoring observations of aweak-lined T Tauri star, V410 Tau, were reported. The authors obtained spectrawith a wavelength coverage of 3850 Å - 9050 Å and a resolution of ~12,000 and
identified hot spots on the photosphere by producing Doppler images of the object.
Spectroscopic observations are useful for further understanding of the brightness variations of YSOs. [7] conducted the photometric observation of HD
288313. This object showed light variations, which was considered to be attributed to change of coverage of cool spots. Distribution of the chromospheric active regions was examined by spectroscopic observations. The Hα equivalent
width shows rotational modulation only at occasional epochs. It was proposedthat the chromospheric active regions spread across the stellar surface.
AA Tau is another example of YSOs exhibiting light variation. In [8], the authors reported high-resolution spectroscopic observations of AA Tau, which exhibits light variation due to obscuration of the photosphere by the inner edge ofa magnetically-warped disk. When the star is faint, a red-shifted absorption line
appears on the broad emission line of the Balmer series. The redshifted absorption line indicates infalling motion of cold material in front of the photosphere,suggesting an accretion flow of circumstellar material onto the photosphere.
A classical T Tauri star, RW Aur A, is an irregular variable with a large amplitude. [9] searched for periodicities in the variations of the brightness and colors of RW Aur A over three decades. With the spectroscopic observations, they
insisted the accretion of the magnetosphere. Study of the circumstellar environments of YSOs helps in the discussion of the formation of planets [10]. In particular, objects that display large photometric dimming events caused by circumstellar disks give us the opportunity to study the evolution of the circumstellar environments of young stars.
Photometric observations of V409 Tau were carried out with the Kilodegree
Extremely Little Telescope North (KELT-North) [10]. V409 Tau is a Class II object [11] with the spectral type of K8--M0 [11] or M1.5 [12]. They observed two
separate dimming events; one from January 2009 to October 2010 and anotherfrom March 2012 until at least September 2013. In the latter event, the depth of
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H. Akimoto, Y. Itohthe dimming was 1.4 magnitude in the V-band. They estimated the upper limitof the duration of the 2009 dimming event to 630 days. The interval between the
beginnings of the two dimming events was 1130 days. By combining data fromthe Combined Array for Research in Millimeter Astronomy, the All Sky Automated Survey, the Catalina Real-time Transient Survey, and the Wide Angle
Search for Planets, the authors constructed the spectral energy distribution,
which was fitted with the star and disk SED model with a disk inclination angleof 81 degrees. They indicated that an almost edge-on circumstellar disk was distorted and that the dimming was caused by the obscuration of light from thecentral star by the distorted disk.
V409 is a UX Orionis candidate [10]. UX Ori type variability is observed in
Herbig Ae/Be stars and some T Tauri stars with early K spectral types [1]. The
variability is characterized by large-amplitude light variations with no evident
veiling or effective temperature variations. This is interpreted to be caused byvariable obscuration by circumstellar dust. [1] constructed an accretion disk
model in which a large amount of accretion material heats gas in the circumstellar disk, making the disk flaring. They calculated the probability of observationsof the star through the circumstellar disk. The result shows that high mass-accretion rate stars are more likely to be observed through the circumstellar disk thanlow mass-accretion rate stars. They suggested that accretion is driving UX Ori
variability.
In this paper, we present the spectroscopic monitoring observations of V409
Tau, which exhibits irregular variability. A previous photometric study [10]suggested obscuration of the photosphere by a distorted circumstellar disk as thecause of the variability.

