Compilation error when using Unique_ptr versus Shared_ptr as Class member

I am planning on porting some Python code to C++. I want to use C++11 techniques.

I have defined this class:

class Scenario
{

public:
    Scenario();
    ~Scenario();
private:
    std::shared_ptr<Scheduler> scheduler;
};

With empty constructor and destructor. This compiles.

However when changing shared_ptr to unique_ptr, compilation fails with a message that the default constructor is deleted and not accessible.

Question 1: Why?

The following:

class Scenario
{

public:
    Scenario();
private:
    std::unique_ptr<Scheduler> scheduler;
};

does compile however.

Question 2: Why does defining a destructor causes the constructor to be deleted.