Free Access
Issue
A&A
Volume 564, April 2014
Article Number A118
Number of page(s) 7
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/201322997
Published online 15 April 2014

© ESO, 2014

1. Introduction

The UX Ori (UXOr) phenomenon of Herbig Ae/Be stars (HAeBes) is attributed to obscuration by circumstellar dust in an inclined disk (Grinin et al. 1994; Natta et al. 1997; Grinin et al. 2001; Dullemond et al. 2003) or unsteady accretion (Herbst & Shevchenko 1999). The Herbig Ae star V1026 Sco (HD 142666) has a spectral type of A8Ve (Dominik et al. 2003) and is classified as a UX Ori object (Meeus et al. 1998). The UX Ori variability has been confirmed by Zwintz et al. (2009). Dominik et al. (2003) and van Boekel et al. (2005) report distances of 116 pc and 145 ± 43 pc, respectively. We adopt the Hipparcos-based measurement of 116 pc and the associated parameters for our work. The object V1026 Sco shows large, non-periodic (Lecavelier des Etangs et al. 2005) brightness variations (>1.2 mag) and a pulsational variability on the milli-magnitude level (Zwintz et al. 2009). It reddens with decreasing apparent magnitude (Meeus et al. 1998). These authors suggest that dense dust clouds in an inclined disk cause the stellar reddening. Alecian et al. (2013a) report on the magnetic properties of V1026 Sco (and several other Herbig Ae/Be stars). The object V1026 Sco belongs to the Meeus group IIa (Juhász et al. 2010) and might, therefore, have a self-shadowed disk. The stellar parameters (Dominik et al. 2003) of V1026 Sco are listed in Table 1. By modeling the spectral energy distribution (SED), Dominik et al. (2003) found that the circumstellar disk of V1026 Sco has an inclination of approximately 55°. Monnier et al. (2005) have performed Keck Interferometer (KI) measurements of V1026 Sco and found an inner disk diameter of 2.52 mas (0.29 AU at 116 pc). In a recent publication, Schegerer et al. (2013) have reported mid- and near-infrared (NIR) interferometric observations (archival MIDI/VLTI & IOTA data), and performed radiative transfer modeling of V1026 Sco, and derived a disk structure with a gap from 0.35 AU to 0.80 AU.

In this paper, we analyze the circumstellar environment around V1026 Sco by taking new interferometric NIR VLTI/AMBER and archival mid-infrared (MIR) VLTI/MIDI measurements into account. We describe our observations and the data reduction in Sect. 2. The modeling is presented in Sect. 3, and our results are discussed in Sect. 4.

Table 1

Adopted stellar parameters of HD 142666.

2. Observation and data reduction

We observed V1026 Sco on three different nights with the NIR three-beam VLTI/AMBER instrument (Petrov et al. 2007). The observations were performed in low-resolution mode (spectral resolution R = 30). Table 2 lists the observational parameters. The uv coverage of our 2009 and 2011 AMBER observations with the published KI and archival MIDI observations of V1026 Sco used in this study are shown in Fig. 1.

We reduced the AMBER data with amdlib-3.0.21 (Tatulli et al. 2007; Chelli et al. 2009). To improve the calibrated visibility, we processed only 20% of the frames (object and calibrator) with the best fringe signal-to-noise ratio (Tatulli et al. 2007). In addition, we equalized the histograms of the optical path differences (OPD) of the calibrator and the object data, because different histograms (due to OPD drifts caused, for example, by errors of the OPD model) can lead to visibility errors. Histogram equalization can reduce these visibility errors. This method is described in detail in Kreplin et al. (2012).

We were able to extract H and K band visibilities (cf. Fig. 2). Within the error bars, the obtained closure phase is zero for all nights (Fig. 3), which is consistent with a centro-symmetric brightness distribution.

We also used archival low-resolution data obtained with the MIR two-beam combiner MIDI (Leinert et al. 2003; Schegerer et al. 2013). The data (see Table 2) were reduced using our own IDL codes (see Appendix A of Kishimoto et al. 2011 for details), which utilize a part of the standard software EWS2 and also implement an average over a relatively large number of frames to determine the group-delay and phase-offset tracks with a good signal-to-noise ratio. This is important when dealing with sub-Jy sources, such as our target here. The MIDI visibilities are shown in Fig. 4 (right).