2. Observations and Data Analysis
We conducted a series of spectroscopic observations of V409 Tau over 18 nights
between November 2015 and March 2016. Observations were carried out withthe medium- and low-dispersion long-slit spectrograph (MALLS) mounted on
the Nasmyth platform of the Nayuta telescope at Nishi-Harima Astronomical
Observatory, Japan. With a grating of 1800 lines/mm and a 0.8'' slit, we obtained
spectra with a wavelength resolution of ~10,000 between 6280 Å and 6720 Å.
This wavelength range was chosen for investigating the Hα emission line which
is an index of accretion, and metallic absorption lines originating from the photosphere. The exposure time ranged from 900 - 1200 s, and 1 - 17 spectra weretaken each night. The goal was to reach S/N of 20 on good observation conditions. Flat frames and comparison frames were acquired with a halogen lamp
and an Fe-Ne-Ar lamp in the instrument, respectively. Dark frames were also
taken. Details of the observations are shown in Table 1.
The image analysis software IRAF (Imaging Reduction and Analysis Facility)
was used for image processing. First, the overscan and dark current were subtracted from the raw data. Next, we performed flat-fielding with the normalized
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H. Akimoto, Y. Itoh
Table 1. Observation date, exposure time, and S/N.
Date (JST)Exp. Time (s)

S/N

Date

Exp. Time (s)S/N2015 Nov 29

3600

23

2016 Jan 7

6000

11

2015 Nov 30

3600

19

2016 Jan 10

8400

16

2015 Dec 1

3600

25

2016 Jan 12

4800

2

2015 Dec 4

3600

20

2016 Jan 13

4800

4

2015 Dec 7

3600

29

2016 Jan 16

7200

5

2015 Dec 19

3600

8

2016 Mar 7

1200

8

2015 Dec 27

6000

5

2016 Mar 11

1200

2

2015 Dec 29

3600

10

2016 Mar 12

4500

11

2016 Jan 1

20,40039

2016 Mar 26

1800

20

flat frames, wavelength calibration and distortion correction with comparison
frames, and background subtraction. The spectrum was extracted and binned
based on the slit width. We combined the spectra acquired for each night, andfinally, we normalized the continuum level of the spectrum. We shifted the wavelengths of the spectra so that the wavelengths of the five absorption lines described below matched to the wavelengths of the lines in vacuum.

3. Results
We obtained spectra for 18 nights. The spectra exhibited the Hα emission line
and several absorption lines. The Hα line displayed an inverse P Cygni profile.
Among the spectra, those acquired for eight of the nights exhibited a signal-to-noise (S/N) ratio greater than 15, enabling clear confirmation of the absorption lines (Figure 1). The equivalent widths of five absorption lines (Ca I at
6439 Å, Ca I and Co I at 6450 Å, Ca I at 6463 Å, Li and Fe I at 6708 Å, and Ca I
at 6718 Å) were measured using the SPLOT task in IRAF. The lines were fittedwith a Gaussian function, and the error in the equivalent width was estimatedfrom the root-mean-square of the continuum region adjacent to the line in the
spectrum.
Table 2 shows the equivalent widths of the absorption lines and the Ic-band
magnitudes of the object. The Ic-band magnitudes were taken from the databaseof the Kamogata/Kiso/Kyoto wide-field survey (KWS) [13]. The light curve of
V409 Tau is shown in Figure 2. If the Ic-band magnitude was not available for aparticular observation date, we used photometric data taken within one day ofthat date. We do not further discuss the spectrum taken in March 2016 because
no photometric data are available within 30 days from the observation date.
During the observation period, V409 Tau changed by 1 magnitude in the

Ic-band; however, no significant changes were observed in the equivalent widthsof the absorption lines.
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H. Akimoto, Y. Itoh

Figure 1. Optical spectra of V409 Tau. The horizontal axis represents the wavelength andthe vertical axis presents the normalized flux + constant. Spectra with S/N ratios greaterthan 15 taken in eight nights are shown. Prominent absorption lines are Ca I at 6439 Å,
Ca I and Co I at 6450 Å, Ca I at 6463 Å, Li and Fe I at 6708 Å, and Ca I at 6718 Å. The Hαemission line shows an inverse P Cygni profile that changes with time. A spectrum of anM0 dwarf (HD156274) is also shown [33].

Figure 2. KWS light curve of V409 Tau. The filled circles indicate the Ic-band magnitudes. The open circles indicate the V-band magnitudes.
Table 2. Observation date, Ic-band magnitude, and equivalent widths of the absorption
lines.