Table 2

Observation log.

thumbnail Fig. 1

The uv coverage of all interferometric measurements used (AMBER, MIDI, Keck; see Table 2).

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3. Analysis

thumbnail Fig. 2

AMBER visibilities (see Table 2) and temperature-gradient model B3: the panels show the wavelength-dependent H and K band visibilities of our AMBER observations. Each panel displays one of the three baselines of each measurement (nights I-IV, cf. Table 2). The red line indicates the corresponding best-fit temperature-gradient model curves (model B3 in Table 5) in all plots.

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3.1. Geometric modeling

To estimate the characteristic size of the NIR emission region, we fit geometric models to the visibilities. The model for the NIR data consists of an unresolved stellar contribution and an inclined ring with a width of 20% of its inner radius rring,in (which is equal to the semi-major axis in the model). We averaged the different visibility measurements (shown in Fig. 2) of the spectral channels within the H and K bands to obtain wavelength-averaged H- and K-band visibilities (Fig. 5). The resulting visibilities within each band were fit with the model visibilities of the two-dimensional, inclined star-ring model.

In the K band, we included literature data from the Keck interferometer (Monnier et al. 2005, see Fig. 1) in the fit. Because of the almost constant projected baseline length and position angle of the five KI measurements, the data points were averaged.

To fit the visibilities, we derived the NIR flux contribution of the star (fstar) from our SED fit (Fig. 6 left) and obtained approximately 0.33 (Monnier et al. 2005 report 0.39) in the K band and 0.53 in the H band. For the total visibility, we obtain (1)where fstar + fdisk = 1 and the unresolved star has Vstar = 1.

We found a semi-major axis (rring,in) of 1.30 ± 0.14 mas (or 0.15 ± 0.06 AU for a distance of 116 pc) in the H band and 1.57 ± 0.09 mas (or 0.18 ± 0.06 AU) in the K band (Fig. 5). All fitted parameters are listed in Table 3. In the H band, the inclination angle i (angle between the system axis and the viewing direction) is 50 ± 11°, and the position angle ϑ of the semi-major axis of the disk is 179 ± 17°. In the K band, we derived i = 49 ± 5° and ϑ = 163 ± 9°, respectively.

thumbnail Fig. 3

Wavelength dependence of the closure phases of all AMBER measurements from Table 2. The curves are offset from each other by 40°. The respective zero line is indicated as a solid horizontal line.

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3.2. Temperature-gradient model

To fit all available wavelength-dependent visibilities (AMBER, MIDI, and KI, see Figs. 2, 4) and the SED (Fig. 6) simultaneously, we used a temperature-gradient model. This model consists of many thin rings emitting blackbody radiation at a local temperature T(r) = T0·(r/r0)q, where r0 denotes the inner disk radius, T0 the effective temperature at r0, and q the power-law index. Other free parameters are the inclination i and the position angle ϑ of the semi-major axis of the disk. A more detailed description of our modeling of temperature-gradient disks can be found, for example, in Vural et al. (2012) or Kreplin et al. (2012).

We computed models for all mathematical combinations of the parameter values listed in Table 4. We chose the model with the lowest value as the best-fit model. The given error bars are 3σ errors.

We first attempted to fit the data with a model consisting of a stellar point source (distance, L, and T from Table 1) and a temperature-gradient disk (model A in Table 4). The model has six free parameters: the inner ring radius rin,1, the width of the ring Δrin,1 = rout,1rin,1, the temperature at the inner radius Tin,1, the power-law index q1, the inclination i, and the position angle of the semi-major axis of the disk-like object ϑ. The best-fit parameters are listed in Table 5. However, no successful fit could be found that is able to reproduce all observations simultaneously ().

Therefore, we adopted a two-component model consisting of the star (same parameters as above) and two inclined concentric ring-shaped disks (model B, see Table 4 and 5). There are ten free model parameters: four for the inner disk (rin,1, Δrin,1, Tin,1, q1), four for the outer disk (rin,2, Δrin,2, Tin,2, q2), and two for the whole disk system (i, ϑ). We computed all combinations of these parameters within the parameter ranges (and for the described N step values) defined in Table 4. We first calculated the models with a rough grid (model B1) and then with finer grids (model B2 and B3) around the -minimum of the previous run. In total (for all models described in Table 4), we computed ~700 million models.