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Date

I c ( mag )EW6439 ( Å )EW6450 ( Å )EW6463 ( Å )EW6708 ( Å )EW6718 ( Å )2015 Nov 29

11.45 ± 0.48

0.38+−0.07
0.06

0.39+−0.13
0.09

0.59+−0.19
0.19

0.53+−0.10
0.06

0.29+−0.07
0.122015 Nov 30

11.19 ± 0.15

0.42+−0.11
0.09

0.37 +−0.11
0.08

0.49+−0.16
0.13

0.56+−0.14
0.11

0.26+−0.79
0.102015 Dec 1

11.19 ± 0.15

0.43+−0.05
0.05

0.41+−0.08
0.07

0.53+−0.08
0.08

0.59+−0.07
0.06

0.68+−0.20
0.222015 Dec 4

10.97 ± 0.12

0.51+−0.19
0.11

0.57 +−0.33
0.21

0.52+−0.12
0.15

0.59+−0.09
0.08

0.27 +−0.71
0.022015 Dec 7

11.14 ± 0.13

0.41+−0.10
0.08

0.41+−0.10
0.09

0.48+−0.40
0.11

0.63+−0.09
0.08

0.36+−0.50
0.042016 Jan 1

11.59 ± 0.24

0.38+−0.04
0.04

0.45+−0.01
0.10

0.45+−0.07
0.07

0.43+−0.04
0.04

0.18+−0.03
0.082016 Jan 10

12.01 ± 0.36

0.26
0.16+−0.05

0.36+−0.11
0.09

0.54+−0.20
0.23

0.50+−0.27
0.27

0.35+−0.04
0.27

2016 Mar 26-

0.45+−0.11
0.09

0.41+−0.25
0.13

0.43+−0.36
0.10

0.59+−0.13
0.11

0.27 +−0.46
0.13

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4. Discussion
4.1. Absorption Lines
V409 Tau showed a 1-magnitude variation in the Ic-band during the observation
epoch. To specify the cause of the light variation, we constructed three simple
models, in which the variation of the equivalent widths of the absorption lines
was investigated. Since the SED of V409 Tau indicates little contribution of thecircumstellar disk in the optical wavelength range [10], we assume that the absorption lines in the optical region originate from the photosphere.
First, we investigated the possibility that the luminosity changes resulted fromthe coverage of cool spots on the stellar surface. The spots have a temperatureapproximately 1000 K lower than the temperature of the photosphere [5]. Assuming a constant stellar radius and an effective temperature of 3800 K [14], theradiation of the central star, I λ , is the sum of the blackbody radiation, Bλ (T ) ,of the normal photosphere and that of the cool spots, as follows;=
I λ Bλ ( 3800 ) ⋅ (1 − β ) + Bλ ( 2800 ) ⋅ β ,(1)where β ( 0 ≤ β ≤ 1 ) represents the coverage of the cool spots. The calculated
radiation was multiplied by the transmittance of the Ic-band filter [15]. We set
β = 0 for the brightest epoch (11 magnitude in the Ic-band) and calculated the
Ic-band magnitude by changing β . For the faintest epoch (12 magnitude in the
Ic-band), we set β = 0.5 . The equivalent width of an absorption line is the sumof the equivalent widths of the absorption lines for the 3800-K and 2800-K spectra.

=
EW EW ( 3800 ) ⋅ (1 − β ) + EW ( 2800 ) ⋅ β(2)The model spectra were calculated using the BT-NextGen model [16]. We assumed the surface gravity to be log g = 3.5 and the metallicity to be the same as
the Sun. The spectra for temperatures of 3800-K and 2800-K were calculated andcombined with the coverage factor, β , and we smoothed the model spectrum
to the spectral resolution of the observed spectra. Table 3 shows the calculated
equivalent widths of the absorption lines. The equivalent widths decrease or are
constant with increasing the spot coverage.
Table 3. Equivalent widths of the absorption lines expected for luminosity changes
caused by cool spots on the stellar surface.