In our best-fitting model B3 (, see Figs. 2, 4, and 6), the inner disk spans from 0.19 ± 0.01 AU to 0.23 ± 0.02 AU (with a temperature of K at the inner radius rin,1) and the outer disk between AU and >4.3 AU ( K at rin,2) with a gap between both components. The very narrow disk width of 0.04 AU makes the inner disk region appear rather ring-like in the NIR (see Fig. 6, right). We cannot constrain the temperature gradient q1 because the inner narrow ring-shaped component is basically emitting only at one uniform temperature Tin,1. The inclination (angle between the system axis and the viewing direction) is °and the position angle of the disk is °, which is approximately consistent with our geometric model in Sect. 3.1. We emphasize that the structure of the outer disk is a result of the longer wavelength data – that is, the MIDI data and the MIR/far-infrared (FIR) SED. The inclination and position angle of the system are mainly determined by the NIR interferometric data but consistent with the MIDI data. In the case of temperature-gradient models with more than one component, please note that simultaneous modeling of the visibilities and the SED is able to constrain the inner temperatures (Tin,1, Tin,2) of the single components rather than the exact shape of the single temperature gradients (q1, q2).

Table 3

Parameters of the best-fit geometric model.

thumbnail Fig. 4

Keck and MIDI observations and temperature-gradient model B3. Left: Keck visibility. Right: MIDI visibilities.

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thumbnail Fig. 5

Geometric inclined ring-fit models (ring width =20% of rring,in) of the H- (left) and K-band (right) visibilities. The right plot contains our AMBER data and the KI measurements. The wavelengths are averaged over the whole respective spectral band. The model consists of the unresolved stellar contribution and an inclined ring (see Sect. 3.1). We simultaneously fit all visibilities with a two-dimensional visibility model. The model curves are plotted for all position angles for which visibilities were measured. The color sequence (blue to red) describes the decreasing difference between the PA of the measurement and the disk’s fitted semi-major axis ϑ.

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thumbnail Fig. 6

Left: SED and temperature-gradient model B3: the SED of V1026 Sco was constructed using published SED observations (black dots). Some of the points represent binned observations; the size of the error bars is on the order of the size of the dots. The resulting fit curve (red) consists of the stellar contribution (Kurucz model, black dashed line), the inner ring (black solid line), and the outer ring model SED (black dash-dotted line). Right: two-dimensional intensity distribution of our best-fit model (B3) at 2 μm. The narrow bright ring is the inner ring-shaped disk in the model. The central star is not shown here. Please note that the intensity scale is logarithmic; the outer disk contributes only insignificantly to the NIR flux. The actual geometry of the outer disk remains poorly constrained because only two MIDI uv points exist.

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Table 4

Scanned range of the parameters of the computed temperature-gradient models (A, B1, B2 and B3).

4. Discussion

To compare the disk size of V1026 Sco with other young stellar objects, we plot the K-band radius (0.18 ± 0.06 AU, see Table 3) obtained with a geometric inclined-ring fit into the size-luminosiy diagram (Fig. 7). The derived radius is ~1.5 times larger than expected for a dust sublimation temperature of 1500 K and is located approximately on the 1200 K line. Within the error bars, it still agrees with a dust sublimation temperature of 1500 K expected for dust consisting mostly of silicates (Natta et al. 2001; Dullemond et al. 2001; Monnier & Millan-Gabet 2002).

The derived best-fit temperature-gradient model B3 consists of a two-component disk model; all parameters are listed in Table 5. The inner component is a ring-shaped disk with an inner radius of 0.19 ± 0.01 AU and an inner temperature of K. The outer ring-shaped disk extends from AU to >4.3 AU with K at the inner edge. Between the hot inner and cool outer components, our best-fit model shows an approximately 1.1 AU-wide gap. The inclination of °is similar to the inclination derived with our geometric model in Sect. 3.1 (~49°and ~50°) and to the value of 55° found by Dominik et al. (2003). Neither of the power-law indices could be constrained. In the inner component, the ring width is too small to allow us to constrain q1. In the outer component, there seems to be a degeneracy between q2 and rout,2. The parameters of the inner disk (Table 5) are similar to the ones obtained with the geometric fit of the NIR visibilities (Table 3) and the size-luminosity diagram (Fig. 7). A possible explanation for the lack of FIR emission in the SED of some HAeBes and the apparent disk gap is self-shadowing by the puffed-up inner rim (Dullemond & Dominik 2004).

The stellar rotation of V1026 Sco is vsini = 65.3 ± 3.1 km s-1 (Alecian et al. 2013a), leading to a maximum rotational velocity of ~86 km s-1 if the inclination of the star is comparable to the disk inclination. This is similar (Boehm & Catala 1995) or slightly lower (Alecian et al. 2013b) than the average velocity of low-mass Herbig Ae/Be stars but higher than measurements of T Tauri stars (Weise et al. 2010).

Magnetic braking can reduce the rotational velocity as observed in T Tauri stars (Koenigl 1991; Weise et al. 2010; Johnstone et al. 2014). In Herbig stars, strong magnetic braking is less likely than in T Tauri stars because the observed magnetic fields are weaker. In V1026 Sco, a magnetic field has not been detected (Alecian et al. 2013a), which could mean that it is weak, as expected for Herbig Ae stars (Weise et al. 2010). Even if a magnetic field exists, but could not be detected, the high value of the stellar rotation velocity of V1026 Sco suggests that rotational braking via disk locking (Koenigl 1991; Stȩpień 2000) is much weaker than in T Tauri stars.

Our findings can be described with the standard disk theories for Herbig Ae stars, which postulate passive circumstellar disks with inner holes and puffed-up inner rims (Natta et al. 2001; Dullemond et al. 2001). In addition, the derived inclination might be large enough to explain the UXOr variability of V1026 Sco in the context of current proposed theories, as we discuss in the following. Theories about partial obscuration of the stellar light by hydrodynamic fluctuations of the inner rim need high inclination angles for explaining the UXOr variability (Dullemond et al. 2003). With the measured intermediate inclination of V1026 Sco, the rim fluctuations would have to be twice as high as the theoretical fluctuation height. Therefore, this model is less suitable in our case and also for several other UXOrs (e.g. Pontoppidan et al. 2007). The unsteady accretion model by Herbst & Shevchenko (1999) is inclination independent and cannot be disproven with our measurements. For intermediate to high disk inclinations, orbiting dust clouds might intercept the line of sight toward the star (Grinin et al. 1994; Natta et al. 1997). For this case, the derived inclination of V1026 Sco is still within the range predicted for UXOrs (45°–68°) by Natta & Whitney (2000). Dust clouds in centrifugally-driven disk winds (Vinković & Jurkić 2007; Bans & Königl 2012) can also explain the UX Ori type variability of V1026 Sco, as they also are consistent with intermediate to high disk inclinations.

Table 5

Parameters of best-fit temperature-gradient models A and B3.

thumbnail Fig. 7

Size-luminosity diagram. The K-band ring-fit radius of V1026 Sco (semi-major axis derived with an inclined ring fit in Sect. 3.1) is plotted as a green filled square. For comparison, we also plot a sample of Herbig Ae stars (unfilled black triangles, Monnier et al. 2005) and a sample of TTS (filled black triangles, Pinte et al. 2008). The theoretical relation between the ring radius in the NIR and the luminosity (Monnier & Millan-Gabet 2002) is shown for different temperatures. The 1500 K line is plotted in red; the gray lines indicate curves for temperatures between 500 K (highest line) and 2000 K (lowest line) in 100 K steps.

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5. Conclusion

We observed the UX Ori star V1026 Sco with VLTI/AMBER in the H and K bands. With a geometric ring-shaped model consisting of the star and an inclined ring, we found a radius of rring,in = 0.18 ± 0.06 AU in the K band. In the context of the size-luminosity diagram, this radius is found to be consistent with the theory of a passive circumstellar disk with an inner hole and a rim at the dust sublimation radius. We further derived an inclination of 50 ± 11° and 49 ± 5° and a PA of the semi-major axis of the inclined disk of 179 ± 17° and 163 ± 9° in the H and K bands, respectively.