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β

0.0

0.1

0.2

0.3

0.4

0.5

I c ( mag )

11.00

11.35

11.45

11.56

11.83

11.99

EW6439 ( Å )

1.04

0.99

0.94

0.89

0.84

0.79

EW6450 ( Å )

0.25

0.23

0.21

0.19

0.17

0.15

EW6463 ( Å )

0.88

0.83

0.79

0.74

0.69

0.65

EW6708 ( Å )

0.79

0.79

0.79

0.79

0.79

0.78

EW6718 ( Å )

0.30

0.30

0.29

0.29

0.29

0.29

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H. Akimoto, Y. Itoh

We next considered unsteady material accretion from the circumstellar disk
onto the photosphere. When the material accretion rate increases, the boundary
layer connects to the inner part of the disk [17] and emits continuum light in theultraviolet and optical wavelengths [18]. We assumed constant intensities of thephotosperic absorption lines. When the intensity of continuum increases by α ,the equivalent width EW ′ is S ( I + α=
) EW (1 + V ) , where I is the intensityof continuum, S is the intensity of absorption line, and V = α I . We set V = 0
and used the 3800 K spectrum in the BT-NextGen model when the star is 12
magnitude. The EWs of the absorption lines were calculated until V increased
and the Ic-band magnitude reached 11 magnitude.
We also considered the obscuration of light from the photosphere by the distorted disk. AA Tau-like variables show periodic dips in their light curves, which
are thought to be caused by the obscuration of light from the photospherecaused by warps in the inner disk. The periods of the dips are typically five to tendays, corresponding to a distance of 0.1 AU between the central star and the inner disk for Keplerian rotation [19]. In the case of AA Tau, the luminosity dropsby 1.2 magnitude with a period of 8.2 days [8] [20]. It is proposed that the innerdisk is distorted by the magnetic field and thus obscures the photosphere. As
light from the central star passes through the disk, the intensity decreases uniformly within a certain wavelength range. As the absorption line weakens, theadjacent continuum light also weakens. As a result, the equivalent width of theabsorption line does not change when the photosphere is obscured.
Figure 3 shows the relationship between the equivalent widths of the absorption lines and the broadband magnitudes. The observed equivalent widths areroughly constant at all magnitude. In the case of the cool spots, the equivalent
widths decrease slightly or are constant with decreasing luminosity. In the caseof accretion, the equivalent widths increase with decreasing luminosity. In thecase of the disk, the equivalent widths are constant. Figure 3 indicates that thecause of the 2015 dimming event is the case where the coverage of cool spots on
the stellar surface has changed or the case where the light from the photosphereis obscured by a distorted disk. V409 Tau has a periodic photometric variation of
4.754 days, corresponding to the rotation period of the photosphere with star
spots [21]. This period is significantly shorter than the duration of the 2015
dimming event. The cool spot model indicates that the large portion of the photosphere is covered by the spot when the star was faint. The temperature of the
spot is as low as 2800 K, so that the spectrum would show absorption features at
6650 Å and 6680 Å. However, no clear absorption features appeared in the observed spectra. Therefore we claim the occultation by the disk as the cause of thelight variation.

Figure 4 is a (V − I c , I c ) color-magnitude diagram. The Ic-band data aretaken from the KWS database. We acquired V-band photometric data from
ASAS-SN [22] taken on the same day as the KWS data. The arrow in the figure
indicates interstellar extinction. Photometric variation can be interpreted as the
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H. Akimoto, Y. Itoh

Figure 3. Relationship between the equivalent widths of the absorption lines and the
Ic-band magnitudes. The equivalent widths of the five absorption lines at 6439 Å, 6450 Å,
6463 Å, 6708 Å, and 6718 Å were measured (filled circles). Dashed-dotted line: equivalent
widths expected if the luminosity changes result from variations in the coverage of coolspots on the stellar surface. Solid line: equivalent widths expected if the luminosity
changes result from unsteady material accretion from the circumstellar disk onto thephotosphere. Dotted line: equivalent widths expected if the luminosity changes result
from obscuration by the distorted disk. The equivalent widths of the obscuration model
are shifted to match the equivalent width of the cool spot model at 11 magnitude. The
measured equivalent widths are roughly constant for all absorption lines regardless of the
Ic-band magnitude.

Figure 4.

(V − I c , I c ) color-magnitude diagram. We used Ic-band photometric datafrom KWS and V-band photometric data from ASAS-SN. The arrow indicates interstellar
extinction. Photometric variation can be interpreted as the intersteller extinction.
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H. Akimoto, Y. Itohintersteller extinction. This further supports that the dimming event occurred
due to the obscuration by the disk.
[10] presented the light variation of V409 Tau. They reported that the star experienced two dimming events in 2009-2010 and 2012-2013. The authors discussed the number of obscuring objects and their distances from the central star.
In the discussion, they presented the possibility that obscuring objects of 2009
and 2012 are different. This is because V409 Tau did not experience significant
dimming in 2002 and 2006. Other occultation happened in the 1960s [23]. Theysuggested that the obscuring object is located at farther 10.7 AU from the central
star if the dimming events are attributed to different obscuring objects. They
argued that if it is a single feature, it implies that the duration and depth ischanging. In this case, the distance between the central star and the obscuring
object was determined as 1.7 AU.
In this paper, the duration of the dip cannot be precisely estimated due to thesparsity of the photometric data. According to the KWS data, the Ic-band magnitude of the star increased to 11.2 mag on September 22, 2014 and then varied
within the range of 0.5 mag. The magnitude dropped to 12.9 mag on December
28, 2015 and then returned to 11.2 mag on October 26, 2017. We assume a single
obscuration object, with dimming events ending on September 22, 2014 andOctober 26, 2017. If we adopt 0.57M  and 1.11R [10] for the mass and radius of the central star, respectively, the distance between the center star and the
obscuration object is 1.76 AU. In our observations, the maximum duration of
the dimming event was ~668 days (December 28, 2015 to October 26, 2017),
similar to that of the 2009-2010 dimming event. However, the beginning of the
2012-2013 dimming event is not consistent with the orbital period of the obscuration object. For a precise determination of the shape and number of obscuring
objects and the distance from the central star, more frequent photometric observations are required.

4.2. Hα Emission LineThe Hα emission line of V409 Tau shows an asymmetric profile with time variation. The Hα line had an S/N > 5 in the spectra acquired for 13 nights. Amongthese spectra, nine displayed an inverse P Cygni profile (Table 4).
In [24], the authors reported on low- and high-resolution spectroscopy of a T
Tauri star, T Cha. The Hα emission line profile changed from pure emission to
an inverse P Cygni profile in less than one day. The authors claimed that a
red-shifted absorption component appeared in the optically thin Hα emission
when an episodic mass accretion was caused. [25] claimed that the redshifted
absorption arises from an accretion flow from the inner disk to the stellar surface along the magnetospheric field lines. As suggested by the relatively highprojected rotational velocity of the star [26], T Cha is seen in an almost edge-on
geometry, as is the case for V409 Tau [10]. We propose that the redshifted absorption for V409 Tau arises from the accretion flow and is overlaid on the
broad emission line.
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Table 4. Observation date and equivalent widths of the blueward and redward components in the Hα emission line.
date (JST)EWblue ( Å )EWred ( Å )inverse P Cygni