We found a two-component-disk temperature-gradient model that is able to reproduce all visibilities and the SED. The inner radius of the inner disk is 0.19 ± 0.01 AU and similar to the one found with a geometric ring fit. The two disk components are separated by a gap, which may be explained by a shadow cast by a puffed-up inner rim and agrees with the type II classification of the object. The derived inclination of °and the PA of °are consistent with the values found by geometric modeling. Our inclination of ~49° is probably not consistent with a model where rim fluctuations cause the UXOr variability, because the expected rim height is not high enough, as discussed above. The unsteady accretion theory cannot be excluded with our measurements, because the model is inclination-independent. Finally, the measured intermediate disk inclination is within the range predicted from UXOr models with orbiting dust clouds in the disk or in centrifugally-driven disk winds.


Acknowledgments

We would like to thank K. R. W. Tristram for helpful discussions and suggestions and also our colleagues at Paranal for their excellent collaboration. We thank the anonymous referees for the helpful comments.

References

All Tables

Table 1

Adopted stellar parameters of HD 142666.

Table 2

Observation log.

Table 3

Parameters of the best-fit geometric model.

Table 4

Scanned range of the parameters of the computed temperature-gradient models (A, B1, B2 and B3).

Table 5

Parameters of best-fit temperature-gradient models A and B3.

All Figures

thumbnail Fig. 1

The uv coverage of all interferometric measurements used (AMBER, MIDI, Keck; see Table 2).

Open with DEXTER
In the text
thumbnail Fig. 2

AMBER visibilities (see Table 2) and temperature-gradient model B3: the panels show the wavelength-dependent H and K band visibilities of our AMBER observations. Each panel displays one of the three baselines of each measurement (nights I-IV, cf. Table 2). The red line indicates the corresponding best-fit temperature-gradient model curves (model B3 in Table 5) in all plots.

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In the text
thumbnail Fig. 3

Wavelength dependence of the closure phases of all AMBER measurements from Table 2. The curves are offset from each other by 40°. The respective zero line is indicated as a solid horizontal line.

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In the text
thumbnail Fig. 4

Keck and MIDI observations and temperature-gradient model B3. Left: Keck visibility. Right: MIDI visibilities.

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In the text
thumbnail Fig. 5

Geometric inclined ring-fit models (ring width =20% of rring,in) of the H- (left) and K-band (right) visibilities. The right plot contains our AMBER data and the KI measurements. The wavelengths are averaged over the whole respective spectral band. The model consists of the unresolved stellar contribution and an inclined ring (see Sect. 3.1). We simultaneously fit all visibilities with a two-dimensional visibility model. The model curves are plotted for all position angles for which visibilities were measured. The color sequence (blue to red) describes the decreasing difference between the PA of the measurement and the disk’s fitted semi-major axis ϑ.

Open with DEXTER
In the text
thumbnail Fig. 6

Left: SED and temperature-gradient model B3: the SED of V1026 Sco was constructed using published SED observations (black dots). Some of the points represent binned observations; the size of the error bars is on the order of the size of the dots. The resulting fit curve (red) consists of the stellar contribution (Kurucz model, black dashed line), the inner ring (black solid line), and the outer ring model SED (black dash-dotted line). Right: two-dimensional intensity distribution of our best-fit model (B3) at 2 μm. The narrow bright ring is the inner ring-shaped disk in the model. The central star is not shown here. Please note that the intensity scale is logarithmic; the outer disk contributes only insignificantly to the NIR flux. The actual geometry of the outer disk remains poorly constrained because only two MIDI uv points exist.

Open with DEXTER
In the text
thumbnail Fig. 7

Size-luminosity diagram. The K-band ring-fit radius of V1026 Sco (semi-major axis derived with an inclined ring fit in Sect. 3.1) is plotted as a green filled square. For comparison, we also plot a sample of Herbig Ae stars (unfilled black triangles, Monnier et al. 2005) and a sample of TTS (filled black triangles, Pinte et al. 2008). The theoretical relation between the ring radius in the NIR and the luminosity (Monnier & Millan-Gabet 2002) is shown for different temperatures. The 1500 K line is plotted in red; the gray lines indicate curves for temperatures between 500 K (highest line) and 2000 K (lowest line) in 100 K steps.

Open with DEXTER
In the text

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