2015 Nov 29

0.63+−0.23
0.16

0.01+−0.10
0.01

yes

2015 Nov 30

1.22+−0.33
0.25

0.32+−0.28
0.17

yes

2015 Dec 1

0.99+−0.22
0.16

0.06+−0.09
0.04

yes

2015 Dec 4

0.90+−0.38
0.25

0.18+−0.20
0.13

yes

2015 Dec 7

1.03+−0.33
0.25

0.43+−0.38
0.25

yes

2015 Dec 19

2.59+−1.18
0.85

0.88−+1.02
0.54

no

2015 Dec 29

1.58−+0.97
0.65

0.59+−0.78
0.39

no

2016 Jan 1

1.28+−0.23
0.17

0.03+−0.09
0.02

yes

2016 Jan 7

3.64+−0.66
0.52

0.42+−0.54
0.30

no

2016 Jan 10

1.07 −+0.55
0.40

0.48−+0.43
0.27

yes

2016 Mar 7

1.15+−0.71
0.54

0.60−+0.65
0.43

yes

2016 Mar 12

1.42+−0.54
0.40

0.58+−0.53
0.36

no

2016 Mar 26

0.78+−0.31
0.24

0.12+−0.22
0.10

yes

In [27], the shape of the Hα emission line of 63 YSOs was analyzed. The authors measured the equivalent widths of the blueward and redward componentsof the line. For the relatively weak emission line stars [10 Å < EW(Hα) < 70 Å],
the equivalent widths of the blueward and redward components are comparable.
In contrast, among the strong emission line stars [EW(Hα) ≥ 70 Å], the redward
components are stronger than the blueward components for many objects.
We measured the equivalent widths of the redward and blueward componentsof the Hα emission line of V409 Tau between −500 km/s and +500 km/s (Figure5). We set 6563 Å to 0 km/s. We defined redward component as the velocity
range from 0 km/s to +500 km/s and blueward component from 0 km/s to −500
km/s. We did not measure the equivalent width of the absorption line. The total
equivalent width of the redward and blueward components of V409 Tau varies
from 0.64 Å to 4.08 Å. For the Hα emission line of V409 Tau, the blueward
component is dominant. The equivalent widths of both the blueward and redward components vary with time. The time-variable inverse P Cygni profile ofthe Hα emission line of V409 Tau indicates unsteady mass accretion from thecircumstellar disk to the central star. Based on the correlation between the massof T Tauri stars and the mass accretion rate [28], the mass accretion rate of V409
Tau is estimated to be 10−8 M  yr . Assuming that the inner radius of the accretion disk is five times the stellar radius [29], Rs = 1.11R and M s = 0.57 M 
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H. Akimoto, Y. Itoh[10] [30], we determined that the accretion luminosity is 0.127L . Thus, theluminosity of the object increases by 0.4 magnitude when the accretion phenomenon occurs. Because continuum excesses caused by accretion shocks primarily contribute to the blue and ultraviolet flux [31], we expect that the light variation due to unsteady mass accretion is less than 0.4 magnitude in the Ic-band.
Figure 6 shows the relationship between the Ic-band magnitudes and theequivalent widths of Hα emission line. Except for one data, one may find a tendency that the equivalent widths increase with decreasing the broad-band
brightness. This implies light variation due to veiling. However, most of the Hαlines show the inverse P Cygni profile. Since we are measuring only the emission
line part, it is necessary to construct a detailed model that takes self-absorption
into account (e.g. [32]).

Figure 5. Equivalent widths of the redward and blueward components of the Hα emission line of V409 Tau. The open circle indicates the inverse P Cygni profile. The blueward
components are dominant for all spectra, and the inverse P Cygni profile appeared when
the Hα emission was weak.

Figure 6. Equivalent widths of the Hα emission line of V409 Tau and Ic-band magnitudes. Most of the Hα lines show the inverse P Cygni profile (open circles).
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5. Summary
We conducted optical spectroscopic monitoring of a T Tauri star V409 Tau.
Medium-resolution spectra were obtained over 18 nights in 2015 and 2016.
While the Ic-band magnitude of the object changed by 1 magnitude during theobservations, the equivalent widths of the absorption lines remained nearly constant. We constructed three simple models to investigate the variation of theequivalent widths of the absorption lines. We concluded that the light variationarises from the distorted disk. If only one obscuring feature is present, the distance between the central star and the obscuring feature is estimated as 1.76 AU.
The Hα emission line profile of V409 Tau showed a time-variable inverse P
Cygni profile, indicating unsteady mass accretion from the circumstellar disk to
the central star.

Acknowledgements
H. A. is supported by the Iue Memorial Foundation.

Conflicts of Interest
The authors declare no conflicts of interest regarding the publication of this paper.

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